4.5 Cloud analysis and forecasting

Forecasters are frequently called upon to make forecasts of clouds over areas where synoptic observations are not readily available, or over areas where clouds above the lowest layer are obscured by a lower cloud deck.

This section is designed to acquaint the forecaster with the principles of detection and analysis of clouds from rawinsonde data. A complete discussion of this problem is beyond the scope of this training manual. Further information on this subject may be found in the practical training publication, "Use of the Skew T Log P Diagram in Analysis and Forecasting", NAVAIR 50-1P-5.

4.5.1 IMPORTANCE OF RAWINSONDE OBSERVATIONS (RAOB) IN CLOUD ANALYSIS

Cloud observations regularly available to forecasters in surface synoptic reports leave much to be desired as a basis for cloud forecasting.
Rawinsondes, which penetrate cloud systems, reflect, to some extent (primarly in the humidity trace), the vertical distribution of clouds. If the humidity element were perfect, there would usually be no difficulties in locating cloud layers penetrated by the instrument. Because of the shortcomings in the instrument, however, the relationship between indicated humidity and cloud presence is far from definite, and art empirical interpretation is necessary. Nevertheless, rawinsonde reports give valuable evidence that, when compared with other data, aids greatly in determining a coherent picture of stratiform and frontal cloud distributions. Their value in judging air mass cumulus and cumulonimbus distribution is negligible.

4.5.2 INFERRING CLOUDS FROM RAOB

Theoretically, we should be able to infer from the humidity data of RAOBs the layers where the rawinsonde penetrates cloud layers. In practice, the determination that can be made from temperature and dewpoint curves are often less exact and less reliable than desired. Nevertheless, RAOBs give clues about cloud distribution and potential areas of cloud formation. These clues generally cannot be obtained from any other source.

4.5.3 DEWPOINT AND FROST POINT IN CLOUDS

The temperature minus the dewpoint depression yields the dewpoint, which is defined as the temperature to which the air must be cooled at a constant vapor pressure for saturation to occur. The FROST POINT (that is, the temperature to which the air has to be cooled or heated adiabatically to reach saturation with respect to ice) is higher than the dewpoint except at 0°C, where the two coincide. In the graph shown in figure 4-15, the difference between dewpoint and frost point is plotted as a function of the dewpoint itself.

In a cloud with the temperature above freezing, the true dewpoint will coincide closely with the true air temperature, indicating that the air between the cloud droplets is practically saturated. Minor discrepancies may occur when the cloud is not in a state of equilibrium (when the cloud is dissolving or forming rapidly, or when precipitation is falling through the cloud with raindrops of slightly different temperature than the air); but these discrepancies are very small. In the subfreezing portion of a cloud, the true temperature is between the true dewpoint and the true frost point, depending on the ratio between the quantities of frozen and liquid cloud particles. If the cloud consists entirely of supercooled water droplets, the true temperature and the true dewpoint will, more or less, coincide. If the cloud consists entirely of ice, the temperature should coincide with the frost point. Therefore, we cannot look for the coincidence of dewpoint and temperatures as a criterion for clouds at subfreezing temperatures. At temperatures below -12°C, the temperature is more likely to coincide with the frost point than the dewpoint. The graph shown in figure 4-15 indicates that the difference between the dewpoint and frost point increases roughly 1°C for every 10°C that the dewpoint is below freezing. 

For example, when the dewpoint is -10°C, the frost point equals -9°C; when the dewpoint is -20°C, the frost point is -18°C; and when the dewpoint is -30°C, the frost point is
-27°C. Thus, for a cirrus cloud that is in equilibrium (saturated with respect to ice) at a (frost point) temperature of -40°C, the correct dewpoint would be -44°C, (to the nearest whole degree).
We can state, in general, that air in a cloud at temperatures below about -12°C is saturated with respect to ice, and that as the temperature of the cloud decreases (with height), the true frost point/dewpoint difference increases. Any attempt to determine the height of cloud layers from humidity data of a RAOB is, there fore, subject to error. It is possible to overcome some of these errors by a subjective interpretation of the Rawinsonde Observations (RAOBs), as discussed in the following sections.

