■ I I -v.- ^m %> C/ aso gUg^ AEROGRAPHER'S MATE THIRD CLASS (OBSERVER) f '•*■ ^^^* ^ '^fiB, jl^^^^^^^^^^r -^E^^B ' *•' '.' ■ ' ^■■^^ ■ 4y AvSj 201.45. IB Figure 1-1-5. — Altocumulus. or filaments. The forms of CI cloud that is most readily identified is the hook-shaped CI. This type of CI, figure 1-1-6, is in very fine strands which are shaped into the form of a hook by the wind. HI through H6 in foldout 1-1-3 (appearing at the end of this lesson) show eight different types of CI cloud formations. Cirrus clouds of a denser variety, as shown on foldout 1-1-3, H3, frequently evolve from the dissipation of other basic cloud forms such as CB. The cirriform remains of a cloud may spread out to a great extent and completely lose its former identity (anvil shape). Cirrus clouds also form from middle cloud layers that are forced aloft. Cirrus and cirrostratus clouds are often combined in one layer as shown on foldout 1-1-3, H5, and H6. When an extensive cirrostratus layer approaches the station from the distant horizon, the leading edge is usually CI clouds. As the layer continues to approach, the cloud layer becomes more uniform and usually thickens. This situation is quite common in advance of a warm front. Halo phenomena, figure 1-1-7, can occur with CI clouds, but this is relatively rare. When a halo is present with cirrus, it is usually only a partial halo because of the characteristics of cirrus (strands, filaments, etc.). When the halo is a complete circle, you should suspect the presence of cirrostratus. C H -1 (CIRRUS).— High cloud "1" is a CI cloud in the form of filaments, strands, or hooks that do not progressively invade the sky. This cloud type is often present with other CI clouds. In this case, you classify the cloud type as HI only when the total amount of hooks, filaments, or strands is greater than the combined total of the other CI clouds present. Whatever the situation, remember that HI does not progressively invade the sky. 1-1-13 AEROGRAPHER'S MATE THIRD CLASS 69.108.1A Figure 1-1-6. — Cirrus. C H -1 (CIRRUS).— High cloud "2" is a dense cirrus cloud that is in patches or entangled sheaves which usually do not increase in size and which sometimes seem to be the remains of the upper part of a cumulonimbus. An H2 cloud can also be CI with sproutings in the form of small turrets or battlements or CI having the appearance of cumuliform tufts. This dense CI cloud does not originate from CB clouds, although the patches are sometimes rather opaque and have borders of entangled filaments. This can give the erroneous impression that the cloud patches are the remains of cumuliform clouds. When an H2 cloud is present with other CI clouds, the H2 characteristics must pre- dominate the clouds to be encoded as such. H2 and H3 clouds are often mistaken for each other. When it is certain that the cloud evolved from a CB cloud, the cloud is classified as H3. Ctf-3 (CIRRUS).— High cloud type "2" is a dense cloud that is often in the form of an anvil, which is the remains of the upper parts of a CB cloud. The best guide to classify this cloud type is to observe the upper part of a CB cloud as it transforms into dense CI. However, if you have sufficient evidence that the dense CI cloud evolved from cumuliform clouds, you may classify dense CI clouds as H3 even though you do not actually see the transformation. This evidence may come from pilot sightings of CB clouds near your area or the unmistakable features associated with the dissipation of cumuliform clouds (M6 for example). C/j-4 (CIRRUS).— High cloud type "4" is a CI cloud (in the form of hooks and/or filaments) that progressively invades the sky and becomes more dense. This cloud type is very similar to HI except that an H4 cloud progressively invades 1-1-14 Unit 1— Lesson 1— SKY CONDITION AND VISIBILITY 209.448 Figure 1-1-7.— Halo. the sky and becomes more dense. These clouds appear to fuse together near the horizon where they first appear, but no cirrostratus clouds are present. When cirrostratus conditions are present, you should examine the clouds closely to deter- mine whether or not to classify the type as H5 or H6. Cirrostratus (CS) A CS cloud is a whitish veil very similar in appearance to CI clouds. The primary difference is the great horizontal extent of CS and its more veil-like appearance. Cirrostratus clouds usually produce a halo when the cloud composition is thin enough. Cirrostratus often appears as AS on the distant horizon. In this case, you should consider the speed of movement of the cloud (a CS cloud appears to move more slowly) and the slower changes in form and appearance that are characteristic of CS. Cirrostratus clouds on the horizon are sometimes confused with haze. You can distinguish the haze by its dirty yellow-to-brown color. A CS cloud is never thick enough to prevent objects on the ground from casting shadows when the sun is higher than 30 degrees above the horizon. Observing the effect that the sun has on CS can be one of your greatest aids in determining the type of CS cloud present. For example, a CS layer may be so thin that only the presence of a halo reveals its presence, as shown in H7 of foldout 1-1-3. C H -5 (CIRRUS AND CIRROSTRATRUS OR CIRROSTRATUS ALONE).— High cloud type "5" is CI and CS clouds or CS clouds only. (The CI clouds are often in bands converging towards one point or two opposite points of the horizon.) In either case, they progressively 1-1-15 AEROGRAPHER'S MATE THIRD CLASS invade the sky and generally grow more dense, but the continuous veil does not reach 45 degrees above the horizon. Usually, the leading edge of this cloud type is in the form of CI filaments or hooks and, occasionally, resembles the skeleton of a fish. When this cloud type progresses to 45 degrees above the horizon, it is classified as H6. C H -6 (CIRRUS AND CIRROSTRATUS OR CIRROSTRATUS ALONE).— High cloud type "6" has the same appearance and features of H5 but extends to more than 45 degrees above the horizon, without the sky being actually covered. Similar to H5, it progressively invades the sky and grows more dense. When the cloud layer covers the entire sky, it is classified as H7. Ctf-7 (CIRROSTRATUS).— High cloud type "7" is a veil of CS clouds covering the celestial dome. This dome is uniform in structure, showing few distinct details. On occasion, the continuous veil of H7 is so thin (transparent) that the only indication of its presence is a halo phenomenon. When lower clouds obscure parts of an overcast CS layer, you may still classify it as H7 if you have evidence that the layer covers the sky. If the CS layer does not cover the sky, classify the cloud type as H8. C//-8 (CIRROSTRATUS).— High cloud type "8" is CS which is not or is no longer pro- gressively invading the sky and which does not completely cover the celestial dome. When H8 is present with other cirriform cloud types, it must be predominant to be classified as H8. Though the definition of this cloud type specifically states that the CS clouds are not progressively invading the sky, this refers to the continuous veil form of the CS formation. When CS is in patches (not CI) H5, H6, and H7 are not appropriate classifications. Classify patches of cirrostratus as H8 regardless of whether they are increasing, even though CI and cirrocumulus clouds may also be present but not predominant. Cirrocumulus (CC) Cirrocumulus clouds (H9) are very much like the regularly arranged elements of high AC clouds. The basic difference, however, is their size and composition. To be CC clouds, the element must have an apparent width of less than 1 degree. You can measure this by extending your little finger at arm's length. If the element you are evaluating is not larger than your finger, the cloud type is probably CC. Again, this guide is only reliable when the cloud element is higher than 30 degrees above the horizon. Cirrocumulus clouds consist primarily of ice crystals, but they can also consist of minute super-cooled water droplets that are usually replaced rapidly by ice crystals. Cirrocumulus clouds are observed with a slight corona phenomenon which adds to the beauty of the cloud. When this cloud is present, the sky is often referred to as a mackerel sky because of the cloud layer's resemblance to the scales of a fish. Some observing terms used to identify this cloud are pebbles on a beach, honeycomb, and netlike. Some forms of CC clouds are similar to altocumulus castellanus clouds. They appear as small tufts or turrets; however, they must be less than 1 degree in width to be classified as CC. H9 in foldout 1-1-3 shows an example of CC development with other cirriform clouds. Some of the elements appear so small that they are difficult to discern with the naked eye. High cloud type "9" are CC clouds by themselves or accompanied by CI and/or CS clouds, but the CC clouds must be predominant. Be sure that you remember that the elements of CC must have an apparent width of less than 1 degree. 