Thermal is a form of updraft that occurs when solar radiation heats the earth's surface and, as a result, the air close to the ground. During the day, this convection transports air heated by the sun to greater heights and at the same time brings cooler air to the ground from a height of several hundred meters. Mountaineers often feel this updraft during the afternoon descent as a warm headwind from the valley, motorless pilots such as glider pilots , hang- gliders and paragliders appreciate it as a so-called " beard " to gain altitude. Thermal is u. a. also responsible for the development of valley winds and dust devils .
In order to develop, thermals require sufficient solar radiation and suitable soil conditions. Put simply, a growing layer of air on the ground warms up. As a result of inhomogeneities or slight disturbances, a bulge forms at one point in the warm air layer, where the warm air begins to collect and to push upwards. When a sufficient volume of warm air has accumulated, the package begins to rise. There are two possibilities of thermals that can now occur:
- Packet-like thermals: here the air parcel detaches from the ground (the so-called detachment occurs, the observer standing on the ground recognizes this by the rapidly refreshing wind, which is caused by the (cold) air flowing in and does not come from the main wind direction). A glider pilot notices strong fluctuations in the rate of climb, in which altitude gains are repeatedly interrupted by periods of descent, although one's own location is not changed.
- Thermal hose : here there is a continuous supply of heated air on the ground, so that the air escaping upwards constantly flows in near the ground, is heated quickly enough and in sufficient quantities and escapes upwards. The climb is almost constant.
Since the air rises slightly , especially in an unstably stratified atmosphere , and initially cools down via the dry adiabatic temperature gradient , it can eventually reach the level of condensation and the formation of clouds . Cumulus clouds are a visible sign of thermals. If the air is too dry so that no cloud formation occurs, one speaks of blue thermal - the sky remains cloudless and blue. It can only be recognized by pollen, dust and circling birds or gliders . In contrast to this, cumulonimbus clouds and thunderstorms can form from cumulus clouds in the course of the day if the thermal strength is appropriate .
Often the resulting thermal “flows” along a slope opposite to the fall line up to a trailing edge - this can be a kink in the terrain or a change in the nature of the ground. There, the warm air pack separates from the floor and rises like a large soap bubble . In the lowlands, light to moderate wind helps them detach from the ground at the edges of terrain or forest edges and so that they can rise.
The thermal updraft ends when no more warm air flows in from the floor. Depending on the amount of sunshine, it can take some time until there is enough warm air again and it can rise again. With these recurring updrafts in the same place, one speaks of pulsating thermals.
Barriers such as inversion or the tropopause stop the rising air at altitude.
Thermal intensity factors
The intensity of the thermal depends u. a. on solar radiation , the nature of the earth's surface , humidity and the angle of radiation. A dry grain field can give off more heat than a moist meadow, a mountain slope inclined towards the sun is warmed up more than the plains. This is due to the different heat storage capacity as well as moisture and evaporation of the substrate. The ideal thermal floor should
- Reflect as little sunlight as possible (small albedo value ),
- evaporate little water and
- Dissipate little heat into the soil, but get hot to heat the air above.
If the soil conducts heat downwards (e.g. clay soil), it only heats up slightly. A poor conductor of heat, such as dry sand or a plowed field, on the other hand, heats up. Also, for example, a wet soil, if it is colder than the air, can store more heat than a dry one, because the water can absorb heat from the air in addition to the matter of the soil. If a wet soil is colder than the air, a large part of the solar energy that hits the soil is converted into so-called evaporation cold, correspondingly the air close to the ground is cooled by the soil compared to the directly higher air layers. Plants can reduce thermals depending on their species, growth status and density. The forest plays a special case: during the day it reduces the thermal rises through evaporation, but in the evening the canopy is warmer than the surrounding area and gives off weak thermal rises. In contrast, clearings and forest edges are good thermal sources and trailing edges. Does the soil store a lot of heat, such as B. forests or cities, it can release them back into the air at different times and leads to thermals in the late afternoon to evening.
