cloud base indicates a deeper cloud and, therefore, a higher  likelihood of a thermal underneath. Also, several thermals  can feed one cloud, and it is often well worth the deviation  to those darker areas under the cloud. At times, an otherwise  flat cloud base under an individual Cu has wisps or tendrils of  cloud hanging down from it, producing a particularly active  area. Cloud hanging below the general base of a Cu indicates  that the air is more moist, and hence more buoyant. Note the  importance of distinguishing features under Cu that indicate  potential lift from virga. Virga is precipitation in the form of  rain, snow, or ice crystals, descending from the cloud base  that is evaporating before it strikes the ground. Virga often  signals that the friendly Cu has grown to cumulus congestus  or thunderstorms.  [Figure 10-2] Another indicator that one area of Cu may provide better lift  is a concave region under an otherwise flat cloud base. This  indicates air that is especially warm, and hence more buoyant,  which means stronger lift. This can cause problems for the  unwary pilot, since the lift near cloud base often dramatically  increases, for instance from 400 to 1,000 feet per minute  (fpm). When trying to leave the strong lift in the concave  area under the cloud, pilots can find themselves climbing  rapidly with cloud all around—another good reason to abide  by required cloud clearances. See Title 14 of the Code of  Federal Regulations (14 CFR) part 91, section 91.155, Basic  VFR Weather Minimums. After a thermal rises from the surface and reaches the  convective condensation level (CCL), a cloud begins to form.  At first, only a few wisps form. Then, the cloud grows to a  cauliflower shape. The initial wisps of Cu in an otherwise blue  (cloudless) sky indicate where an active thermal is beginning  to build a cloud. When crossing a blue hole (a region anywhere  from a few miles to several dozen miles of cloud-free sky in  an otherwise Cu-filled sky), diverting to an initial wisp of Cu  is often worthwhile. On some days, when only a few thermals  are reaching the CCL, the initial wisps may be the only cloud  markers around. The trick is to get to the wisp when it first  forms, to catch the thermal underneath. Lack of Cu does not necessarily mean lack of thermals. If the  air aloft is cool enough and the surface temperature warms  sufficiently, thermals form whether or not enough moisture  exists for cumulus formation. These dry, or blue thermals  as they are called, can be just as strong as their Cu-topped  counterparts. Glider pilots can find blue thermals, without  Cu markers, by gliding along until stumbling upon a thermal.  With any luck, other blue thermal indicators exist, making  the search less random. One indicator of a thermal is another circling glider. Often  the glint of the sun on wings is all that can be seen, so finding  other gliders thermaling requires keeping a good lookout,  which glider pilots should be doing anyway. Circling birds  are also good indicators of thermal activity. Thermals tend to  transport various aerosols, such as dust, upward with them.  When a thermal rises to an inversion, it disturbs the stable air  above it and spreads out horizontally, thus depositing some  of the aerosols at that level. Depending on the sun angle and  the pilot’s sunglasses, haze domes can indicate dry thermals.  If the air contains enough moisture, haze domes often form  just before the first wisp of Cu. On blue, cloudless days, gliders and other airborne indicators  are not around to mark thermals. In such cases, pay attention  to clues on the ground. First, think about previous flight  experiences. It is worth noting where thermals have been  found previously since certain areas tend to be consistent  thermal sources. Remember that weather is fickle, so there  is never a guarantee that a thermal currently exists where one  existed before. In addition, if a thermal has recently formed,  it takes time for the sun to reheat the area before the next  thermal is triggered. Glider pilots new to a soaring location  should ask the local pilots about favored spots—doing so  might save the cost of a tow. Glider pilots talk about house  thermals, which are simply thermals that seem to form over  and over in the same spot or in the same area. Stay alert for other indicators, as well. In drier climates, dust  devils mark thermals triggering from the ground. In hilly or  Other Indicators of Thermals A B C Figure 10-2.  Photographs of (A) cumulus congestus, (B) cumulonimbus (Cb), and (C) virga. 10-3