4.5.4 INTERPRETATION OF RAOB LAYERS WITH RESPECT TO CLOUD LAYERS

The following diagrams (figs, 4-16, 4-17, and 4-18) illustrate the behavior of a rawinsonde during cloud penetration. These diagrams are correlated with aircraft observations or the heights of cloud bases and tops from aircraft flying in the vicinity of an ascending rawinsonde. The difference in time and distance between the aircraft and sounding observations was usually less than 2 hours and 30 miles, respectively. Some of the aircraft reported only the cloud observed above 15,000 feet; others reported all clouds. In figure 4-16 through 4-18, the aircraft cloud observations are entered in the lower left corner of each diagram under the heading cloud; the surface weather report is entered under the aircraft cloud report.

Figure 4-16.-Example of inferring clouds from a RAOB with an active warm front approaching from the south.

Where the low cloud was not reported by the aircraft, the height of the cloud base may be obtained from the surface reports, Aircraft height reports are expressed in thousands of feet, pressure-altitude.

The temperature, frost point, and dewpoint curves are indicated by T, Tf , and TD respectively. In figure 4-16, a marked warm front is approaching from the south. Moderate continuous rain fell 2 hours later. At 1830 UTC, an aircraft reported solid clouds from 1,000 to 44,000 feet (tropopause). The 1500 UTC sounding shows an increasing dewpoint depression with height and no discontinuity at the reported cloud top of 15,000 feet. A definite dry layer is indicated between

Figure 4-17.-Example of inferring clouds from a RAOB with a middle layer and no precipitation reaching the surface
Figure 4-18.-Example of lnferring clouds from a RAOB showing layer clouds with their Intermediate clear layers not showing in the humidity trace.

18,300 and 20,000 feet. The second reported cloud layer is indicated by a decrease in dewpoint depression, but the humidity element is obviously slow in responding. The dewpoint depression at the base of the cloud at 21,000 feet is 14°C and at 400-hPa; after about a 3-minute climb through the cloud, it is still 10°C. From the sounding, clouds should have been inferred to be from about 4,500 feet (base of the rapid humidity increase) to 500-hPa and a second layer from 20,000 feet up. In view of the rapid falling of the cloud free gap between 15,000 and 21,000 feet that followed as the warm front approached, the agreement between reported and inferred conditions is good. Figure 4-17 shows a middle cloud layer with no precipitation reaching the surface. This is a case of a cloud in the 500-hPa surface with no precipitation reaching the surface; the nearest rain reaching the surface was in Tennessee. The evidence from the sounding for placing the cloud base at 12,200 feet is strong, yet the base is inexplicably reported at 15,700 feet. The reported cloud base of 15,000 feet was probably not representative, since altostratus, with bases 11,000 to 14,000 feet, was reported for most stations over Ohio and West Virginia.

Figure 4-18 shows layered clouds with their intermediate clear layers not showing in the humidity trace. There is good agreement between the sounding and the aircraft report. The clear layer between 6,000 and 6,500 feet is not indicated on the sounding. Thin, clear layers, as well as thin cloud layers, usually cannot be recognized on the humidity trace.

In addition to the above analysis, another study was made to determine how reliable the dewpoint depression is as an indicator of clouds. The results are summarized in figure 4-19. Each graph shows the percent probability of the existence of a cloud layer in January for different values of dewpoint depression. On each graph one curve shows the probability of clear or scattered conditions as a function of the dewpoint depression; the other curve shows that of broken or overcast conditions.

Separate graphs are based on 1,027 observations, which are enough to indicate the order of magnitude of the dewpoint depressions at the base of winter cloud layers. Minor irregularities in the curves were not smoothed out because it is not certain that they are all due to insufficient data. The graphs are applicable without reference to the synoptic situation.

For a given winter sounding, you can estimate from the graph the probability of different sky cover conditions with cloud bases between 1,000-hPa and 600-hPa for layers of given minimum dewpoint depressions.

