Thermal popping off from a “thermal trigger”
Really this is more for soaring pilots who look for the least little indication that a thermal may be about to “pop.” And if they’re low enough to be looking for these cues, they’re only a few dozen seconds from committing to landing.
Chapter 24 details the Daily Cycle that breeds thermals, including the times and conditions where they thrive. Most motor pilots avoid thermals since the swirling violence near strong lift can cause significant wing maladies, especially for less experienced flyers. But even highly skilled and experienced paraglider pilots have met their end dancing with the strongest conditions.
While surface tension is a property of liquids that doesn’t apply here, there is a similarity with liquids that does apply – different air densities do not readily mix. So warmer air, being lighter, will not readily mix with cooler air right above it. Just like heavy liquid will layer below a lighter liquid. So too, does cool air seek out the lowest level.
One process for triggering a thermal starts with a flat bubble of air that is warmed and pushes upward (1). If something on the ground pushes enough to get a vertical flow going, such as the depicted tractor, the rest of the warm air starts heading in that direction and it all starts oozing up through the new opening. If the warm air mass gets blown over an obstruction, like a hill, the same thing can happen.
As the warm air oozes into the rising column it sucks air in from all sides. Soon the warm air is depleted and whole shebang starts rising as a column. The warm ground starts heating a new batch of air and the process repeats. Surfaces that are sufficiently large or warm (like a fire) can keep the process going continuously, especially in light winds.