Looking at the Sky: Cumulus Clouds - A Glider Pilot's Perspective
If you are flying along and you see these beautiful white puffy clouds ahead, you may start to regret talking your spouse into this flight for lunch. I am sure most can associate these cumulus clouds with a not so pleasant flight. Some of the times you are lucky enough that you could climb above and potentially cruise in the smoother air above them. This picture was taken on a day that normally a glider pilot only dreams about. When you look at the clouds, there are few things to look at. Look at the base of the clouds you can see a darker base. That darker section is where the thermal is rising up into the cloud. Another good section will be the upwind side of the cloud. Now it doesn’t matter how long you stare at that cloud you are not going to see it move because you are in the same airmass, moving with it. However the clouds cast great shadows which give away two things, first it shows you which way they are drifting showing winds aloft showing the upwind side. The second thing the cloud shadow is useful for is how far away the clouds are. Ok so you take into account sun angle, but it is by far easier to do than looking at the cloud and guessing if you can glide to it or not. Another consideration is the sunny side of the cloud, that generally gets better heating and produces the stronger lift closer to the cloud (of course always maintaining VFR distance from clouds). The bases will also form a shelf or concave base, you want to be under that higher shelf or where the cloud is concave. Where it is lower it is generally falling.
With all this said. Those are the rules of thumb I use for trying to determine where the strongest thermal is going to be. Those might be areas to avoid if you are looking for that smoother ride, however it is always nice to be able to throttle back and climb in the power plane. Following underneath the clouds I have managed to do glides over 200miles and never having to turn in a thermal to climb, just connecting each of the thermals, slowing down and climbing then diving and racing off to the next one. My longest flights have been in thermals with flights over 600miles and just over 8hours. I still hold a record at a camp I flew at in Australia hosted by 3-time World Champion George Lee, it was 350km at 150kph with a thermalling percentage of 9%. Flying gliders is all about using the energy of the air.