4.5.5 HUMIDITY FIELD IN THE VICINITY OF FRONTAL SYSTEMS

Studies of the humidity field throughout frontal zones indicate there is a tongue of dry air extending downward in the vicinity of the front, and sloping in the same direction as the front. One study found that such a dry tongue was more or less well developed for all frontal zones investigated. This dry tongue was best developed near warm fronts; it extended, on the average, down to 700-hPa in cold fronts and to 800-hPa in warm fronts. In about half the fronts, the driest air was found within the frontal zone itself on occasion it was found on both the cold and warm sides of the zone. About half the flights through this area showed a sharp transition from moist to dry air, and the change in frost point on these flights averaged about 20°C in 35 miles. Some flights gave changes of more than 20°C in 20 miles.

As a frontal cloud deck is approached, the dewpoint or frost point depression starts diminishing rapidly. At distances beyond 10 to 15 nm, this variation is much less. You should keep this fact in mind when attempting to locate the edge of a cloud deck from rawinsonde data. Linear extrapolation or interpolation of dewpoint depressions cannot be expected to yield good results. For instance, when one station shows a dewpoint depression of 10°C and the neighboring station shows saturation, the frontal cloud may be anywhere between them, except within about 10 nm of the driest station.

Since the frontal cloud masses at midtropospheric levels is usually surrounded by relatively dry air, it is possible to locate the edge of the cloud mass from humidity data on constant pressure charts. This is so because the typical change in dewpoint depression in going from the cloud edge into cloudfree air is considerably greater than the average error in the reported dewpoint depression.

4.5.6 500-hPa ANALYSIS OF DEWPOINT DEPRESSION

Figure 4-20 shows an analysis of the 500-hPa dewpoint depression field superimposed upon an analysis of areas of continuous precipitation, and of areas of overcast middle clouds. The 500-hPa dewpoint depression isopleths were drawn independently of the surface data. The analysis shows the following:

1. The regions of high humidity at the 500-hPa level coincide well with the areas of middle clouds and the areas of precipitation.

2. The regions of high humidity at the 500-hPa level are separated from the extensive dry regions by strong humidity gradients. These gradients are, in all probability, much stronger than those shown on this analysis.

3. A dewpoint depression of 4°C or less is characteristic of the larger areas of continuous precipitation, and also of the larger areas of overcast middle clouds.

Figure 4-20.-Surface fronts, areas of continuous precipitation, areas covered by 8/8 middle clouds, and isolines of 500-hPa dewpoint depression at 0300 UTC, 7 September 1995

Since the 500-hPa level dewpoint depression analysis agrees well with the surface analysis of middle cloud and precipitation, the possibility exists of replacing or supplementing one of these analyses with the other. The characteristics of the 500-hPa level dewpoint depression analysis, outlined above, make it a valuable adjunct to the surface analysis. These analyses can be compared and, by cross-checking, each can be completed with greater accuracy than if they were done independently.

4.5.7 THREE-DIMENSIONAL HUMIDITY ANALYSIS-THE MOIST LAYER

Using a single level (for example, the 500-hPa level dewpoint depression analysis) to find probable cloud areas does not indicate clouds above or below that level. For example, if the top of a cloud system reached only to 16,000 feet, and there was dry air above at 500-hPa (approximately 18,000 feet), you wouldn't suspect, from the 500hPa analysis, the existence of clouds below the 500-hPa level.

However, an analysis of the extension of the moist layers in three dimensions can be obtained simply by scrutinizing individual RAOBs. Those selected should be in the general vicinity of, and the area 500 to 1,200 miles upstream of, the area of interest, depending on the forecast period. The heights of the bases and tops can be indicated, though there is little advantage in indicating a dry layer 2,000 to 3,000 feet thick sandwiched between thicker moist layers. Usually, it is sufficient to indicate the entire moist layer, without bothering about any finer stratums. A survey of the cloud field is made easier by writing the heights of the bases and tops in different colors.

A moist layer for the sake of simplicity may be defined as a layer having a frost point depression of 3°C or less (i.e., a dewpoint depression of 4°C at -10°C; 5°C at -20°C; 6°C at -30°C).

