On Sep 22, 2:00*am, Derek Copeland wrote:
Can I agree with Andy. In the UK we often seem to get really strong
thermals close to a large lake or reservoir such as Grafham Water. The
thermals are coming off the surrounding land of course, but the
temperature contrast between the hotter ground and the cooler water seems
to act as a trigger mechanism or a mini front. I have often found that
discontinuities such as the edge of a forest, a ridge line (especially if
pointing into sun), or a ploughed field next to one with crop often seem
to give thermals. Provincial towns and active power stations are also good
ground sources.

However the most reliable thermal markers are circling birds of prey such
as buzzards or kites, or insect eaters such as swallows and swifts.

Derek Copeland *

At 22:28 21 September 2009, Andy wrote:

On Sep 21, 1:52=A0pm, danlj *wrote:
I hope this turns into a thread on "How I keep going" - there must
be
scores of pilots more excellent than I who can say more than me.

I well remember that years ago when a visiting German pilot asked for
advice on finding thermals in Arizona I told him to look above and
downwind of cattle tanks. *These are known as stock ponds in other
places. *His reaction, that I must be a complete idiot for suggesting
that pools of water could trigger thermals, has made me cautious about
offering that advice to anyone. *I hoped he landed out but I don't
think he did.

Andy

Andy's observation is a secret well-known to cross-country pilots in
Arizona. Sometimes the most ridiculously small and almost dry water
holes will be the source of strong thermals. My theory is that water
holes inject enough moisture into the hot dry air to significantly
increase its buoyancy. We also have small hills consisting of black
volcanic rock that are also good sources. Dry river beds (called
washes in the southwest US) also can be useful sources.

Another issue with clouds here is that on fairly dry days the top of
the lift is often a thousand feet or more below cloud base. However,
a strong thermal will often continue for a short time above the top of
lift fueled by the extra heat released by condensation and eventually
form a cumulus. Once this short thermal pulse has decayed, the area
immediately below the cloud then has no lift, although the original
thermal may still be found a few thousand feet below. I call these
clouds "cumulus decoyus" - decoy cumulus!


Mike