Spotting Thermals

In article <CB6wyp.CK8@tug.com>, andrew@tug.com (Andrew Beattie) wrote:
> I've cross posted this to rec.aviation.soaring, because those guys are *bound*
> to know, so I'll explain the problem in full:
> We are discussing the problem that I had of flying a 15 square meter light
> weight kite at an altitude of 300' (with CAA clearance).  I hit a thermal
> and was lifted a short distance off the ground.  I was fortunate to
> notice the problem and collapse the canopy before I got into trouble, but
> since I am using this kite to power my kite buggy (an idea similar to land
> yachting), and could easily have rushed into the updraft at 30mph and been
> too far clear of the ground before I realised what was going on.  So:
> I'd like to know how to spot a thermal, just by observation.

First of all, if you find a way to really spot a thermal just by
observation, you could become a rich person by selling the device to
sailplane and hang glider pilots around the world.  However, there are a
few techniques that I am aware of from my years of hang gliding, some of
which may be usefull to you.

A thermal is nothing more than a rising column of warm air.  As such, it is
impossible to detect with normal vision, unless it happens to be carrying
something up with it.  Flying in desert areas, typically the base of the
thermal is marked by a dust devil (rising column of dust) which looks like
a mini tornado.  At your ground perspective, and probably non-desert
conditions, I'd look for bits of leaves being picked up.  Also, as the
thermal moves across the ground, it creates quite a bit of its own local
wind.  If you could observe the wind direction in your immediate area from
tree leaves, or perhaps flags put up to mark the wind direction, you could
perhaps detect where one is (sort of).

In the air, we also look for other indicators of thermals.  A mature
thermal will often have it's top marked by a cumulus cloud (probably not of
use for your application).  Also, at altitude, you may find birds in a
thermal chasing the bugs that got sucked up from the ground (or maybe a
hawk, eagle, or other pilot climbing out).

Since thermals are warmer than the surrounding air, a simple temperature
based thermal detector can be used with some success at altitude,  mostly
for slow flying aircraft such as hang gliders.  You might want to try
putting a sensor on you kite, although I don't know if you could detect
much change at 300'.

Thermals also have a different electrical charge than the surrounding air,
but no one has been able to build a detection system that can compensate
for the electrical charge buildup on the hang glider/sailplane enough to
allow detection the faint field generated by the thermal.

I suppose if someone had unlimited funds they could build a thermal vision
device with enough sensitivity to actually see the thermal, but this is
certainly out of the range of your typical consumer.

I doubt if much of this helps, but it may give you some ideas.

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