As regards cities I think it depends...

I once went on a lead and follow course with Andy Davis
in blue or partially blue weather. In briefing before
flying he would nominate where he was planning to take
climbs. In the case of larger towns and cities (Bath,
Worcester and Swindon come particularly to mind) he
specified where the best thermal source would be and
would then take us there as low as possible. If a
strong core wasn't there when we arrived he parked
us in weak lift and nosed around until he found the
next strong pulse. It seemed odd at first to take
so much time to do that but it was worth it because
he had saved so much more time by ignoring weak lift
in the cruise to reach his nominated best thermal source
with the minimum of delay and at an altitude low enough
to take best advantage of the good climb.

So I think that cities will be likely to have at least
one source that is better than the multitude of little
trigger points

John Galloway

At 21:24 04 January 2004, K.P. Termaat wrote:
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
thlink.net...

'Andy Durbin' wrote in message
om...
'Roger Worden' wrote in message
news:...
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