EAS 308, Meeting 6

The organization of cumulus clouds

The arrangement of cumulus sometimes looks haphazard but there are usually rules governing their locations:

  • They usually form over dry surfaces rather than moist surfaces
  • They usually form over sunny slopes rather than shady slopes
  • A group of large cumulus tends to suppress smaller nearby cumulus .

Thermal leaving the surface:

Last meeting, we learned about the shapes of thermals (Figs. 56 and 57). They can be described a “plume” and “puffs”. Now, we learn about how you can determine if thermals are forming and leaving the surface.

Figure 70 illustrates the response of the temperature of the air just above the surface and wind speed to the “launching’ of thermals. The temperature slowly rises as the wind speed decreases, it becomes still and hotter. Energy is being absorbed by the air in contact with the surface. When the bubble breaks loose of the surface, the temperature drops dramatically as cooler air rushes in to the replace the rising bubble.

The phenomenon illustrated in Fig. 70 is familiar to me as I rig my glider in preparation for a days flying; the procedure takes about 1 to 2 hours. The rigging begins in the early morning when it is cool and still. But, mid-way through the preparation, the first gusts occur indicating the day’s first thermals are forming.

Distribution of cumulus:

Figure 71 illustrates the typical distribution of cumulus over fairly uniform level ground during the day. In the morning the first cumulus are similar in size and evenly spaced. By mid-day, cloud base has risen and a distribution of sizes is present. By mid-afternoon, when the surface temperature is highest, cloud base is highest and only the largest clouds have survived.

An important rule: On average, the lifetime of thermals and the space between the them increases as the CBL becomes deeper. This rule is consistent with the lift in cloudless thermals illustrated in Fig. 65 (pg. 54).

“Blue holes”:

When the surface is not uniform (Fig. 72 and Fig. 74) regions occur called “blue holes” where cumulus will not form. Glider pilots do not cross these regions because they know that thermals are not forming. The surface is too cool for thermals to form.

In-class exercise leading to Assignment 6:

The web-site you visited in Assignment 5 () is a rich source of cumulus cloud images, especially the formation, maturation and dissipation of fields of cumulus clouds. To illustrate this sequence, I downloaded a sequence of hourly visible and infrared images for 17 September 2006. Let’s have a look focusing on central Pennsylvania:

You see valley fog dissipate and cumulus form first along the ridges in the visible images. The infrared images show little temperature differences between the cumulus and surface indicating the cumulus are not tall but shallow. The fog dissipation and first cumulus formation occurs dramatically between 15Z and 18Z. Notice the suppression of cumulus along the wet Susquehanna RiverValley in SE Pennsylvania. By 21Z, the cumulus begin to dissipate and the valley fog begins to re-form.

We can study the evolution of the CBL is the vicinity you these cumulus using atmospheric soundings from the web-site . We go to the Archived meteorology and enter a position of 41N and 77W. We go to Soundings and NAM, 12 km. I downloaded the 12, 15, 18 and 21Z soundings for the 17th and 00Z for the 18th:

The 15Z and 18Z soundings show a dramatic increase in depth of the CBL, consistent with the evaporation of the valley fog and the formation of the cumulus. I will demonstrate how you can apply your knowledge of determining the depth of the CBL from the soundingsusing the 21Z sounding:

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