Lift Due to Convergence
Convergence lift is most easily imagined when easterly and westerly winds meet. When two opposing air masses meet, air is pushed aloft by the two opposing winds. Air does not need to meet head on to go up, however. Wherever air piles up, it leads to convergence and rising air.
Sea air often has a higher dewpoint than drier inland air. As shown in Figure 9-29, a curtain cloud sometimes forms, marking the area of strongest lift. Due to the mixing of different air along the sea-breeze front, at times the lift can be quite turbulent. At other times, weak and fairly smooth lift is found.
Several factors influence the sea-breeze front character (e.g., turbulence, strength, and speed of inland penetration, including the degree of inland heating and the land/sea temperature difference). For instance, if the land/sea temperature difference at sunrise is small and overcast cirrus clouds prevent much heating, only a weak sea-breeze front, if any, forms. Another factor is the synoptic wind flow. A weak synoptic onshore flow may cause quicker inland penetration of the sea-breeze front, while a strong onshore flow may prevent the sea-breeze front from developing at all. On the other hand, moderate offshore flow generally prevents any inland penetration of the sea-breeze front.
If the sea-breeze front is well defined and marked by a curtain cloud, the pilot can fly straight along the line in fairly steady lift. A weaker convergence line that is not well-defined often produces more lift than sink and the pilot must fly slower in lift and faster in sink. Soaring pilots must be aware that convergence zone lift can be turbulent especially if the colder and warmer air is mixing and be prepared that flying the thermals may be difficult at times.
Convergence can also occur along and around mountains or ridges. In Figure 9-30A, flow is deflected around a ridgeline and meets as a convergence line on the lee side of the ridge. The line may be marked by cumulus or a boundary with a sharp visibility contrast. The latter occurs if the air coming around one end of the ridge flows past a polluted urban area, such as in the Lake Elsinore soaring area in southern California. In very complex terrain, with ridges or ranges oriented at different angles to one another, or with passes between high peaks, small-scale convergence zones can be found in adjacent valleys, depending on wind strength and direction. Figure 9-30B illustrates a smaller-scale convergence line flowing around a single hill or peak and forming a line of lift stretching downwind from the peak.
Convergence can also form along the top of a ridgeline or mountain range. In Figure 9-31, drier synoptic-scale wind flows up the left side of the mountain, while a more moist valley breeze flows up the right side of the slope. The two flows meet at the mountain top and form lift along the entire range. If clouds are present, the air from the moist side condenses first, often forming one cloud with a well-defined step, marking the convergence zone. For this scenario, the better lift conditions will be found on the west side where the air is dryer rather than the east side where clouds are more likely to form.
As a final example, toward evening in mountainous terrain and as heating daytime abates, a cool katabatic, or drainage, wind flows down the slopes. The flow down the slope converges with air in the adjacent valley to form an area of weak lift. Sometimes the convergence is not strong enough for general lifting, but acts as a trigger for the last thermal of the day. In narrow valleys, flow down the slope from both sides of the valley can converge and cause weak lift. [Figure 9-32] Many local sites in either flat or mountainous terrain have lines or zones of lift that are likely to be caused or enhanced by convergence. Chapter 10, Soaring Techniques, covers locating and using convergence.