Spring Thermals

It is now getting flyable at last!
The time has come to warn newcomers that spring thermals can be rough, and that particular care is needed when flying near to the hill on cold, clear bright days. These days are when the condition of the air favours small, fast moving thermals that can tip your glider to angles that you may not have flown at before.
Avoiding the technicalities of thermal formation and behaviour you must be aware of the following:
When a bubble of air starts to rise, a circulation is set up in its outer “skin” by the combination of friction between the rising air and the air surrounding it and convection caused by the warm thermal being cooled by contact with the cooler air around it.


In addition, as the warm air rises, cooler air descends to replace it. Thus, loosely speaking, there are three areas to concern us.

A. In the middle where the air is rising

B. Around the edges of a thermal, where the air is turbulent and where the circulation causes the net upward flow to be much less then in the core

C. Where the airflow is downwards.

So, when you fly along a radius into a thermal that is ‘out in space’ you will usually feel sink first, followed by the turbulence with some lift then the really useful up flowing air in the middle.
What happens when this thermal up the face of a hill. The shape is probably distorted as shown, and depending on the gradient of the hill, the thermal may break away from the face part way up.


In the occasional extreme case, we can have a situation as
above where the circulation at the ridge side of thermal may be augmented by downward flowing air being sucked into the bottom.
When the thermal is large, your glider may be wholly or mainly in one of the regions A, B or C with a fairly gentle transition from one to the other. When the thermals are small your glider may span all three of these regions.


If you are flying close to the ridge and you pass tangentially through a thermal, as shown above, your glider will be tipped violently towards the hill. Even if there is no down flow between the thermal and the hill, the first time you experience strong lift under one wing tipping you towards the ridge you will probably wish you had tried golf instead.


The above information has been condensed from articles by John Klunder, Bill Walmsley and Jonathan Gill.

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