1-1-16 Unit 1— Lesson 1— SKY CONDITION AND VISIBILITY EXERCISE (1-1-3) Supply the appropriate high cloud classification for the descriptions and characteristics given below. 1. This cloud type is often referred to as a mackerel sky. 2. This cloud type is a dense cloud that is often in the shape of an anvil. 3. This cloud type is uniform in structure, covers the celestial dome, and a halo may be the only indication of its presence. 4. This cloud type is in the form of filaments, strands, or hooks that do NOT progressively invade the sky. 5. This cloud type progressively invades the sky and generally grows more dense, but the continuous veil does NOT reach 45 degrees. 6. This cloud type can have sproutings in the form of small turrets or battlements. 7. This cloud type is in the form of hooks and/or filaments that progressively invade the sky and become more dense. 8. This cloud type progressively invades the sky and generally grows more dense. The continuous veil extends to more than 45 degrees. 9. This cloud type is NOT (or no longer) progressively invading the sky and does NOT completely cover the celestial dome. OROGRAPHIC CLOUD that you recognize and report these unusual FORMS clouds. The most common orographic clouds belong Certain types of clouds are formed as a result to the same class as AC, SC, and CU clouds, of air moving over rough terrain. These clouds Listed below are the orographically produced indicate the presence of a mountain wave clouds that are related to a mountain wave: condition in the atmosphere; therefore, they are significant in flight operations. A mountain • Lenticular — AC wave condition consists of turbulent air and strong updrafts and downdrafts. Flight opera- • Rotor (roll)— CU tions in these conditions pose a serious threat to flight safety. As an observer, it is important • Foehnwall (cap, collar) — SC. 1-1-17 AEROGRAPHER'S MATE THIRD CLASS Learning Objective: Given descriptions and characteristics of Orographic Clouds, supply the name and subtype number for each. Lenticular The lenticular cloud is an AC cloud (M4) which is almond or fish-shaped. The cloud is observed in patches at one or more levels, and the elements are continually changing in appearance but generally remain stationary in spite of the high wind speeds. They constantly form on their windward side and dissipate on their downwind side. Since the cloud patches are of limited horizontal extent and their elements are continually changing, these clouds are usually semitransparent rather than opaque. The patches, as a whole, may have the form of large lenses and are NOT progressively invading the sky. M4, in foldout 1-1-2, shows an example of standing (stationary) lenticular clouds. are stationary and are constantly forming on their windward side and dissipating on the leeward side. Because of their vertical develop- ment and cumuliform appearance, they are usually encoded as low cloud type "2". In addition to classifying the lenticular and rotor clouds for cloud code group encoding, you must append remarks (concerning these clouds and their direction from the station) to your weather observation, such as the following: ACSL OVHD AND W FEW ACSL FRMG W-NW APRNT ROTOR CLDS OVR MTNS The first remark indicates that you observed altocumulus (AC) standing lenticular (SL) overhead (OVHD) and to the west (W) of your station. In the second and third remarks, "FRMG" is the contraction for forming and "APRNT" is the contraction for apparent. Foehnwall Rotor Rotor clouds are cumuliform in appearance and are found on the leeward side of the mountain range. Rotor clouds, similar to lenticular clouds, The foehnwall cloud is SC in appearance and is usually classified as low cloud type "5". This cloud hugs the top of the mountain and sometimes flows down the leeward side of the mountain, producing the appearance of a waterfall. EXERCISE (1-1-4) Supply the name and subtype number for each of the following orographic clouds: 1. 2. 3. .This orographic cloud type is observed in patches at one or more levels, and the elements are continually changing in appearance but generally remain stationary in spite of high wind speeds. .This orographic cloud type hugs the top of a mountain and gives the appearance of a waterfall. .This orographic cloud type is found on the leeward side of a mountain range and has vertical development. 1-1-18 Unit 1— Lesson 1— SKY CONDITION AND VISIBILITY CLOUD CODE GROUP Cloud recognition and identification is only the first part of your job. You must also know how to encode the cloud types so that your observation can be transmitted and used by other weather office personnel to plot on a weather chart. Your cloud code group, correctly encoded, provides anyone who understands the code with a snapshot of the clouds over your station. Learning Objective: For given cloud types, encode the correct cloud code group. Encoding IClCmCh Group (Column 13) Each 3- and 6-hourly observation must have a cloud code group appended to it. Of course, when the sky is clear or completely hidden by surface obscuring phenomena, a cloud code group is not appended. The IClCmCh group is entered in column 13 of MF 1-10. (The seqeunce of entry for observations is discussed in a later section of this text.) Presently, the concern is how to encode the cloud types correctly. Whenever there is only one cloud type present for each cloud division of the atmos- phere (Cl, Cm, and C//), you merely enter the correct type for each division. If no clouds are present in a division, enter a zero for that division. Whenever you have more than one cloud type in a division, you select the type that is the most significant. Table 1-1-1 shows the order of priority for encoding clouds in the IC l CmC h group. Table 1-1-1.— Order of Priority for Encoding \C l CmC h Group Order of Priority Low Cloud Middle Cloud Cat High Cloud 1st 9 CB (anvil) 9 AC (chaotic) 9 CC (predominant) 2nd 3 CB 8 AC (turrets) 7 CS (covers sky) 3rd 4 SC (from CU) 7 AC (with AS or NS) 8 CS (not covering or invading) 4th 8 SC & CU 6 AC (from CU) 6 CS (invading, over 45 °) 5th 2 CU (large) 5 AC (invading) 5 CS (invading, less than 45 °) 6th 1 CU 4 AC (changing) 4 Cl (invading) 7th 5 SC (not from CU) 7 AC (two levels) 3 Cl (from anvil) 8th 6 ST 7 AC (opaque) 2 Cl (dense patches predominant) 9th 7 STFRA, CUFRA 3 AC (semi-transparent) 1 Cl (filaments predominant) 10th 2 NS or AS 11th 1 AS (semi-transparent) 1-1-19 AEROGRAPHER'S MATE THIRD CLASS Suppose you determine that the following cloud types are present during a state-of-the-sky evaluation: Cl-2 (towering cumulus) Cl-5 (stratocumulus at a different level) Qvf-3 (altocumulus) Q/-1 (cirrus) C//-8 (cirrostratus) How is this cloud observation encoded for the cloud group entry in column 13 of MF 1-10? Table 1-1-1 shows that L2 takes priority over L5 when low cloud types are encoded. But it is not as simple as this. Generally, you enter the code of the cloud type that has priority; how- ever, when L2 and L5 are both present (at different levels), the low cloud type is classified as L8. This example for encoding low cloud types illustrates the importance of knowing the definitions of the 27 international cloud types. An inexperienced observer might select L2 for encoding. Only one middle cloud (M3) is present in this example; therefore, the cloud group code up to this point is 183. The cirriform cloud types are classified as HI and H8; therefore, you need to determine from table 1-1-1 which cloud must be encoded. In this case, high cloud 8 takes priority over HI . The correct entry in column 13 of MF 1-10 for this particular state of sky is 1838. You may make the air traffic controllers aware of the presence of the towering cumulus by the remark TCU in column 13 and the direc- tion from the station. When you cannot determine the middle or high clouds because of lower clouds and/or obscuring phenomena, a slant is entered for CmCh, or both, as appropriate. If there is less than 10/10 but more than 9/10 sky cover (breaks in an otherwise overcast sky) and no higher clouds are visible, classify this condition as 9/10 sky cover and enter a broken symbol in column 3. When there is less than 10/10 but more than 9/10 sky cover (breaks) and higher clouds are visible, assign a height and sky cover symbol for the higher cloud. For a trace of sky cover (less than 1/10) as the first layer, assign a height and classify as 1/10 sky cover. Enter this condition as a layer in column 3 and a "1" in column 21 if it is the only layer. EXAMPLES Column 3 Column 13 Column 21 25 SCT M30 BKN 100 TCU W/1838 10 BKN 200 OVC M20 BKN 1500 9 15 SCT E200 OVC 1501 10 E80 BKN 1070 8 M15 OVC 15// 10 EXERCISE (1-1-5) For the following cloud types, encode the correct cloud code group. 