When a moving air mass meets another, convergence results. Convergence often occurs:
1. At mountain peaks when upslope breezes rise up on
both sides of the mountain.
2. When a sea breeze meets a light prevailing wind.
3. When downslope winds encounter each other as they flow down opposite sides of a valley.
Flying in convergence areas is usually very smooth with widespread ascending currents, since such a phenomenon usually occurs in light weather conditions.
Let's deal with this very important topic in a concise manner: A thermal is a warm upward motion in the air. Downdrafts often exist around a thermal since the air must move down to replace the rising thermals.
Instability results in thermals. Unstable days with humidity produce cumulus clouds which are a good sign of the presence of thermals. The so-called "blue thermals" do not produce clouds, since they are low humidity thermals. All thermals of reasonable strength reach the same altitude the dew point or cloud base. Weaker ones dissipate at a lower altitude.
Strong thermals form a column.
Strong thermals reach cloud base.
The formation of a thermal cloud
The best way to spot a thermal is to watch cumulus clouds forming or birds circling under them. In addition, thermal sources can be assessed by studying the terrain. An example will illustrate where thermals can be found in general: if we turn the earth's surface upside down after having it thoroughly flooded and observe where the last droplets of water are coming from, we will see that it is from these points or ridges that thermals are likely to originate.
terrain: Especially when surrounded by dark terrain. This is a fundamental source.
• Hills: The ridges of hills are ideal producers of thermals, even in late afternoon.
• Rocks: Despite taking time to heat up, they are good producers of thermals, even in late afternoon.
• Trees and flora: Evening suppliers of thermals.
• Villages and houses: Excellent sources during the day.
• Lee side thermals: Also excellent. But remember lee side means downwind side, and in any significant wind it is advised not to seek them out as its is dangerous. If you lose a leeside thermal you will land in the turbulence of the lee side area.
• House thermals: Reliable thermals commonly known to local pilots, which almost invariably emerge from the same point on the terrain.
• Magic Lift: Common parcels of rising air in a valley late in the afternoon (sometimes in combination with wonder winds). They are steady thermal suppliers.