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Before we look at mountain thermals we will have a quick recap on how thermals work before we look at what is different with mountain thermals.
The atmosphere, particularly the lower atmosphere (troposphere) is largely transparent to radiation from the sun. The sun heats the ground and the ground warms the atmosphere from the bottom. Warm air is less dense than cool air, so it rises and we call this upflow a thermal. As the air rises the pressure reduces so the air expands and as it expands it cools. If the air it is rising into is cooler, the air will keep rising. When the air surrounding it is the same density, it will stop rising.
If the warm air rises to the dewpoint, cloud will form as the moisture condenses out.
When a thermal develops on a slope, the thermal will tend to rise up the slope and release from the top of the slope.
As the thermal starts rising, it needs inflow to replace the rising air. On a slope, this inflow will come from beside or below, not from above. The rising air follows the slope, continuing to gain heat from the warm slope. This reduces the cooling as the air expands with altitude. So a mountain thermal releases from a more predictable location and has more energy than a flat-land thermal.
As the mountain thermal tends to be more concentrated than a flatland thermal, it tends to be smaller as well as more powerful than a thermal that has had a chance to get established fully over flat ground. Often you are working these thermals a few hundred feet above the mountain top. The thermal hasn't had time to get well organised so it is often rough and unstructured, just as a thermal would be over the flat if you were working it from a few hundred feet above ground. This often necessitates an aggressive technique when working these thermals.
Mountains are often found in ranges, long ridges that will have thermals popping off along the top of the ridgeline. On a nice day it is possible to cruise along above the ridge crest in an almost continuous line of lift. The lift is sometimes very narrow, and may be difficult to circle in. Often it is best to fly along the lift slowly, maintaining or gently climbing, until you find a hotspot that is worth circling in.
The strength of mountain thermals depends on a number of factors.
The stability of the atmosphere affect thermal production in mountain thermals just as flatland thermals. Due to the mountain ranges seperating airmasses, and valley winds, the atmosphere is often more complicated around the mountains. We are on an island and get a lot of cool marine air coming inland. When the cool marine air gets into a valley it can shut down thermal production from the bottom of the valley, leaving only the upper slopes to drive the thermals.
The fetch is the amount and type of surface area available to catch the energy from the sun and transfer it to the air.
Bare dark rocks are more efficient collectors of insolation and more efficient at transfering the energy to the atmosphere than a heavily vegetated surface. Exposed river gravel is more efficient than grass and scrub river flats, and the river itself is the less efficient. Probably the worst is a lake.
A slope facing the sun will collect more insolation than one at an angle. We're nearly 45° south, so a steep slope facing north will get more sun than south face. An easterly face will see sun in the morning, a westerly face will see it in the afternoon.
The height of the mountain and the size of the area at the foot of the slopes will affect the fetch.
A wind blowing onto a slope can trigger thermals even if the slope would otherwise not have enough energy for thermal generation. If the local airmass is unstable the orographic lifting will trigger a thermal. This happens even when the wind is quite gentle, not strong enough for soarable ridge lift. If the wind is blowing onto the sunny aspect of the slope this reinforces the likelyhood of a thermal. If the wind is blowing onto the shady aspect, it is less likely to trigger a thermal, but still may.
The strength of the sun depends on the time of day, the time of year, and the amount of insolation that is reflected away by cloud. The lower atmosphere is largely transparent to the radiation from the sun but it still blocks some. Dust or smoke in the atmosphere blocks more.
Heating is better in the summer because the sun is more directly overhead and the days are longer. Each day starts cool, warms until midday (solar midday, when the sun is at its highest) then cools as the sun sets.
Mountain valleys exaggerate this by shading the valley in the morning and evening resulting in a 'shorter' day in the deep valleys.
Briefing prepared by Phil Plane.
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