1. Cl-4, Cl-5, Cm-6, C/j-1, C H -S 2. Cm-7 (not overcast) 3. C L -9, C L -2, Cm-3 (not overcast) 4. Overcast (10/10) Cjr-5 5. Less than 10/10 SC but more than 9/10 (breaks) with no higher clouds visible 6. Less than 10/10 SC but more than 9/10 (breaks) with CI visible through breaks. 1-1-20 Unit 1— Lesson 1— SKY CONDITION AND VISIBILITY SKY CONDITION Determining the sky condition is largely subjective and requires, above all, practical experience. There is one important reason for a careful evaluation of the sky: almost all changes in surface weather are preceded or accompanied by clouds. For example, frontal passages give advance warning of their presence by a series of changes in clouds and sky conditions. The forecaster interprets the significance of these changes from your observation. Sky condition is observed and evaluated in layers. During your observation, you consider amount, transparency, and height for each layer. Looking at a series of observations, you can see sky cover transitions by the changes in the observed layer. A change in the amount of a layer from 0.8 to 0.6 may appear unimportant from one observation to the next. However, when this minor change is regarded within a trend and in relation to all the other sky data, an approaching weather situation may be foretold. In observing the sky condition, first you consider the layers of sky cover. Learning Objective: Given cloud diagrams, determine the cloud layers and heights. Layers A layer is defined as "clouds or obscuring phenomena which have bases at the same approximate level." A layer may appear as continuous cover, such as stratus, or it may appear as detached elements, such as fair-weather cumulus. Also, both continuous and detached elements may combine to form a layer. The essential requirement is that their bases be at the same approximate level. The upper portions of a cumulonimbus cloud are often spread horizontally by wind and form a dense cirrus or altiform clouds. These horizontal extensions of the cumulonimbus clouds are regarded as separate layers only if their bases appear horizontal and at a different level from the parent cloud. Otherwise, the entire cloud system should be regarded as a single layer at a height corresponding to that of the base of the cumulonimbus. A layer can be a combination of cloud types or obscuring phenomena at the same level. Obscuring phenomena, such as haze, are often present in the atmosphere but are not considered as a layer unless they have an apparent base. Having divided the state of the sky into layers of clouds, obscuring phenomena, or both, next determine the amount of each layer. Amount Though you observe the amount of sky covered by each layer in terms of tenths of sky, contractions are used to describe the sky cover. Table 1-1-2 gives the sky cover contractions and their meanings. These sky cover contractions are entered in column 3 of MF 1-10 for each layer of clouds or obscuring phenomena — surface-based or not. Each contraction represents the portion of the sky that is covered at that layer and below. Figure 1-1-8 illustrates this "at and below" con- cept of assigning sky cover contractions. The dif- ference between layer and sky cover also is shown. 4/10 ALTOSTRATUS 3/10 CIRROSTRATUS 2/10 STRATUS Figure 1-1-8. — Layer and Sky Cover. 1-1-21 AEROGRAPHER'S MATE THIRD CLASS Table 1-1-2.— Sky Cover Contractions Summation Amount of Sky Cover Symbol Contraction Remarks 1/10 to less than 10/10 surfaced- based obscuring phenomena. -X -X No height assigned to this condition. Vertical visibility is not completely restricted. . 10/10 surfaced-based obscuring phe- nomena. X X Always preceded by a vertical visi- bility (height) value. Height value preceding this symbol is normally prefixed with the ceiling designator W. Clear CLR This symbol (contraction) is not used in combination with any other. Less than 1/10 thru 5/10, half or more thin. H u. O < & m >< < H O Oh 2 o H Q W oo D >H _) Z o -SCT Less than 1/10 thru 5/10, more than half opaque. SCT Height values preceding these sym- bols (contractions) are never desig- nated as ceiling layers. 6/10 thru less than 10/10, half or more thin. -BKN 6/10 thru less than 10/10, more than half opaque. BKN Height value preceding this symbol (contraction) is prefixed with a ceiling designator (M or E), provided a lower ceiling layer is not present. 10/10, half or more thin. -OVC Height value preceding this symbol (contraction) is never designated as a ceiling layer. 10/10, more than half opaque. OVC Height value preceding this symbol (contraction) is prefixed with a ceiling designator (M or E), provided a lower broken ceiling layer is not present. This symbol (contraction) is used in combination with lower overcast layers only when such layers are classified as thin. 1-1-22 Unit 1— Lesson 1— SKY CONDITION AND VISIBILITY In figure 1-1-8, the first layer (2/10 stratus) is entered in column 3 of MF 1-10 as scattered (SCT). This 2/10 amount also represents the total sky cover at this level. The next layer tells a different story. Though the altostratus covers 4/10 sky as a layer, the total sky cover up to this point is 6/10 because of the combined amounts of the first two layers. Thus, the contraction for the altostratus layer is broken (BKN) because of the concept described as "at and below" sky cover. The highest layer (3/10 cirrostratus) is also assigned a broken con- traction because the combined total equals 9/10 sky cover. 4/10 ALTOSTRATUS ■*#>:*# o/io ***:**# FOG Figure 1-1-9. — Surface-Based Sky Cover. You can understand how meaningless it would be to enter three separate scattered contractions in column 3 to report these individual layers. To a pilot flying above the highest layer and looking for ground navigational aids, your report of "scattered" sky cover would hide 9/10 of the ground from his view. By reporting a broken sky cover, you have more accurately described the sky condition to the pilot. Symbols for reporting surface-based obscuring phenomena are also provided. Table 1-1-2 shows that an - X symbol describes a partly obscured condition. Figure 1-1-9 shows another typical sky condition. What sky cover symbols/contractions are entered in column 3 of MF 1-10 for this example? If you selected - X for the first layer, BKN for the next layer (cumulus fr actus), and OVC for the highest layer, you are correct. Figure 1-1-9 illustrates two principles. First, the 6/10 fog, even though surface-based, hides the sky just as if it were a cloud aloft. Second, the trace of cumulus fr actus must be treated as a layer. Even though this layer covers less than 0. 1 sky, it is a layer by definition and also meets the criteria for broken sky cover. This is true because the total at and below that level (including the 6/10 fog) hides enough sky to require the broken contraction. When sky cover layers are advancing or receding on the horizon, you use the left-hand column of table 1-1-3 as a guide to determine the Table 1-1-3.— Sky Cover Evaluations Angle of Advancing or Receding Layer Edge Tenths of Sky Cover 0°to 25° 26° to 45° 1 46° to 59° 60° to 72° 2 3 73° to 84° 85° to 95° 4 5 96° to 107° 108° to 119° 6 7 120° to 134° 135° to 154° 155° to 180° 8 9 10 Angular Elevation of Layer Surrounding Station 0° to 2° 3° to 8° 9° to 14° 15° to 20° 21° to 26° 27° to 33° 34° to 40° 41° to 48° 49° to 58° 59° to 71° 72° to 90° 1-1-23 AEROGRAPHER'S MATE THIRD CLASS number of tenths of the sky that is covered by a layer. When a layer of sky cover surrounds the station, use the right-hand column of table 1-1-3 as a guide to determine the number of tenths of sky coverage. Table 1-1-3 takes much of the guesswork out of estimating sky coverage at difficult angles of observa- tion. During your observation of sky cover, be alert for layers that occur directly beneath another layer. In this case, you cannot add the amounts of both layers to arrive at total sky cover because they hide the same section of the sky; for example, when 0.3 of cumulus is below 0.5 of altocumulus. Together, these two layers hide 0.5 of the sky and, therefore, are both scattered layers. The few samples discussed here help to illustrate the layer versus sky cover principle and entries for sky cover amounts. Another feature that you must consider when observing sky condition is the transparency of the layer. 10/10 CIRROSTRATUS TRANSPARENT <£? 2/10 CUMULUS 4/10 ALTOCUMULUS 3/10 TRANSPARENT Figure 1-1-10.