Primary wave Secondary wave Tertiary wave Wind Figure 10-40.  Possible flightpath while transitioning from the tertiary into the secondary and then into the primary. •  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. 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. Caution: Flights under a rotor cloud can encounter high sink  rates and should be approached with extreme caution. 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  Soaring Convergence Zones 10-23