Thermals drift with the wind on blue days as well, and similar  techniques are required to locate thermals using airborne  or ground-based markers. For instance, if heading toward  a circling glider but at a thousand feet lower, estimate how  much the thermal is tilted in the wind and head for the most  likely spot upwind of the circling glider.  [Figure 10-5]  When  in need of a thermal, pilots might consider searching on a  line upwind or downwind once abeam the circling glider.  This may or may not work; if the thermal is a bubble rather  than a column, the pilot may be below the bubble. It is easy  to waste height while searching in sink near one spot, rather  than leaving and searching for a new thermal. Remember that  a house thermal will probably be downwind of its typical spot  on a windy day. Only practice and experience enable glider  pilots to consistently find good thermals. or a circling bird. When down low, accept weaker climbs.  Often the thermal cycles again, and hard work is rewarded. Cool, stable air can also drift with the wind. Avoid areas  downwind of known stable air, such as large lakes or large  irrigated regions. On a day with Cu, stable areas can be  indicated by a big blue hole in an otherwise Cu-filled sky. If  the area is broad enough, a detour upwind of the stabilizing  feature might be in order.  [Figure 10-6] When searching for lift, use the best speed to fly, that is,  best L/D speed plus corrections for sink and any wind. This  technique allows glider pilots to cover the most ground with  the available altitude. See Chapter 3, Aerodynamics of Flight,  to review shifting of the polar for winds and sink. Once a thermal has been located, enter it so you do not lose  it right away. The first indicator of a nearby thermal is often,  oddly enough, increased sink. Next, a positive G-force is felt,  which may be subtle or obvious, depending on the thermal  strength. The “seat-of-the-pants” indication of lift is the  quickest, and is far faster than any variometer, which has  a small lag. Speed should have been increased in the sink  adjacent to the thermal; as the positive G-force increases,  reduce speed to between L/D and minimum sink. Note the  trend of the variometer needle (should be an upswing) or the  When the sky is full of Cu, occasional gliders are marking  thermals, and dust devils move across the landscape, the  sky becomes glider pilot heaven. If gliding in the upper part  of the height band, it is best to focus on the Cu, and make  choices based on the best clouds. Sometimes lower altitudes  cause glider pilots to go out of synch with the cloud. In that  circumstance, use the Cu to find areas that appear generally  active, but then start focusing more on ground-based  indicators, like dust devils, a hillside with sunshine on it,  Figure 10-6.  Blue hole in a field of cumulus downwind of a lake. Lake Wind Entering a Thermal  The Big Picture Wind Wind Wind Wind A B Sink Sink Figure 10-5.  Thermal tilt in shear that (A) does not change with height, and that (B) increases with height. 10-5