— Thin Sky Cover. Transparency This fancy term means "capable of being seen through." A window is transparent. Opaque is the opposite of transparent. Occasionally, when we talk about certain clouds, such as altocumulus, we use the term semitransparent. That is a proper and accurate description for clouds. For sky cover it is not proper. These semitransparent layersj though they permit the passage of light, do not permit a clear picture of higher layers. Therefore, for practical purposes, consider them opaque when you are deciding between transparency and opaqueness for encoding. To accurately encode transparent sky cover, you must again recognize the difference between a layer and sky cover. That is, the "at and below" concept importantly affects your decision. Transparent layers are classified as thin. Column 3 entry (MF 1-10) would be -BKN. The minus (-) sign indicates that the layer is thin enough to reveal higher clouds or sky above. When you observe multiple layers, use the "at and below" concept to obtain "total" opaque and transparency amounts. Figure 1-1-10 shows an example of opaque and transparent layers coexisting in the sky. To solve this problem, start with the lower layer and work up. Let's arrange the amounts for each layer in order and add the transparency totals for each layer. You can count three layers in figure 1-1-10; thus, you need three sky cover con- tractions. As you add each layer to the total sky cover, the first layer is SCT, second BKN, and Table 1-1-4. — Sky Cover Height Values Feet Reportable Values (Coded in Hundreds of Feet) 1 5000 or less To nearest 100 feet 5001 to 10,000 To nearest 500 feet Above 10,000 To nearest 1 ,000 feet 'Code heights that are halfway between reportable values as the lower of the two heights. 1-1-24 Unit 1— Lesson 1— SKY CONDITION AND VISIBILITY third OVC. Decide now, at which layers the sky cover is thin. Below, you can see the informa- tion for each layer arranged in table form: Layer Amount Total Total Total Sky Cover Opaque + Transparent ° = Sky Cover Contraction 1st 0.2 0.2 - 0.2 SCT 2nd 0.4 0.3 0.3 0.6 -BKN 3rd 1.0 0.3 0.7 1.0 -OVC It is easy to see that the total transparent sky cover becomes one-half of the total sky cover at the second layer. It, then, is reported as thin broken (-BKN). The sky cover remains thin at the third layer. This is so because the transparent sky cover is well over one-half of the total cover. It is impossible to report more than one overcast contraction in column 3. The only rule to observe in this case is that only the highest layer may be classified opaque. The lower over- casts must be thin. As a final step in reporting sky cover, you ascribe a height to each reported layer. Height Heights of layers must be reported according to established reportable values. Table 1-1-4 shows the reportable values that can be entered in column 3. For example, during the evaluation of sky cover, suppose you detect four opaque layers: 0.2 surface-based fog 0.3 stratocumulus at 4,780 feet 0.0 altocumulus at 9,300 feet 1.0 altostratus at 16,500 feet. If you use table 1-1-4 correctly, the height entries in column 3 for each layer should be: -X 48 SCT 95 SCT 160 BKN. This example does not indicate a ceiling designator which we discuss separately. Notice that the last layer (altostratus) is exactly halfway between two reportable values. In this case, select the lower height. In the above example, each height represents the base of the layer above the surface. There is one situation when height represents the vertical visibility into the layer. This applies only to a surface-based layer completely obscuring the sky (X). Since this layer is a ceiling, the discussion of how to obtain its height is discussed later. For all sky coverage, whether scattered, broken, or overcast — ceiling or nonceiling — thin or opaque — clouds or obscuring phenomena, you must use the height that is obtained from the most reliable method. Several methods are available for obtaining heights. You must take into consideration not only the reliability of the height data but also the distance from the observation point, the height of the layer, and the time of observation. Do not enter in column 3 (MF 1-10) the method by which you obtained the height measurement, unless you have a broken or overcast layer that is classified as a ceiling. However, the same rules for obtaining heights apply for all layers, regardless of amount. When finally you have the amounts, transparency, and heights of the layers, your last decision involves the sky cover ceiling. 1-1-25 AEROGRAPHER'S MATE THIRD CLASS EXERCISE (1-1-6) 1. Determine cloud layers and reportable heights from diagram below. 4/10 AITOSTRATUS 12.500' 3/10 CIRROSTRATUS 23,000" 2/10 STRATUS 900' 2. Determine cloud layers and reportable heights from diagram below. 10/10 CIRROSTRATUS TRANSPARENT 25,500' 4/10 ALTOCUMULUS 2/10 CUMULUS 1,250' 3/10 TRANSPARENT 16.4831 3. Determine cloud layers and reportable heights from diagram below. 4/10 ALTOSTRATUS 14,670' 6/10 FOG CEILINGS In many cases, ceiling layers are the con- trolling factor for aircraft departures, landings, or the diversion of aircraft to another airfield. Low ceilings demand the most accurate measure- ments possible. Sometimes a difference of 100 feet in the ceiling layer determines whether or not an aircraft can safely land or whether the pilot must seek an alternate field. Therefore, 1-1-26 Unit 1— Lesson 1— SKY CONDITION AND VISIBILITY two important responsibilities in observing sky conditions are that you correctly judge the presence of 0.6 sky cover or more and that you assign an accurate height to the ceiling layer. In column 3 of MF 1-10, ceiling heights are provided with a ceiling designator. These letter designators (listed below) indicate the method by which you obtain the ceiling height: Designator Meaning M Measured ceiling E Estimated ceiling W Indefinite ceiling Normally, the cloud height set (AN/GMQ-13), commonly called the rotating beam ceilometer (RBC), is used for determining layer or ceiling heights. But the cloud height set has limitations. Let's investigate the methods of obtaining heights to see how and when each one should be used. Learning Objective: State the methods of obtaining a measured ceiling and the procedures for obtaining cloud heights from the rotating beam ceilometer. Measured Ceiling Heights A ceiling is the height ascribed to the lowest opaque broken or overcast layer aloft, or the vertical visibility in a surface-based layer of obscuring phenomena. Ceiling heights are pre- fixed with an "M" designator whenever they are obtained by a rotating beam ceilometer (AN/GMQ-13), ceiling lights, or known heights of unobscured portions of abrupt, isolated objects (buildings, towers, etc.) within 1 1/2 nautical miles of a runway. Values obtained from either the RBC or ceiling light must be less than 10 times the baseline to be classified as a measured ceiling. When you use the RBC for obtaining ceiling heights, the following procedures should be followed: 1. During outages of the RBC, if an RBC is available for an alternate runway, it may be used provided, that in your judgment, the measure- ments are considered to be representative of conditions an aircraft encounters during landing approach. 2. When reactions from the RBC scope for a single broken or overcast layer are present, consider the spot of maximum deflection on the scope as an instantaneous height value. Determine a mean height value by averaging as many angular readings as possible during the period of evaluation. 3. For scattered clouds, use as many scope reactions as are available during the period of evaluation to obtain an average height. 4. When multiple layers are present, supple- ment scope height indications with visual obser- vations. Average only those reactions which are considered applicable for the layer whose height is being determined. EXERCISE (1-1-7) 1. Define a "ceiling." 2. What are the three methods for obtaining a measured ceiling? 3. What procedures are used when the RBC is out of service? 4. What procedure should be used to obtain cloud heights when multiple layers of clouds are present? 5. What procedure should be used to obtain cloud heights when scattered clouds are present? 1-1-27 AEROGRAPHER'S MATE THIRD CLASS Learning Objective: Identify and correct false statements concerning estimated (E) and indefinite (W) ceiling heights. Estimated Ceiling Heights (E) There are times when you cannot obtain a measured ceiling from your RBC or ceiling light. For example, heights obtained from these measuring sets that are equal to or greater than 10 times their baseline must be classified as estimated (E). The following procedures are used to classify a ceiling as estimated: AIRCRAFT. — Ceiling heights reported by a pilot (converted from height above mean sea level (MSL) to height above surface) can be classified as estimated when they are: 1. Within 1 1/2 nautical miles of a runway of the airfield and within 15 minutes of the actual time of observation for noncirriform layers. These layer heights need not be used if, in your judgment, they are not representative of conditions over the airfield. 