On the other hand, the temperature gradient (vertical temperature decrease) of the ambient air plays an important role for the intensity of a thermal updraft , which can be between 0.65 ° C and 1.35 ° C per 100 m altitude. Since air cools down constantly at 1 ° C per 100 m when it rises until it reaches the condensation level, air rising below 1 ° C would soon become colder than the surrounding air (stable stratification). With a gradient of 1 ° C (indifferent stratification), the temperature difference remains the same with increasing altitude and leads to moderate to good thermals with constant climb rates. If the gradient is above 1 ° C (unstable stratification), the temperature difference increases with altitude - as does the climb rates and thermal strength.
Accordingly, the thermals can increase significantly with cold air advection . It occurs when cooler air masses in higher air layers are brought about from another place, e.g. B. after the passage of a cold front , the so-called backside weather . As a result, even a slight warming of the floor is sufficient to give the heated air a temperature advantage over the ambient air and to cause it to detach and quickly ascend. Such weather conditions are often used by thermal pilots for extended cross-country flights.
Other effects support the upward trend:
When clouds form, additional heat of condensation is released, which can lead to a further temperature advantage over the surroundings and thus to a further rise in the air parcels - the thermal increases.
In the edge zones of the updrafts, dry and cooler air is mixed in through entrainment. Particularly in the case of moisture convection , i.e. thermal clouds, the thermal effects can increase even further due to the evaporation cold , as a thin layer of cold air surrounds the cloud.
In aviation , the strength of the thermal is measured as the speed of the rising air. This is between 0.1 and 10 meters / second, and considerably more under cumulonimbus clouds . The variometer is used as a measuring instrument in an aircraft .
The spatial distribution of thermals in the atmosphere can also be measured as follows:
- Measurement of the wind field (by Doppler shift) using various radar techniques , RADAR , LIDAR , SODAR
- Indirect measurement of the temperature distribution of an air volume by measuring the thermal radiation (infrared). The thermal can be inferred from the temperature distribution
Thermal updrafts u. U. ions move, which changes the electric field of the atmosphere. The measurement of the electric field, respectively. its gradients within the volume of air (out of an aircraft) allow conclusions to be drawn about the presence of thermals.
In motorless flying, such as gliding , hang-gliding and paragliding , thermals are used to gain altitude (1,000 to 3,000 meters in the flat, even higher in the mountains). The upper, usable limit of the thermal is the cloud base . Depending on national law, glider pilots with a cloud flying license may also continue to climb within a cloud, although approval by air traffic control may be necessary. Flying in a cloud, however, involves risks and is rarely practiced. For motorized aviation, on the other hand, thermals are more of a nuisance because they can cause unpleasant turbulence . It can even be dangerous for hot-air balloons , as thermals cause the balloon to sink due to the lower temperature difference (balloon envelope to the environment).
Thermal power plants try to convert the energy contained in the thermal into electrical energy.
Strength of the thermal
For the thermal strength , i.e. the speed of the rising air, differences in density between thermal and ambient air play a decisive role. The difference in density depends largely on the relative humidity , i.e. the dew point difference . The thermal is more humid and therefore lighter than the ambient air. On the other hand, differences in temperature between thermal and ambient air are negligible for the differences in density: At around 200 m above ground, the temperature difference is on average less than 0.3 ° C, at an altitude of 600 m it is often only 0.15 ° C.
The thermal formula for calculating the lift speed is:
The individual symbols stand for the following quantities:
|Updraft speed in m / s,|
|Temperature of the ambient air in ° C,|
|Dew point temperature of the ambient air in ° C,|
|Dew point temperature of the thermal bubble in ° C.|
A vertical profile , as measured or calculated by weather services for different places in the world, shows the temperature and dew point temperature of the ambient air at different altitudes. This also contains the line of the constant saturation mixture ratio of the soil dew point, which corresponds to the dew point temperature of the thermal. This means that all relevant parameters for calculating the thermal strength at the respective altitude can be read from the vertical profile.
- Winch and wind systems
- Lee waves can generate updrafts on the leeward slopes of a mountain range.
- Reverse thermal creates updrafts when thermal activity subsides and the valley wind turns into mountain wind .
- Zero slide
- "Where do you go to the thermal?" - Presentation by Prof. Alfred Ultsch.
- Thermal map of Germany by D. Müller, Christoph Kottmeier, Manfred Kreipl and Bernd Frettlöh.
- Thermal checklist - interactive website for classifying thermal quality.
- Derivation of the thermal formula - physical background for calculating the thermal strength.