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• Rotor turbulence can be severe or extreme. Caution is needed on tow and when transitioning from smooth wave flow (lift or sink) to rotor. Rotors near the landing area can cause strong shifting surface winds of 20 or 30 knots. Wind shifts up to 180° sometimes occur in less than a minute at the surface under rotors. 

• Warm, moist exhaled air can cause frost on the canopy, restricting vision. Opening air vents may alleviate the problem or delay frost formation. Clear vision panels may also be installed. If frost cannot be controlled, descend before frost becomes a hazard. 

• In wet waves, those associated with a great deal of cloud, beware of the gaps closing beneath the glider. If trapped above cloud, a benign spiral mode is an option, but only if this mode has been previously explored and found stable for the glider. 

• Know the time of actual sunset. At legal sunset, bright sunshine is still found at 25,000 feet while the ground below is already quite dark. Even at an average 1,000 fpm descent, it takes 20 minutes to lose 20,000 feet. 

Caution: Flights under a rotor cloud can encounter high sink rates and should be approached with extreme caution. 

## Soaring Convergence Zones 

Convergence zones are most easily spotted when cumulus clouds are present. They appear as a single, straight, or curved cloud street, sometimes well defined and sometimes not. The edge of a field of cumulus can mark convergence between 

a mesoscale air mass that is relatively moist and/or unstable from one that is much drier and/or more stable. Often, the cumulus along convergence lines have a base lower on one side than the other, similar to that in Figure 9-31 . 

With no cloud present, a convergence zone is sometimes marked by a difference in visibility across it, which may be subtle or distinct. When there are no clues in the sky itself, there may be some clues on the ground. If lakes are nearby, look for wind differences on lakes a few miles apart. A lake showing a wind direction different from the ambient flow for the day may be a clue. Wind direction shown by smoke can also be an important indicator. A few dust devils, or—even better—a short line of them, may indicate the presence of ordinary thermals versus those triggered by convergence. Spotting subtle clues takes practice and good observational skills, and is often the reason a few pilots are still soaring while others are already on the ground. 

The best soaring technique for this type of lift depends on the nature of the convergence zone itself. For instance, a sea-breeze front may be well defined and marked by curtain clouds; the pilot can fly straight along the line in fairly steady lift. A weaker convergence line often produces more lift than sink; the pilot must fly slower in lift and faster in sink. An even weaker convergence line may simply serve as focus for more frequent thermals; normal thermal techniques are used along the convergence line. Some combination of straight legs along the line with an occasional stop to thermal is often used. 

