definition of the term thermals
Thermals are the updrafts caus by solar radiation, which warm the earth’s surface and thus also the air above it. If the surrounding air is somewhat cooler, the heat air rises as a “bubble” and continues to rise as the temperature decreases with altitude. As it rises, the “air bubble” cools down because it expands due to the decreasing air pressure. This process ends when the air bubble reaches the ambient temperature. If the humidity of the air is high enough, a cumulus cloud is form.
Schematic representation of the detachment and rise
Schematic representation of the detachment and rise of a heat air parcel. (Source: Michael FH Krutina)
This rising air is us primarily by gliders, hang gliders and paragliders to gain altitude without an engine and, in good conditions, to fly longer distances. The thermals are also responsible for the formation of
valley winds
or conversely, during the evening cooling for mountain winds.
Conditions for good thermals
The best thermal conditions here are usually from April to June, because then there are often colder air masses at higher altitudes, which in turn allows for a high level of instability. In midsummer, the warming increasingly spreads to higher layers and the instability tends to decrease until inversion layers can gradually form in the autumn.
High instability means a strong decrease in temperature with altitude. In the free atmosphere, the decrease can be as much as 1°C/100m in dry air, and this value can even be exce near the ground. If the air is very dry, we also speak of “blue thermals” (no cloud formation). If clouds form (condensation), this vertical temperature gradient is still around 0.65°C/100m, as heat is releas senegal phone number library through the condensation process. We now know that instability is higher in dry air than in humid air.
What about the rate of rise of an air parcel? The rate of rise depends mainly on the dew point difference and the mixing ratio at a certain altitude, assuming dry adiabatic stratification (1°C/100m).
Calculation of the vertical spe
Calculation of the vertical spe (thermal strength) at a certain altitude bas on a vertical profile. (Source: Oliver Prelli)
Usable thermals and cross-country flight
Thermal conditions depend on other factors in addition to solar radiation and vertical stratification. The general weather situation can influence the quality of thermals, particularly due to the current. For example, a leave a website exit intent technology tracks strong bise situation leads to “broken” thermals, which means that the thermal bubbles are divid into smaller bubbles awb directory depending on the wind strength. This makes it more difficult because you cannot make optimal use of these narrow updrafts in flight. Or if thicker clouds form at higher altitudes, the thermals can quickly be dampen or come to a complete standstill.
Thermals also depend on the topography. Thermals are generally stronger in the Alps than in the lowlands. This is why people fly more often in the Alps than in the lowlands. On the one hand, the air warms up more quickly due to the lower volume in the Alpine valleys, and on the other hand, air bubbles in the mountains rise from a higher point and thus reach greater heights before condensing than over the lowlands.
