Recognizing Thermals

Author: Adam Woolley

One of the key skills of gliding is to recognise a thermal as we approach it; there are many ways to do this using ground features, clouds, off mountainous terrain, smoke from a fire, etc. Naturally, we need to find a way to recognise a thermal as we approach it.

Thermal Form
Thermals rise through our atmosphere because of a change in temperature, as hot air rising. We must also acknowledge that when a thermal rises, it’ll leave a wake of turbulence in its path upwards. So, the first sign of lift will be some turbulence as we approach it, followed by an increasing amount of sink. What goes up, must go down too right? Often if the sink is really bad, it’s a good sign that the thermal ahead will be a strong one – at least, that’s what I tell myself! As we approach the rising air of a thermal, we can safely assume that the sink will progress from a well sink to a lift. In between this sink to lift, we can expect some turbulence again.

Feeling the Thermal
As we transition to this phase, we should think about slowing down to feel our way further into the thermal. Remember, though, to keep your cruise speed up enough to ensure effective aileron control so you can take advantage of the thermal quickly should you need to. A Discus or the like glider, 55-65kts, is ideal, for a ballasted 18m glider, perhaps 65-80kts. The next thing you’ll notice is a surge, the tail coming up and the airspeed increasing ever so slightly, or one of the wings raising, perhaps both at the same time!

Seat of the Pants
The key to this next phase is to realise that your personal senses are now the key detection system, as almost all instruments have a delay in them before they register. Our senses are within our inner ear which takes care of our balance, the other is the nerve endings in our backside. Our brain connects all these via a complex algorithm, then advises us of the acceleration, the feeling of lift once we become tuned to it. You’ve probably heard it before, “your seat of the pants” feedback system has an almost instant response time, listen to it!

Sadly, our body can only detect the rate of change, so that’s the initial sink or lift of the glider oneself. Once we are established in a thermal, this is where the audio and visual indicators of the vario system come into play, to show us the rate of thermal. All thermals feel different and the trick to the game is to learn which thermals feel better than the others on any given day, as this is what separates the really fast pilots from the fast ones. I almost forgot to mention, once you determine that you’re in one of these elusive thermals, turn towards the rising wing and try to stay in it – happy thermalling! Source: ‚Adam Woolley / Wings & Wheels‚.

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