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.
Convergence zone lift can at times be somewhat turbulent, especially if air from different sources is mixing, such as along a sea-breeze front. The general roughness may be the only clue of being along some sort of convergence line. There can also be narrow and rough (but strong) thermals within the convergence line. Work these areas like any other difficult thermals, using steeper bank angles and more speed for maneuverability.
Combined Sources of Updrafts
Finally, lift sources have been categorized into four types: thermal, slope, wave, and convergence. Often, more than one type of lift exists at the same time, such as thermals with slope lift, thermaling into a wave, convergence zones enhancing thermals, thermal waves, and wave and slope lift. In mountainous terrain, it is possible for all four lift types to exist on a single day. The glider pilot needs to remain mentally nimble to take advantage of various types and locations of rising air during the flight.
Nature does not know that it must only produce rising air based on these four lift categories. Sources of lift that do not fit one of the four lift types discussed probably exist. For instance, there have been a few reports of pilots soaring in travelling waves, the source of which was not known. At some soaring sites, it is sometimes difficult to classify the type of lift. This should not be a problem. Simply work the mystery lift as needed, then ponder its nature after the flight.