2. Within 50 nautical miles and during the past hour preceding the actual time of observa- tion, for cirriform layers. BALLOON. — If you cannot determine a ceiling height with a ceiling light, RBC, radar, or pilot report, it should be determined by balloons whenever necessary. For example, if the ceiling is at or below the minimum height for VFR operations or the ceiling height is 2,000 feet or less and the presence of a stratus-type cloud layer makes estimation difficult, a balloon may be used to estimate the ceiling. A balloon ceiling is based on the known ascension rate of a pilot or ceiling balloon. Ascension rates are fixed by the amount of lift given to the balloons. Proper balloon inflation (neither over- nor under-inflated) controls the lift. When using a balloon to determine ceiling heights, use the following procedures: 1. Choose a balloon of the appropriate color — red for thin clouds and blue or black balloons under all other conditions. 2. Watch the balloons continuously, deter- mining with a stop watch (or any watch having a second hand) the length of time that elapses between the release of the balloon and its entry into the base of the cloud layer. The point of entry is midway between the time the balloon first begins to fade and the time of complete disappearance. 3. Then determine the height above the surface from prepared tables in the FMH-1B. The accuracy of the height obtained by a balloon is decreased when the balloon does not enter a representative portion of the cloud base, is used at night with a light attached, or is used during the occurrence of hail, ice pellets, any intensity of freezing rain, or moderate-to-heavy rain or snow. CONVECTIVE CLOUD HEIGHT DIA- GRAM. — This method is not suitable for stations in mountainous or hilly terrain. It should be used only when the clouds present are formed by active surface convection in the vicinity of your station. The diagram (figure 1-1-11) is usually most accurate when estimating the height of cloud bases at 5,000 feet or less. Recent dew- point and free air temperature readings must be available. To use the diagram in figure 1-1-11, locate the dewpoint temperature at the base of the diagram (vertical solid lines) and the dry-bulb temperature (sloping solid lines), and follow these lines to the point where they intersect. Follow this intersection point horizontally to the right side of the diagram, and read the estimated cloud height. This value represents the base of the convective clouds at your station. For example, assume that you have a dewpoint temperature of 58 °F and a free air temperature of 75 °F. The estimated cloud height is 4,000 feet. One important fact to remember when you use this method is that as changes in the dewpoint and temperature occur, you should recompute the height. NATURAL LANDMARKS OR OB- JECTS. — Known heights of unobscured portions of natural landmarks or objects more than 11/2 nautical miles from any runway of an airfield can be used to estimate a ceiling height. 1-1-28 Unit 1— Lesson 1— SKY CONDITION AND VISIBILITY f J § § | I § | § I § § | | § § § °u: On O E at 8? "5 i I o 8 * 3 s y s U, *S L *» 5 1-1-29 AEROGRAPHER'S MATE THIRD CLASS Most weather offices have visibility charts that provide you with the heights of any hills, mountains, TV towers, etc., that are within the area of your base. If, for example, there is a hill three miles from your base with a known height of 600 feet and the cloud base that you are trying to evaluate is touching the top of the hill, you can estimate the height of the ceiling as 600 feet. OBSERVATIONAL EXPERIENCE.— You can estimate a cloud height by observational experience provided the sky is not completely hidden by surface-based obscuring phenomena and other guides are lacking or, in your judgment, are unreliable. You can also consider the persistence of heights previously classified as measured. Your estimations should be checked, whenever possible, against a reliable method of measurement. This comparison tells you whether you usually estimate high or low under different sky cover conditions. RBC OR CEILING LIGHT.— You can estimate ceiling heights from an RBC or ceiling light when their values equal or exceed 10 times the baseline used. For example, if the baseline of the RBC is 400 feet, an angular reading of 84 degrees would equal 3,800 feet. Therefore, any angular reading over 84 degrees can only be used as an estimated height. WEATHER-SURVEILLANCE RADAR CEILING HEIGHTS.— The range height indica- tor (RHI) scope of the AN/FPS-106 can be used to estimate cloud heights. However, such height indications seldom compare well with indications from cloud height measuring equipment for heights below 10,000 to 12,000 feet. RHI scope displays are also not reliable for detecting the heights of cirriform clouds. Ordinarily, RHI scope indications only aid in evaluating the heights of middle clouds. Indefinite Ceiling Heights (W) Ceiling values are classified as "indefinite" when the vertical visibility in a surface-based obscuring phenomena is: 1. The distance that you can see, from the ground, vertically into an obscuring phenomena which completely conceals the sky. 2. Based on the visible portions of nearby objects (buildings, control towers, etc.) on the airfield complex. 3. Based on height equivalent to a ceilometer upper limit reaction. Consider the point at which deflection on the scope of the RBC becomes zero deflection as an evaluation of the vertical visibility. Use the average value obtained from at least four consecutive sweeps as a representative (W) ceiling height. 4. Based on the top of a ceiling light beam, or the height at which a balloon completely disappears. 5. Based on the maximum vertical height from which a pilot can see the ground. The report must occur with 1 1/2 nautical miles of the runway and with 15 minutes of the actual time of an observation. Pilot reported values need not be used if, in your judgment, they are not representative of conditions over the air- field. 1-1-30 Unit 1— Lesson 1— SKY CONDITION AND VISIBILITY EXERCISE (1-1-8) From the list of statements below, concerning estimated and indefinite ceiling heights, identify and correct those that are false. 1. A ceiling height reported by a pilot is coded as estimated in column 3, MF 1-10, in height above MSL. 2. An aircraft ceiling can be classified as estimated if the report is within 1 1/2 nautical miles of the airfield and within 15 minutes of the actual time of observation for noncirriform layers. 3. The approximate color of balloon to use for estimating a thin cloud layer is black. 4. The convective cloud height diagram is suitable for use in mountainous or hilly terrain. 5. When using the convective cloud height diagram, you should recompute the cloud layer heights as changes in the dewpoint and temperature occur. 6. When an indefinite (W) ceiling height is based on the upper limit of a ceilometer reaction, you use at least six consecutive sweeps as a representative ceiling height. 7. Known heights of unobscured portions of natural landmarks or objects more than 1 1/2 nautical miles from any runway or an airfield can be used to estimate ceiling height. 8. Estimated ceiling heights from an RBC or ceiling light can be used only when their values are less than 10 times the baseline used. 9. Height indications from the RHI scope of the FPS-106 can be used for estimating heights of cloud layers below 10,000 feet. 10. An indefinite (W) ceiling is the distance you can see, from the ground, horizontally into an obscuring phenomena that completely hides the sky. 11. RHI scope indications, from the FPS-106 radar, are most useful to you in evaluating the heights of middle clouds. 12. The accuracy of an estimated balloon ceiling height is decreased when the balloon does not enter a representative portion of the cloud base. 13. Pilot reports of ceiling heights within 1 1/2 nautical miles of a runway and 15 minutes of observation time for noncirriform layers need not be used if, in you judgment, they are not representative of conditions over the airfield. 14. If you cannot determine a ceiling height with an RBC, ceiling light, or pilot report, the ceiling should be determined by balloons whenever locally deemed necessary. 15. An indefinite (W) ceiling height can be based on the visible portions of objects (buildings, control towers, etc.) on the airfield complex. 