Schempp-Hirth Ventus under cumulus clouds
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wind thermal moist thermal dry thermal cumulus cloud center thermal

Big and high cumulus days are super fun, especially when all the ducks line up.  That’s the trick, to figure out the day's pattern so that they (ducks) do, as it is on every day and flight.  However, when we fly under large cumulus clouds, it requires a different tact at the day.  Instead of looking down, we must look up, this is to get all available cues from the clouds ahead.  The beauty is that there is no guesswork, as there is already proof that that convection exists!

Cumulus Cloud Cycle

As you’ve probably found, not all clouds produce lift or good thermals under them, others, especially the big ones, often have multiple cores.  Knowing the life cycle of a cloud will help too, hit it in the sweet spot and you’ll have a super-strong climb, get the timing wrong and you’ll be left with something weaker.  So we must build our knowledge on this cycle.  You could get lucky occasionally, but typically luck will only go your way on smaller clouds.

So we have to start studying all the indicators.  Approaching the cloud, generally, we have plenty of time to start studying it.  Firstly we can look at it in general, know our overall track after the cloud, figure out the wind, then plan our attack through the cloud to maximise our opportunity at finding the very best of lift.  Where is it the most active?  As I mentioned earlier, there may be multiple cores, so maximise your search opportunity and go via as many of the potential thermal locations as possible.

Dry vs. Moist Thermals

There are dry thermals that rise to produce clouds, and there are thermals that carry more moisture.  How can we tell the difference?  A thermal that is dry typically punches up into the cloud, which creates a dome-like shape into the usually flat base, it’s sometimes marked by a lighter discoloration too.  The other, the wetter thermal, will produce a ‘daggy’ as we like to call it in Australia.  A ‘daggy’ is effectively a tendril or some condensation that appears below the base.  Both indicate the best options, we should aim to put our gliders here, then turn to stay within or around them!

Cloud Shadows

One thing we must also consider when flying on big cumulus days is the cloud shadow.  This in itself can cut off thermal trigger points, cooling the general area, which in the end will hinder the thermal activity.  So by staying higher, you should be able to thermal in stronger cores.

Wind

The wind is always an important factor to consider too.  What I’ve found in my experience is that thermals seem to elongate or leave long tails downwind of the core.  So in order to feel your way into a core, line the cloud up with consideration of the wind, start from downwind and work your way upwind via all your pre-determined points of likely cores, so as to maximise your time under the cloud and finding the one – naturally this works when working in the opposite direction too...

Finally, when the wind gets stronger with altitude, you’ll often find the climb on the upwind edge of the cloud, even in front of it too, just to keep you on your toes.  As I said at the start of this article, the trick is to figure out the pattern of the day asap, that way you really can bounce along and maximise the huge amount of fun that large CU days bring

Safe circles as always!

Banner photo by John Absolon, 'Got that shot' Images

Adam Woolley  Adam Woolley was born into the gliding world, being the 3rd generation in his family. Going solo at 15, his thirst for efficiency in soaring flight & quest for a world championship title to his name has never wavered. One big passion is sharing his experiences & joy with other glider pilots all around the world. Adam is an airline pilot in Japan on the B767 & spends his off time chasing summer around the globe. He has now won 7 national Championships & represented Australia at 5 WGC's & 1 EGC.