The Skew T gives you, the analyst, the best information to evaluate where cloud layers are and the type of cloud in each layer (short of sending an observer up in an aircraft). Analysis of moisture and winds can help you identify cloud layers missed by your observer at night or hidden by a low overcast. In this section we will see how to identify cloud layers by their moisture content and how you can identify the cloud type. We will also look at the association between cloud temperatures and precipitation, and some limita-tions in the identification of the taller cumuloform and cirriform clouds.

Studies have shown that water vapor becomes visible, as either minute water droplets or ice crystals, well below the saturation point. Other studies have correlated radiosonde observed temperatures and moisture measurements with aircraft observed cloud layers. While most aircraft reported cloud layers were supported by raob data, many thin or scattered cloud layers reported by aircraft often were not evident in the raobs. This was often interpreted as an accepted deficiency due to the lack of sensitivity of the older hygristors in the radiosondes. Nevertheless, the studies did find definite relationships between observed relative humidities and cloud layers.

Figure 6-2-13 shows the relationship between the dew point depression and cloud coverage that was determined by the study. Note that these results relate the dew point depression to the probability of occurrence of a scattered or overcast cloud layer. This study and other similar studies provided the basis for the following rules, which are used to analyze cloud layers on a Skew T.

A cloud base maybe inferred to be located where the dew point depression decreases to 5C or less when the air temperature is above freezing, or where the frost point depression is 5C or less when the air temperature is below freezing. This is especially true if the dew point or frost point depression shows a sudden decrease near the same level.

*A part of the difference is due to deficiencies in the hygristor circuitry of the radiosondes. This discrepancy may be reduced somewhat using the Vaisala Mini Sonde. Manufacturers data indicates that the Humicap humidity element has a much greater sensitivity and less lag time than the hygristor of the widely used Viz J005/J006 sondes and forerunners.

The top of a cloud layer is usually indicated by an increase in dew point (frost point) depression. Once you identify a cloud base, extend the cloud upward until a significant increase in dew point or frost point depression is found. The gradual increase of dew point or frost point depression with height is usually not significant.

Cloud cover may the dew point depression for above freezing temperatures, or from the frost point depres-sion when below freezing temperatures are encountered, by using the following thumb rules:

The relative humidity is an even better guide for cloud coverage. Many forecasters use the following guideline:

Finally, keep in mind that clouds tend to form in areas where some mechanism is providing lift. While we have previously discussed mechanical lift and thermal or convective lift, there is another important lifting mechanism in the atmosphere. This is vorticity. We discussed its effect on the atmosphere in AG2, Volume 1, Unit 8, Lesson 5. While vorticity would be difficult to analyze on the Skew T, a quick look at the plotted wind reports may give you a good idea of what the vorticity pattern may be over your area.

Veering wind directions with height (direction changing clockwise with increasing height) usually indicate a positive vorticity pattern, while backing wind directions with height (directions changing counterclockwise with increasing height) usually indicate a negative vorticity pattern. As you probably remember, positive vorticity indicates lift and an up-ward transport of air, so you should expect cloud layers to increase in density. Negative vorticity, on the other hand, should gradually dissipate cloud layers.

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