1-1-31 AEROGRAPHER'S MATE THIRD CLASS CEILING/SKY REMARKS AND ENTRIES Some facts that the standard column 3 entries MF 1-10, do not reveal are ceilings that vary in height or amount, significant clouds, or other significant features about the sky cover. This significant information is added, when necessary, to the airways observa- tion. Learning Objective: Given simulated sky condition illustrations and descriptions, classify the sky cover amounts into layers, assign reportable heights, select the ceiling layer, record special remarks, and encode a cloud code group. Variable Sky Condition Variable sky condition describes a sky condition which has varied between reportable conditions (e.g., SCT to BKN, BKN to OVC, etc.) during the period of observation (normally the past 15 minutes). This condition is reported in column 13 when the layer is below 3,000 feet. Nothing need be remarked when a layer varies in amount from 4/10 to 5/10 because both amounts qualify as a scattered layer. Enter a remark for those amounts that vary between reportable values — 5/10 to 6/10 (scattered to broken), or when the variability goes from 6/10 to 5/10 (broken to scattered). Enter in column 13, at the time of observa- tion, a "V" and the condition to which it varies during the period of observation. When necessary to distinguish between column 3 entries, include the layer's height; i.e., SCT V BKN, BKN V SCT, 18 OVC V BKN. Variable Ceiling Rapid fluctuations of a ceiling indicate an irregular base; therefore, the height is variable. A variable ceiling is reported whenever the ceiling height is less than 3,000 feet and rapidly decreases and increases by one or more reportable values during the time of observation. The height of the ceiling is the average of all the observed values. A variable ceiling is not based on rapid fluctuations of the instrument readings alone. Visual observation is needed to exclude the possibility that the fluctuation is caused by separate layers. To enter a variable ceiling, average the readings obtained during the ceiling observa- tion. Enter the average (use reportable values only) as the ceiling height in column 3. This average value is suffixed with the letter "V" to indicate that the ceiling is variable; for example, M15V BKN. Whenever you make a variable ceiling entry in column 3, you must enter a remark for the lowest and highest value of the ceiling in column 13, such as CIG 11V19. When considered together, the entries M15V BKN and CIG 11V19 make a complete description of the ceiling layer. Breaks (BRKS) Report breaks or an area absent of clouds in a layer, below 1,000 feet, which covers 6/10 but less than 10/10 of the sky. Enter BRKS in column 13, followed by direction from station. Omit the remark if the breaks are in all quadrants; i.e., BRKS S, BRKS OVR APCH LTS. The remarks BRKS for a broken layer below 1,000 feet is very important to flight operations. This remark discloses to the pilot the location of the clear area in the broken layer. When you know the direction from your observation point to the approach end of the duty runway, you should report BRKS OVR APCH LTS when the remark is appropriate. Approach lights are sig- nificant because they are located off the end of the runway where the pilot makes his landing approach; therefore, if this portion of the sky is free of clouds, you should append this remark to your observation. Check with the tower personnel to find out the duty runway prior to making your observation. Other Remarks Other remarks describe a variety of observed features. Perhaps you might observe that the ceiling or sky condition at a distance from your 1-1-32 Unit 1— Lesson 1— SKY CONDITION AND VISIBILITY station appears to be different. If you can find evidence that this is so, remark it in a fashion that tells exactly what you see. Here are samples of remarks you might use: • CIG LWR OVR CITY— ceiling lower over city • CLD BASES OBSCG MTNS W— cloud bases obscuring mountains to the west • LWR CLDS W APCHG STN— lower clouds west approaching station. OBSCURING PHENOMENA ALOFT.— When obscuring phenomena are aloft rather than surface-based, you must report the height and sky cover symbol with the type. For example, enter a scattered layer of smoke at 1,000 feet as K10 SCT (sky cover contraction from column 3). To enter this remark in column 13, you need to have a corresponding height and sky cover contraction in column 3. SURFACE-BASED OBSCURING PHE- NOMENA. — Whenever you report a sky condi- tion that includes a partly obscured condition (-X), indicate in column 13 the phenomena producing the obscuration. Indicate the tenths of sky obscured following the obscuration symbol, e.g., "F6," "S8," "FK3," etc. No entry is required when the amount of obscuration is zero or ten tenths. Enter direction of breaks or discontinuity in an obscured sky (X); e.g., "THN F NW, BRK IN FOG TO SE," etc. SIGNIFICANT CLOUDS.— The use of cloud remarks with an observation usually produces a variety of opinions. The following cloud remarks are usually considered significant at any location. Therefore, you should be able to report them properly: Cloud Type Towering cumulus Cumulonimbus (no thunderstorm is being reported) Cumulonimbus mamma (with or without thunder) Altocumulus castellanus Standing lenticular or rotor clouds Vertical or inclined trails of precipitation attached to clouds but not reaching the surface Sample Remarks TCU, distance (if known), and direction from station; i.e., TCU NE, TCU 25 SW CB, distance from station (if known, based on radar or pilot report), direction from station, and movement (if known), i.e., CB 20S MOVG NE, CB OVHD MOVG E Same as cumulonimbus, except use CBMAM: i.e., CBMAM 10W MOVG SE ACCAS and direction from station; i.e., ACCAS SE Description and direction from station; i.e., ACSL SW-W, APRNT ROTOR CLDS S, CCSL OVR MTNS S VIRGA and direction from sta- tion; i.e., VIRGA NW. 1-1-33 AEROGRAPHER'S MATE THIRD CLASS EXERCISE (1-1-9) For each of the following diagrams and corresponding descriptions, classify sky cover amounts into layers, assign reportable heights, select the ceiling layer, record special remarks, and encode the appropriate cloud code group. « -*- 1. 3/10 CU of little vertical development. It took a 30-gram balloon one minute and 10 seconds (820) to enter the layer. 4/10 SC, not from cumulus, with a variable reading on the RBC going from 1,200' to 1,300' to 1,200 to 1,400 : a. Ceiling and sky cover: b. Remarks: c. Cloud code group:. 2. 4/10 stratus fractus (3/10 transparent) is present at an estimated height of 300'. 10/10 NS at a height of 650' as determined by the RBC 10 minutes ago. Some precipitation is occurring to the west of the station, but it is not reaching the ground: a. Ceiling and sky cover: b. Remarks: c. Cloud code group:. 1-1-34 Unit 1— Lesson 1— SKY CONDITION AND VISIBILITY EXERCISE (1-1-9)— Continued 3. 2/10 stratus fractus, of bad weather, at a measured height of 150'. 4/10 CBMAM, moving towards the southeast at an estimated height of 1,750. 2/10 CU, of great vertical extent, at an aircraft height of 2,400' above the surface. 2/10 AC from cumulus, at an estimated height of 19,000'. 2/10 CI, the remains of the upper part of a CB, at a height of 42,500' as determined by a radar RHI scope: a. Ceiling and sky cover:. b. Remarks: c. Cloud code group:. 1-1-35 AEROGRAPHER'S MATE THIRD CLASS EXERCISE (1-1-9)— Continued 4. 1/10 cumulus fractus 500'. 1/10 TCU, east at 1,400 . 2/10 AC, from cumulus, at an estimated 6,500 . 2/10 AS, semitransparent and having a base at an estimated height of 9,500 . 2/10 ACCAS at an estimated height of 13,000 '. 1/10 CI, in hooks and strands, not progressively invading the sky, reported by aircraft to be 21,000' above the surface. 2/10 CC at an estimated height of 22,000 .: a. Ceiling and sky cover: b. Remarks: c. Cloud code group: 1-1-36 Unit 1— Lesson 1— SKY CONDITION AND VISIBILITY EXERCISE (1-1-9)— Continued 5. 3/10 CB, with anvil-shaped top, at a height of 1,750'. 2/10 AC in the shape of an almond and having no apparent motion at a height of 17,500 : a. Ceiling and sky cover:. b. Remarks: c. Cloud code group:. 6. 6/10 ST at 240' determined by the known height of a radio tower 1/2 mile to the south. The layer has varied from 6/10 to 5/10 during the period of observation. 3/10 AS, semitrasnsparent, at an estimated height of 7,000 : a. Ceiling and sky cover: b. Remarks: c. Cloud code group: 1-1-37 AEROGRAPHER'S MATE THIRD CLASS EXERCISE (1-1-9)— Continued o 7. 5/10 smoke (3/10 transparent) at 1,000' from the RBC. 1/10 CD, of little vertical development as determined from the convec- tive cloud height diagram (refer to figure 1-1-11). Dewpoint = 52°, temperature = 65 °: a. Ceiling and sky cover: b. Remarks: c. Cloud code group:. 8. 6/10 ST at an estimated height of 700'. There are some small breaks in the layer northeast of the station. 3/10 AS, all transparent, at an estimated height of 7,000': a. Ceiling and sky cover: b. Remarks: c. Cloud code group: 1-1-38 Unit 1— Lesson 1— SKY CONDITION AND VISIBILITY EXERCISE (1-1-9)— Continued 9. 10/10 ST determined by a 30-gram ceiling balloon at a height of 1,000'. There is a small break in the layer to the northwest through which some clouds are visible at an estimated height of 3,500 : a. Ceiling and sky cover: b. Remarks: c. Cloud code group:. C^> 10. 6/10 AC, semitransparent and not changing at an estimated height of 13,000. 10/10 CS can be seen through the areas between the AC elements. The base of the CS is estimated at a height of 46,000'. There are small breaks overhead: a. Ceiling and sky cover:. b. Remarks: c. Cloud code group:. 