Recently I was flying a Piper Arrow at 2500 feet in turbulent air. I was carefully maintaining my altitude with pitch and trim changes. It was a bumpy ride but it was nice watching the puffy cumulus clouds pass by overhead.
Then I began to notice my indicated airspeed was changing despite my unchanged engine settings. First, my speed would be up about 10-15 knots for a minute or so, then it would drop back down for another minute. The next minute it was back up.
Why was this happening?
I thought about it for a few minutes. The engine RPM and manifold pressure were constant, and my altitude was constant.
This could only be caused by updrafts and downdrafts! Yes, the same turbulent air that was creating those clouds was pushing the plane up every time I passed under one.
You can imagine my plane flying in that mass of rising air, and me correcting almost subconsciously to maintain altitude. As the air mass rises, I need to pitch down to stay at the same altitude. This is as good as descending as far as airspeed is concerned.
These updrafts are what glider pilots use to stay airborne and even climb without an engine. A single-engine plane is far too heavy to stay aloft on the updrafts alone, but they do offer us free airspeed which can save on rental or fuel costs.
But how do we take advantage of them?
First, there is terrain: When you have any kind of terrain plan to stay on the upwind side as much as possible. As the wind reaches rising terrain it will rise to go over it and this rising air provides a great updraft that you might be able to take advantage of by altering your course just a little.
Second is cloud streets: Puffy cumulus clouds are formed in rising air so alter course slightly if it will put you under some cumulus clouds. It may be bumpy but if you are between clouds the air will generally be descending to replace the rising air under the cloud. Glider pilots look for cloud streets, which are long rows of clouds that can provide an updraft over a very long distance.
Just think like a glider pilot and consider how the air around you is moving. Tailwinds get all the attention when looking for bonus performance from the environment, but a nice updraft can help any aircraft to go just a little bit faster for the same power.