We’re coming to the time when thermals begin to reappear, though weak and short-lived at first, same as very early on a summer day. And it should be no surprise that they usually rise first above the hills. Often, we see fine local soaring in thermals that wouldn’t even exist without sloping ground of one kind or another.
Slopes can be significant factors for thermal production in lots of ways, even when they’re not facing the sun. Wind-facing slopes often act as ‘triggers’ for incipient thermals that have yet to leave the ground, and tend also to detain those that have already done so, allowing them more time to gather strength. As a result, large slopes – whether high or wide – tend to organize convection into predictable patterns. This can be useful even if the prediction is sink, for recognizing and avoiding sink sources is every bit as important as finding lift!
The endless variety of landforms generates many different kinds of slope-related thermals. In the most general terms let’s distinguish between those spawned by bowls, isolated peaks, or continuous ridges.
Bowl thermals are often large and consistent, and though their lift at any given moment may not be as strong as on some surrounding hills, it might be wider and more reliable. If a bowl is large enough it may gather together nearly continuous clusters of thermals that are easier to locate and work than individual ones. By carefully using only the best lift in such a cluster, it may be possible to climb higher than elsewhere nearby. (In flat country a quarry or other large hole in the ground with steep sides facing the sun also can produce thermals, regardless of wind direction.)
Peak thermals are easy to find and they can be extremely powerful. A large, isolated cone like Mt. Lewis here at Crystal can pull strong thermals together from all sides to meet at the summit where, either right above the peak or just downwind of it, the energy of all those thermals combine into one certifiable boomer. However, if they rise only a short distance into stronger winds aloft and drift downwind of the crest, peak thermals tend to be short-lived. When those break off they might leave you in the strongest of sink, very near and downwind of unlandable high terrain.
Long, continuous ridges may offer a beneficial compromise between the types of thermal sources mentioned above. Their thermals may be smaller and short-lived, but many more might exist, often occurring with convenient regularity. Also, they’re already aligned such that each one leads on to another with relatively little sink between. In conditions where the flat valley is entirely unsoarable, a uniform slope of only a few hundred feet can provide thermals with amazing regularity.
Topography often features also in countless hybrid conditions where it’s possible to transition from one kind of lift to another, such as ridge lift into thermal into rotor into wave… These myriad combinations of effects are much of what makes soaring such a fascinating sport!