1-1-39 AEROGRAPHER'S MATE THIRD CLASS CLOUD HEIGHT MEASURING EQUIPMENT Observation of all weather elements, in some way, involves a measuring device. The primary cloud height measuring set used by AGs is the AN/GMQ-13. Learning Objective: State the normal baseline, the requirements for operation, the turn-on procedures, the calibration procedures, and the limitations of the rotating beam ceilometer. Cloud Height Set (AN/GMQ-13) The AN/GMQ-13 is often called the rotating beam ceilometer (RBC) because the projected light beam rotates through its measuring arc. The RBC offers several advantages. First, a rapid measuring sweep (every other sweep is a measuring sweep) provides a measurement approximately every six seconds. Second, it provides measurements of clouds during all periods of operation, day or night. Third, a dual light system allows height measurements even though one light burns out. Fourth, the baseline length allows height measurements between a range of 50 to 4,000 feet with a reasonable degree of accuracy. The length of the baseline determines the maximum height that clouds can be considered measured with accuracy for observational purposes. Shortening the baseline to less than 400 feet decreases the maximum height of accurate measurements. Increasing the baseline increases the maximum height, but other limiting factors may arise. They include light beam cutoff by low-hanging fragments, attenuation of the light beam intensity by fog or other obstructions to vision, and diffusion of the light beam by water droplets. PERIOD OF OPERATION.— The presence of low clouds or fog governs the period of RBC operation. The RBC should be turned on whenever one of the following conditions exists at your station: 1 . When clouds are present within the height measuring capability of the set or fog is present. 2. When either of the above conditions is forecast or expected to be present within three hours. 3. When a local need exists. When none of these conditions exists or is not expected to occur within three hours, you may keep the RBC in standby. To obtain height readings from the RBC, you must be able to adjust the sweep, read the scales, and interpret the scope. ADJUSTING THE SWEEP.— Figure 1-1-12 shows the controls used to adjust the sweep. After you turn on the POWER and Z MODULA- TION toggle switches, begin the sweep adjustment by turning the BRIGHTNESS control clockwise until the sweep appears on the scope. Use the HORIZ CENTER control to make the sweep run along the vertical centerline of the scale, and adjust the FOCUS to obtain the sharpest beam. Place the CALIBRATE switch in each position and adjust as follows: • Position number 5 . Sweep should appear at degrees. Adjust with the SWEEP LENGTH control. • Position number 4. Sweep should appear at 90 degrees. Adjust with the SWEEP LENGTH control. • Position number 3. Sweep should appear at 45 degrees. Adjust with the VERT CENTER control. • Position number 2. Sweep flashes in each of the rectangles on the scale (18 degrees markers). If not, readjust the other calibration settings. After these adjustments, place the CALIBRATE switch in position number 1 . The sweep should trace the proper length (0 to 90 degrees). Adjust the HORIZ GAIN control until about 1/8 inch of noise (sweep width) is present. During sweep adjustment, the PROJECTOR switch has been OFF. When you turn it on you may find the projector and indicator are not synchronized. 1-1-40 Unit 1— Lesson 1— SKY CONDITION AND VISIBILITY SCALE LIGHT © T TT m ® POWER Z-MODULATION F0CUS HORIZ.GAIN HORIZ. CENTER VERT.CENTER BRIGHTNESS SWEEP LENGTH ( 2 j STNC PROJECTOR ON ON • f (O) (O) CO) (O) (O) (O) (0> ; o # OFF OFF Figure 1-1-12.— GMQ-13 Indicator Controls. The indicator uses a pulse that shows when the indicator sweep is synchronized with the projector. The "SYNC" pulse appears as a short step that is displaced to the right of the sweep of the sweep path. The SYNC pulse should appear at the bottom T degrees of the sweep. This step is commonly called the ZERO STEP, and occurs every fourth sweep. If the SYNC pulse appears anywhere else, push the SYNC button momemtarily. The indicator sweep will come to rest at degree until it becomes synchronized with the projector. During the degree rest time, the SYNC lamp is lit. When synchronization is achieved, the indicator sweep automatically begins its cycle and the SYNC lamp goes out. 1-1-41 AEROGRAPHER'S MATE THIRD CLASS EXERCISE (1-1-10) 1. What is the normal baseline for the RBC? 2. What are the three conditions which require the RBC to be turned on? 3. The calibration switch has five positions. What occurs at each of the positions? How do you make the necessary adjustments? 4. State the turn-on procedures. 5. What position should the calibration switch be in for normal operation? 6. What is the purpose of the SYNC button? 7. What is the main limitation of the RBC? 8. How often does the ZERO STEP occur? 9. How often can a measurement be obtained from the RBC? Learning Objective: From simulated RBC scope indications, determine the height of the base(s) of the cloud(s) and/or vertical visibility. Scale Overlay (400 Feet Baseline) To accurately read the indicator scale on the cathode-ray tube (CRT), you must keep one caution in mind. The measured height changes rapidly as the elevation angle approaches 90 degrees. In other words, a large change in measured height. With a baseline of 400 feet, height indications registered on the scale, above 76 degrees elevation angle, must be carefully observed to avoid misreading the scale by one or more reportable values (see table 1-1-5). Scope Interpretation Interpretation of the patterns on the overlay of the scope requires experience more than anything else. As an aid, a few typical patterns are illustrated in figure 1-1-13. These illustrations present only generalized pictures and do not portray the many variations that can occur. A brief discussion of details A through F of figure 1-1-13 follows: 1. Details A, B, and F are single-cloud indications. As the projector beam shines on the cloud directly over the detector, the scope trace widens. The base of the cloud is at the base of the widest part of the scope trace. Detail A shows an abrupt deflection that places the base at 60 degrees and 700 feet. The trace in detail B widens less abruptly with the widest point at 62 degrees and 750 feet. These two details show clouds whose bases are well defined, such as cumulus clouds. Detail F shows a diffuse or less defined cloud base, such as the base of a status cloud. The scope depicts the base at 75 degrees and 1,500 feet. 2. Detail C apparently presents two cloud layers at 46 and 65 degrees. When multiple layers appear on the scope, you should verify their existence by an outside visual observation, if possible. Do this to avoid reporting a noise signal as a cloud layer. 3. Detail D depicts a low ceiling accompanied by fog at the surface. The fog causes the wide trace at the surface. The base of the cloud is indicated at the widest part of the bulge or about 100 feet. 4. Detail E shows two features. Reflection of the light by falling snow causes a wide trace at the surface. However, enough of the projected light reaches through the snow to strike the cloud base at 60 degrees. Frequently, precipita- tion or dense surface fog reduces the amount of projected light received at the photocell so that 1-1-42 Unit 1— Lesson 1— SKY CONDITION AND VISIBILITY Table 1-1-5.— Height Values for the RBC with a 400-foot Baseline REPORT ACTUAL ANGLE VALUE VALUE REPORT ACTUAL ANGLE VALUE VALUE ANGLE REPORT VALUE ACTUAL VALUE 5 35 33 260 62 752 6 42 34 270 63 800 785 7 49 35 280 36 300 291 64 820 8 56 65 858 9 63 37 301 66 900 898 10 11 71 78 38 39 313 324 67 942 68 990 12 85 13 100 92 14 100 40 41 336 348 69 1000 1042 70 1100 1099 42 360 71 1162 15 16 107 115 43 44 373 386 72 1200 1231 73 1300 1308 17 122 45 400 400 74 1400 1395 18 130 46 414 75 1500 1493 19 138 47 429 76 1600 1604 20 146 48 444 77 78 1700 1900 1733 1882 21 154 49 460 22 162 50 477 79 2100 2058 23 170 51 500 494 80 2300 2269 24 178 52 512 81 2500 2526 25 187 26 200 195 53 531 82 83 2800 3300 2846 3258 54 551 27 204 55 600 571 84 3800 3806 28 213 56 593 85 4600 4572 29 222 57 616 86 5500 5720 30 31 231 240 58 640 59 666 32 250 60 700 693 61 722 only the tapered portion of the trace appears, such as shown in detail E from the surface to 400 feet. A tapered trace should help in estimating the vertical visibility into the phenom- enon. 5. Another feature in details A, B, and C needs to be mentioned. Notice the bulging trace in detail A at about 20 degrees, again, in detail B at 10 and 20 degrees, and also in detail C at 15 degrees. These depict noise signals that are generated either within the set or from external radio or light sources. Noise signals are often characterized by their random patterns; that is, they do not appear as a fixed signal from scan to scan. Also, a signal appearing between measuring scans, when no signal information is being presented, gives a further indication that you are receiving noise. High-intensity flasher interference does cause regularly spaced signal reactions about 15 degrees apart on the indicator scope. Noise signals also show narrow, sharp deflections as well as the gradual bulges shown in the illustrations. Although you cannot eliminate noise signals, you can reduce their effect upon the scope trace by turning the HORIZ GAIN control to a lower setting. 