Yes I agree. Another example is big cities versus small ones.
I almost never find thermals over the larger areas of housings and buildings
belonging to a somewhat larger urban area. Just to many trigger points
producing small and low thermals only. However when over a village or a
group of farm housings it is usually very easy to find the spot where good
thermals are triggered off using the heated air of the direct environment.

Karel, NL


"Bill Daniels" schreef in bericht
hlink.net...

"Andy Durbin" wrote in message
om...
"Roger Worden" wrote in message
om...
In the Jan. 2004 issue of Model Aviation, in the Radio Control Soaring
column, Real Smart Guy candidate Mike Garton proposes a "condensation
analogy" to suggest places to look for thermals to trigger. Imagine
water
condensing on a ceiling: it drips first from the low spots or tiny
bumps.
Now imagine heated, but relatively stable, air along the ground. If
it's
"trying" to rise, might it not "drip up" first from the higher spots,
little
hills, even trees? If it's moving slowly horizontally, and encounters
a
tree
line, it might be forced up enough to trigger a thermal. His
experience
with
models supports the theory on the small scale. Does y'all's experience
support it at the larger scale?

Roger Worden


Yes it seems to work that way. A moving object may also disturb
motionless hot air and start a thermal. I was once low over a local
dirt strip, I think turning base to land, when a truck drove into a
large flat dirt area. It triggered a good thermal that got me up and
home.
Andy (GY)

One thing to keep in mind is that there is a ratio between thermal
triggers
and the heated air available to be triggered.

In other words, in weak conditions over rugged terrain, there is a surplus
of available triggers, but a deficit of heater air to be triggered.
Sometimes there will be no thermal over an obvious trigger site because
the
available bouyant air was already triggered by a lesser, but adequate
trigger upwind. In these cases, potential trigger sites are not a
reliable
thermal indicator.

In strong conditions, over mostly uniform, flat surfaces, the few
available
trigger sites become more important.

Bill Daniels