4.5.8 PRECIPITATION AND CLOUDS

The type and intensity of precipitation observed at the surface is related to the thickness of the cloud aloft, and particularly to the temperatures in the upper portion of the cloud.
The results of a study relating cloud-top temperatures to precipitation type and intensity are as follows:

• From aircraft ascents through stratiform clouds, along with simultaneous surface observations of precipitation, it was found that 87 percent of the cases where drizzle occurred, it fell from clouds whose cloud-top temperatures were warmer than -5°C. The frequency of rain or snow increased markedly when the cloud-top temperature fell below -12°C.

• When continuous rain or snow fell, the temperature of the coldest part of the cloud was below -12°C in 95 percent of the cases.

• Intermittent rain was mostly associated with cold cloud-top temperatures.

• When intermittent rain was reported at the surface, the cloud-top temperature was colder than -12°C in 81 percent of the cases, and colder than -20°C in 63 percent of the cases. From this, it appears that when minor snow (continuous or intermittent) reaches the ground from stratiform clouds, the clouds (solid or layered) extend in most cases to heights where the temperature is well below -12°C, or even -20°C.

This rule cannot be reversed. When rain or snow is not observed at the surface, middle clouds may well be present in regions where the temperature is below -12°C or -20°C. Whether or not precipitation reaches the ground will depend on the cloud thickness, height of the cloud base, and the dryness of the air below the base.

4.5.9 INDICATIONS OF CIRRUS CLOUDS IN RAOB

Cirrus clouds form at temperatures of -40°C or colder. At these temperatures, as soon as the air is brought to saturation, the condensate immediately freezes. The ice crystals often descend in altitude slowly, to levels that have air temperatures of -30°C, and persist if the humidity below the formation level is high enough to support saturation. In general, cirrus clouds are found in layers that are saturated, or supersaturated, with respect to ice at temperatures colder than 0°C.

4.5.10 OBSERVATION AND FORMATION OF CIRRUS

Cirrus, or cirriform clouds, are divided into three general groups: cirrus (proper), cirrostratus, and cirrocumulus. Cirrus clouds, detached or patchy, usually do not create a serious operational problem. Cirrostratus and extensive cirrus haze, however, may be troublesome in high-level jet operations, aerial photography, interception, rocket tracking, and guided missile navigational systems. Therefore, a definite requirement for cirrus cloud forecasting exists. The initial formation of cirrus clouds normally requires that cooling take place to saturation, and to have temperatures near -40°C. Under these conditions, water droplets are first formed, but most of them immediately freeze. The resulting ice crystals persist as long as the humidity remains near saturation with respect to ice. There is some evidence that the speed of the cooling, and the kind and abundance of freezing nuclei, may have an important effect on the form and occurrence of cirrus clouds. Slow ascent starts crystallization at humidities substantially below saturation; this is presumably the case in extensive cirrostratus clouds associated with warm frontal altostratus clouds. If slow ascent occurs in air that has insufficient freezing nuclei, a widespread haze may result, which at -30° to -40°C is predominantly composed of water droplets. In the case of more rapid cooling, there is a tendency for the initial condensation to contain a higher proportion of water droplets, which leads to a "mixed cloud' that will convert to ice or snow in time. Presumably, dense cirrus, fine cirrus, cirrocumulus, and anvil cirrus clouds are of this type. It is assumed that fine cirrus clouds (proper) are formed in shallow layers that are undergoing rapid convection due to advection of colder air at the top of the shear layer.
On the other hand fine cirrus and cirrostratus clouds are so often associated, and cirrostratus clouds are so often reported by pilots as developing from the merging of fine cirrus clouds, that there is a question whether the process of formation in cirrus and cirrostratus clouds are essentially different. Nevertheless, the prevailing crystal types in cirrus and cirrostratus clouds seem to differ, though this may not be universal, or may merely represent different stages in cirrus cloud evolution.
Horizontal visibilities within cirrostratus clouds are generally between 500 feet and 2 nm. Thin cirrus haze, invisible from the ground, often reduces the visibility to 3 nm.

