| Really this is more for soaring pilots who look
for the least little indication that a thermal may be about to
"pop." They're usually desperate, too, because if they're low
enough to be looking for these cues, they're only a few dozen seconds from
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. |
Thermal triggers in action. This depicts the first morning thermal
from an area of dark ground that gets hot easily. Click for larger
version.
|