1-1-43 AEROGRAPHER'S MATE THIRD CLASS Figure 1-1-13.— RBC Scope Interpretations. 1-1-44 Unit 1— Lesson 1— SKY CONDITION AND VISIBILITY EXERCISE (1-1-11) For each of the following figures, determine the angle and height of the base of the cloud(s) and/or vertical visibility. Use table 1-1-5 to determine the reportable value. 1. a. Angle. b. Reportable height. 1-1-45 AEROGRAPHER'S MATE THIRD CLASS EXERCISE (1-1-11)— Continued 40 30 20 10 400 300 200 100 2. a. Angle. b. Reportable height. 1-1-46 Unit 1— Lesson 1— SKY CONDITION AND VISIBILITY EXERCISE (1-1-11)— Continued 3. a. Angle. b. Reportable height. 1-1-47 AEROGRAPHER'S MATE THIRD CLASS EXERCISE (1-1-11)— Continued « i 4. a. Angle. b. Reportable height. 1-1-48 Unit 1— Lesson 1— SKY CONDITION AND VISIBILITY Learning Objective: State when a ceiling light can be used to determine ceiling heights, and list the six steps in determining these heights with a clinometer (ML- 11 9). Ceiling Light Projector (ML-121) Most weather offices have a ceiling light. A ceiling light is a fixed installation consisting of a powerful incandescent lamp with a reflector system and focusing arrangement housed in a weatherproof drum. It projects a concentrated beam of light vertically upon the cloud base (or into a surface-based obscuring phenomena) so that the observer, located at a measured distance from the projector and sighting with a clinometer (ML- 119), can determine the cloud height from the angle of inclination indicated by the clinometer (figure 1-1-14). The ceiling light can be used to determine nighttime sky cover heights and vertical visibility. In order for you to determine a cloud height with the ceiling light, cloud(s) must be directly over the projector beam. When clouds are scattered, or where clearing exists, especially over the observation site, eye estimation must be made. Clinometer (ML-119) The clinometer is a lenseless sighting tube, resembling a beer bottle, with crossed wires at its larger end and a quadrant plate assembly which is graduated in 1 ° intervals from ° to 90 ° (see figure 1-1-14). To determine the cloud height from the clinometer, use the following procedures: CLOUD SPOT BASELINE i PROJECTOR SIGHTING TUBE PEEP SIGHT THUMBSCREW QUADRANT PLATE QUADRANT PLATE COVER PENDANT CLUTCH ASSEMBLY PENDANT CLINOMETER Figure 1-1-14. — Ceiling Ligbt Projector and Clinometer. 3. When the pendant has come to rest, lock it in position without moving the clinometer. 1 . Loosen the pendant clutch on the quadrant plate to allow the pendant to swing freely. 4. Read the indicated angle to the nearest whole degree and release the pendant clutch. 2. Sight through the clinometer and center the intersection of the cross-hair upon the brightest portion of the light beam spot on the cloud base. When the sky is completely obscured by a surface-based layer, sight on the upper limit of the light beam penetration. 5. Repeat steps 1 through 4 three times and determine an average angular reading. 6. Refer to a prepared table applicable to the baseline used for the equivalent height value of this average reading. 1-1-49 AEROGRAPHER'S MATE THIRD CLASS Table 1-1-6. — METAR Cloud and Obscuring Phenomena Types A Clouds Abbreviations Altocumulus AC Altocumulus Castellanus ACCAS Altocumulus (standing lenticular) ACSL Altostratus AS Cirrocumulus CC Cirrocumulus (standing lenticular) CCSL Cirrostratus CS Cirrus CI Cumulonimbus CB Cumulonimbus Mamma (Mammatocumulus) CBMAM Cumulus CU Cumulus Fractus CUFRA Nimbostratus NS Stratocumulus SC Stratocumulus (standing lenticular) SCSL Stratus ST Stratus Fractus STFRA Towering Cumulus TCU Obscuring Phenomena Precipitation : Drizzle (including FZDZ) DZ Hail GR Ice Crystals IC Ice Pellets and Snow Pellets (including PESH) PE Rain (including RASH and FZRA) RA Snow (including SNSH) SN Snow Grains SG Hydrometeors other than Precipitation : Blowing Snow SN Fog (including BR, BCFG, and FZFG) FG Lithometeors : Haze or Dust HZ Sandstorm, duststorm, or blowing dust or sand SA Smoke FU NOTE: This table lists the two-letter abbreviations used to report type of phenomena in sky conditions. In addition, it lists the abbreviations used to report clouds and obscuring phenomena in Remarks of an observation. i 1-1-50 Unit 1— Lesson 1— SKY CONDITION AND VISIBILITY EXERCISE (1-1-12) 1. When can a ceiling light be used to determine ceiling heights? 2. List the six (ML-119). steps used in determining cloud heights with a clinometer Learning Objective: Given simulated data, indicate the entries for sky condition using the METAR code. Sky Condition (Column 3, MF 1-10) Enter each surface-based obscuration and/or each obscuring phenomena aloft as a six-character group (N 5 CCh s h s h 5 ) in ascending order. No entry is made when sky is clear. AMOUNT (N s ). — For each individual layer, enter amounts to the nearest eighth (except enter "9" for a totally obscured sky). Estimate the amount of each layer without consideration for other layers. Enter traces of clouds as one-eighth and overcast with breaks as seven-eighths. When two or more types of phenomena occur with bases at the same level, the amount entered will refer to the total of all types at that level except when cumulonimbus is one of the clouds and it does not represent the greatest amount. TYPE (CC).— Enter type of obscuring phenomena or cloud using the appropriate two- letter abbreviation from table 1-1-6. When two or more types of phenomena occur with bases at the same level, enter the type that represents the greatest amount, or, if equal amounts, enter type considered more significant. When cumulonimbus-type clouds are observed at the same level as other cloud types, and do not represent the greatest amount, enter each type in separate cloud groups (e.g., 3/8 clouds observed at 3,000 feet consisting of 1/8 cumulonimbus and 2/8 cumulus, enter 1 CB 030 2 CU 030). HEIGHT (h s h s h 5 )-— Enter height of layer (vertical visibility for obscured condition) using code figures from table 1-1-7. Enter /// for partially obscured conditions. 1-1-51 AEROGRAPHER'S MATE THIRD CLASS Table 1-1-7.— METAR Reportable Values for Layer Heights Code Feet Meters Code Feet Meters Code Feet Meters 000 0-50 0-15 030 3,000 900 100 10,000 3,000 001 100 30 031 3,100 930 110 11,000 3,300 002 200 60 032 3,200 960 120 12,000 3,600 003 300 90 033 3,300 990 130 13,000 3,900 004 400 120 034 3,400 1,020 140 14,000 4,200 005 500 150 035 3,500 1,050 150 15,000 4,500 006 600 180 036 3,600 1,080 160 16,000 4,800 007 700 210 037 3,700 1,110 170 17,000 5,100 008 800 240 038 3,800 1,140 180 18,000 5,400 009 900 270 039 3,900 1,170 190 19,000 5,700 010 1,000 300 040 4,000 1,200 200 20,000 6,000 Oil 1,100 330 041 4,100 1,230 210 21,000 6,300 012 1,200 360 042 4,200 1,260 220 22,000 6,600 013 1,300 390 043 4,300 1,290 230 23,000 6,900 014 1,400 420 044 4,400 1,320 240 24,000 7,200 015 1,500 450 045 4,500 1,350 250 25,000 7,500 016 1,600 480 046 4,600 1,380 260 26,000 7,800 017 1,700 510 047 4,700 1,410 270 27,000 8,100 018 1,800 540 048 4,800 1,440 280 28,000 8,400 019 1,900 570 049 4,900 1,470 290 29,000 8,700 020 2,000 600 050 5,000 1,500 300 30,000 9,000 021 2,100 630 055 5,500 1,650 310 31,000 9,300 022 2,200 660 060 6,000 1,800 320 32,000 9,600 023 2,300 690 065 6,500 1,950 330 33,000 9,900 024 2,400 720 070 7,000 2,100 340 34,000 10,200 025 2,500 750 075 7,500 2,250 350 35,000 10,500 026 2,600 780 080 8,000 2,400 etc. etc. etc. 027 2,700 810 085 8,500 2,550 990 99,000 29,700 028 2,800 840 090 9,000 2,700 999 100,000 30,000 029 2,900 870 095 9,500 2,850 or more or more 1-1-52 Unit 1— Lesson 1— SKY CONDITION AND VISIBILITY EXERCISE (1-1-13) Indicate the entries for sky conditions in METAR code for following illustrations: 8/8 NIMBOSTRATUS AT A MEASURED 8700 FEET 3. L2 ^ 2. Cumulus congestus. 3. Cumulus congestus. L3 A 4. Cumulonimbus calvus. L5 mmmmmm: — iPWIIIMilllMi in -»— 6. Stratocumulus. L6 7. Stratus. L8 /^ 9. Cumulus humilis and stratocumulus. L8 O 10. Cumulus congestus and stratocumulus. L9 S 11. Cumulonimbus capillatus. 12. Cumulonimbus capillatus. 1-1-63 209.451 301 12101 044 1 77-00 1 FLD00 00030 15°. M8 n togenitus. 22. Altocumulus floccus. M8 n latus. 23. Altocumulus castellanus. 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