A rule of thumb for forecasting or estimating the visibility within thin cirrus or other high cloud (temperatures below -30°C) follows:

Visibility = 1/2 nm times dewpoint depression in degrees C. 

For example; temperature is -35°C, dewpoint is -38°C, and visibility = 1/2 x 3 = 1 1/2 nm.

This rule has been used successfully only in the Arctic where poor visibility in apparently cloudfree air is often encountered.

4.5.11 THE CIRRUS CLOUD FORECASTING PROBLEM

Many forecasters have attempted to forecast cirrus clouds by using frontal or cyclone models. This procedure is not always satisfactory. There are a number of parameters, both surface and aloft, that have been correlated with cirrus cloud formation. A few of the more prominent parameters are mentioned in the following text.

4.5.11.1 Surface Frontal Systems

Frontal and cyclone models have been developed that embody an idealized cloud distribution. In these models, the cirrus clouds are lowering and thickening to form altostratus clouds, which indicates an advancing warm front.

4.5.11.2 Fronts Aloft

Above 500-hPa the concepts of air masses and fronts have little application. Most of the fine cirrus clouds observed ahead of and above warm fronts or lows initially form independent of the frontal middle cloud shield, though later it may trail downward to join the altocumulus and altostratus cloud shields. With precipitation occurring in advance of a warm front, a 60-percent probability exists that cirrus clouds are occurring above. Cirrus clouds observed with the cold front cloud shield either originate from cumulonimbus along and behind the front or from convergence in the vicinity of the upper trough. In many cases there is no post cold front cirrus clouds, probably due to marked subsidence aloft.

4.5.11.3 Contour Patterns Aloft

One forecasting rule used widely states that "the ridge line at 20,000 feet, about 500 hpa, preceding a warm front marks the forward edge of the cirrus cloud shield."

For a typical 500-hPa wave pattern, the following information applies:

• No extensive cirrostratus clouds will occur before the surface ridge line arrives.

• Extensive cirrostratus clouds follow the passage of the surface ridge line.

• No middle clouds appear before the arrival of the 500-hPa ridge line.

• Middle clouds tend to obscure the cirrus clouds after passage of the 500-hPa ridge line.

• When the 500-hPa wave has a small amplitude, the cirrus cloud arrival is delayed and the clouds are thinner.

• The greater the 500-hPa convergence from trough to ridge, the more cirrus clouds between the surface and 500-hPa ridge lines.

4.5.11.4 Cirrus Clouds in Relation to the Tropopause

Experiences of pilots have confirmed that the tops of most cirrus clouds are at or below the tropopause. In the midlatitudes, the tops of most cirrus cloud layers are at or within several thousand feet of the polar tropopause. Patchy cirrus clouds are found between the polar tropopause and the tropical tropopause. A small percentage of cirrus clouds, and sometimes extensive cirrostratus, may be observed in the lower stratosphere above the polar tropopause, but mainly below the level of the jetstream core. The cirrus clouds of the equatorial zone also generally extend to the tropopause. There is a general tendency for the mean height of the bases to increase from high to low latitudes more or less paralleling the mean tropopause height, ranging from 24,000 feet at 70° to 80° latitude to 35,000 to 4,000 feet or higher in the vicinity of the equator. The thickness of individual cirrus cloud layers are generally about 800 feet in the midlatitudes. The mean thickness of cirrus clouds tends to increase from high to low latitudes. In polar continental regions in winter, cirrus clouds are virtually based at the surface. In the midlatitudes and in the tropics, there is little seasonal variation.

4.5.11.5 Cirrus Clouds in Relation to the Jetstream

A discussion of cloud types associated with the jetstream is contained in the AG2 TRAMAN, volume 1. In addition to this information, we will discuss a few studies pertaining to cloud types. All of these studies agree that most of the more extensive and dense clouds are on the equatorward of the jet axis. The observed frequency of high clouds poleward of the jet axis can be accounted for as the upper reaches of a cold front, or cold lows, not directly related to the jetstream.
In some parts of a trough, these high clouds may tend to be dense, and in other areas thin.

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