a. Irradiance Maps
Irradiance maps, however, show an instant situation, whereas heating up the surface and the air causing the thermals takes time. This becomes evident during the afternoon hours. There temperature maps expressing how much heat has been accumulated at a given place and time seemd to be a better indicator for the occurrence of thermals.
For practical reasons TherMap computes the heat accumulation
at the surface using a relatively simple empirical smoothing algorithm,
to approximate the evolution of the surface temperature at the usual
flight altitudes. To simulate the effect of the actual orography Thermap
considers the cooling effect of calculated forest areas and seasonal
vegetation, and makes an approximation for the effect of snow and permafrost
surfaces. With these adjustments the resulting temperature maps seemed
to be more plausible predictors of the location of thermals during the
In his publications the German glider pioneer Jochen von Kalckreuth mentioned that thermals on slopes exceeding about 25 to 30 degrees tend to climb along the slope until they reach a smaller slope or an edge. TherMap also considers the snow and permafrost limits, where thermals climbing along the slope meet the cold air coming from above, an additional reason causing thermals to take off.
The idea of the thermal pressure is based on the insight that any air "bubble" heated above a slope develops a lift force which can be decomposed into two components, namely one along the line of steepest ascent of the slope, and one perpendicaular to the slope. The first one creates a static pressure along the line of steepest ascent. This pressure is basically distributed proportional to the slope angle, but diminishing slowly with the distance from the original bubble, until the residual pressure falls below a critical value or until a takeoff point is reached. The initial lift of the bubbles is derived from the temperature maps, from which the thermal pressure maps can then be derived in an additional computation run based on the principles just described.
The resulting thermal pressure maps show the local
potential for thermalsare indded more precisely. The following picture
illustrates that thermal pressure maps even provide detailed results
for southward facing slopes on hot summer afternoons.
Despite the precision of the thermal pressure maps, one of their limitations is that the part of the areas shaded in green, which have a lower thermal potential, is somewhat redundant. This limits their readability which becomes particularly obvious when generating maps for more southern regions such as the US Sierra Nevada. Since purely topographic maps with a sun elevation of about 22 degrees are easy to read, it turned out to be better to replace part of the less interesting green areas of the thermal pressure maps by the corresponding topographic backdrop view based on the same sun azimuth as the one used for the thermal pressure, but with a constant sun elevation of 22 degrees. Using the same azimuth ensures consistent visual information and the relatively low sun elevation creates good shading contrasts for the topographic backdrops. This presentation mode has therefore been adopted in TherMap 2.0.
Slope angles and mountain ridges have s significant influence on the thermal activities. TherMap therefore also shows slope maps as a complementary view. Their colouring is simply determined by the slope angle. The resulting picture differs from the usual view and can therefore reveal aspects which may otherways remain unnoticed. These maps also remind us of maps drawn by ancient cartographers.
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