When locating and utilizing thermals for soaring flight, called thermaling, glider pilots must constantly be aware of any nearby lift indicators. Successful thermaling requires several steps: locating the thermal, entering the thermal, centering the thermal, and, finally, leaving the thermal. Keep in mind that every thermal is unique in terms of size, shape, and strength.
According to the last chapter, if the air is moist enough and thermals rise high enough, cumulus clouds, or Cu (pronounced like the word “cue”) form. Glider pilots seek Cu in the developing stage, while the cloud is still being built by a thermal underneath it. The base of the Cu should be sharp and well defined. Clouds that have a fuzzy appearance are likely to be well past their prime and probably have little lift left or even sink as the cloud dissipates. [Figure 10-1]
Judging which clouds have the best chance for a good thermal takes practice. On any given day, the lifetime of an individual Cu can differ from previous days, so it becomes important to observe Cu lifecycle on a particular day. A good looking Cu may already be dissipating by the time it is reached. Soaring pilots refer to such Cu as rapid or quick cycling, which means the Cu forms, matures, and dissipates in a short time. The lifetime of Cu often varies during a given day as well; quick cycling Cu early in the day often become well formed and longer lived as the day develops.
Sometimes Cu cover enough of the sky that seeing the cloud tops becomes difficult. Hence, glider pilots should learn to read the bases of Cu. Generally, a dark area under the
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.
Other Indicators of Thermals
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 mountainous terrain, look for sun-facing slopes. Unless the sun is directly overhead, the heating of a sun-facing slope is more intense than that over adjacent flat terrain because the sun’s radiation strikes the slope at more nearly right angles. [Figure 10-3] Also, cooler air usually pools in low-lying areas overnight; taking longer to warm during the morning. Darker ground or surface features heat quicker than grass covered fields. Huge black asphalt parking lots can produce strong thermals. A large tilled black soil field can be a good source of lift if the pilot can find the sometimes very narrow plume of rising air. Finally, slopes often tend to be drier than surrounding lowlands, and tend to heat better. Given the choice, it usually pays to look first to the hills for thermals.
Whether soaring over flat or hilly terrain, some experts suggest taking a mental stroll through the landscape to look for thermals. Imagine strolling along the ground where warmer areas would be found. For instance, walk from shade into an open field where the air suddenly warms. A town surrounded by green fields is likely to heat more than the surrounding farmland. Likewise, a yellowish harvested field feels warmer than an adjacent wet field with lush green vegetation. Wet areas tend to use the sun’s radiation to evaporate the moisture rather than heat the ground. Thus, a field with a rocky outcrop might produce better thermals. Rocky outcrops along a snowy slope heat much more efficiently than surrounding snowfields. Although this technique works better when at lower altitudes, it can also be of use at higher altitudes in the sense of avoiding coollooking areas, such as a valley with many lakes.