An evaluation of directional and speed shear in a moist layer also gives an indication of the type of cloud that maybe present. Relatively little shear indicates a stratiform cloud layer, while larger shear should be associated with a strato-, alto-, or cirro-cumuloform layer.

A thumb rule for finding the type of cloud in a layer uses the thickness of the moist layer as an indicator. If the layer is 1,000 to 4,000 feet thick, the cloud is stratiform; 5,000 to 9,000 feet thick, the cloud is cumuloform; and over 10,000 feet thick, it is towering-cumuloform. This thumb rule should not take precedence over an analytical procedure. Obviously it does not do justice to nimbostratus cloud layers.

Identification of the cloud genera is more straightforward. By definition, clouds with bases below 6,500 feet AGL are low-etage clouds: 6,500 feet to 18,500 feet AGL are mid-etage clouds, and above 18,500 feet AGL are high-etage clouds. Consult your AG3 manual for a further breakdown of the low, mid, and high cloud types.

ASSOCIATION OF CLOUD TOP TEM-PERATURES AND PRECIPITATION. 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 part of the cloud. The processes that cause cloud particles to grow and precipitate out of clouds have received much attention during the past 30 years or so. However, our knowledge of the process is far from complete.

The results of one study relating cloud-top temperatures to precipitation type and intensity are applicable to the Skew T analysis when analyzing stratiform clouds. In 87 percent of the cases where drizzle was reported at the surface, the cloud-top temperatures were colder than 5C. The frequency of rain or snow increased markedly when cloud-top temperatures fell below 12C. During continuous rain or snow, cloud-top temperatures were below 12C in 95 percent of the cases. Intermittent rain or snow fell from clouds with cloud-top temperatures colder than 12C in 81 percent of the cases, and colder than 20C in 63 percent of the cases. From this we can derive the rule that drizzle should be expected only when cloud-top temperatures are colder than 5C and that continuous or intermittent rain or snow should be expected only when stratiform clouds, either layered or continuous, have cloud-top temperatures colder than 12C. We cannot reverse this rule and say that we should expect precipitation if cloud-top temperatures are below 5C. Whether precipitation occurs or reaches the ground depends on (1) the amount of moisture present, (2) if there is some lifting mechanism to bring the moisture to saturation and continued lift to force precipitation of the excess moisture, (3) the cloud thickness, (4) the height of the cloud base above the ground, and (5) the dryness of the air below the cloud base.

LIMITATIONS OF THE DIAGNOSIS OF TOWERING CUMULUS AND CUMULONIM-BUS CLOUDS ON A SKEW T. The towering cumulus and cumulonimbus of summer- and tropical-air-mass situations are generally scat-tered. In many, if not most cases, they do not actually cover even half of the sky. Under such conditions, the probability that radiosondes released once or twice daily from fixed locations will pass up through a cloud of this type appears small. When a sonde does enter such a cloud, it is likely to pass out of the side rather than the top. Electrical charges in active thunderstorms have been seen to affect the sondes circuitry, yielding erratic readings.

Soundings that do pass through these clouds will often show unrepresentative values. The temperature in some parts of these clouds is often colder than that of the ambient, or surrounding, air. The lapse rate will not necessarily be satura-tion adiabatic, because of the effect of downdrafts, snow or hail melt, or entrainment and mixing. While humidity readings will be high, they also will not be representative of the surrounding air.

For these reasons, we cannot rely on a sounding to directly indicate the presence of tall cumulus clouds or the coverage by these clouds. Satellite, radar, or ground observations are our best indication of the presence and coverage of these clouds.

LIMITATIONS OF CIRRUS CLOUD IDENTIFICATION. True cirrus clouds form at temperatures near 40C, or colder. Present rules for encoding Rawinsonde reports establish a cutoff point for reporting the dew point depression at this same temperature. This cutoff temperature was established because the hygristor elements in the older radiosondes were not reliable, and the data received from the sondes after that temperature was considered worthless. With the increasing use of the new humidity-measuring circuitry in radiosondes, this cutoff temperature may be changed to colder temper-atures. Although not routinely reported, data from upper-air soundings indicates cirrus clouds layers are observable by at least an increase in the moisture (decrease in dew point depression) at temperatures colder than 40C, even though the dew point was very low. If you are analyzing your own stations sounding, you should ignore the mandatory dew point cutoff for reporting and have the plotter enter the data on the Skew T. Analysis of this data using frost point calculations may yield some surprising heights for cirrus and may also end the typical standard observer entry for cirrus as either 20,000 or 25,000 feet.

Learning Objective: Define the use of and describe the computation procedures for Skew T.

Privacy Statement - Press Release - Copyright Information. - Contact Us - Support Integrated Publishing