Thermal

Thermals is a form of solar chimney, which arises because solar radiation, the Earth's surface and as a result the air is heated at the bottom. The thermals, scientifically called convection, transported as "Air conditioning" of the Earth's atmosphere during the day by the sun heated air to greater heights and simultaneously brings cooler air from several hundred meters to the ground. Climbers feel this updraft often during the afternoon descent as warm headwind from the valley, motorless flyer as gliders, hang gliders and paragliders appreciate him as a so-called " beard" height recovery. Thermals are, inter alia, responsible for the formation of Talwinden, in extreme form, they are as dust devils ( Dust Devils ) are known.

Formation

Thermals needed to the creation of sufficient sunlight and a suitable soil. In simple terms, a growing layer of air heated at the bottom. Inhomogeneities or slight interference forms at one point the hot air layer a bump, on which the warm air begins to gather and push upwards. If sufficient quantity of hot air is accumulated, the packet begins to ascend. There are two ways of thermals, which can arrive now:

Packet -like thermals: here, the air parcel lifts off the ground ( the so-called separation occurs, the floor standing observer recognizes this by quickly freshening wind caused air through the inflowing (cold ) and does not come from the prevailing wind direction ). A glider pilot noticed strong fluctuations in the rate of climb, are constantly interrupted at the level of profits Sinkperioden, although the own position is not changed.
Thermals: here prevails a continuous supply of heated air at the bottom so that the permanently escaping upward air flows near the ground, is fast enough and heated in sufficient quantity and escapes upward. The climbing is almost constant.

As the air rises easily especially in a unstable stratified atmosphere and cools initially via the trockenadiabatischen temperature gradients, it can eventually reach the condensation level and come to the formation of clouds. Cumulus clouds are thus a visible indication of thermals. If the air is too dry, so no cloud formation occurs, it is called blue thermals - the sky is cloudless and blue. It can be recognized most by mitaufsteigende pollen, dust and circling birds or gliders. In contrast, can form with appropriate thermal strength of cumulus clouds during the day cumulonimbus clouds and thunderstorms.

Often "flows" the resulting thermal along a slope opposite to the case line up to a spoiler - this may be a kink in the road or a change in the soil. There, the warm air package separates the ground and climbs like a big soap bubble. In the lowlands helps it easier to moderate wind, that it may separate and float to terrain edges or edges of woods from the ground.

The thermal updraft ends when the ground nachströmt no more hot air. Depending on the sunlight it may take some time to again enough hot air exists and its value can rise again. These recurring winding at the same position is referred to as pulsating thermal.

Barrier layers such as the tropopause inversion or stop the rising air at altitude.

Factors of the thermal intensity

The intensity of thermals depends, inter alia, of the solar radiation, the nature of the surface, the humidity and the irradiation angle. A dry cereal field can dissipate more heat than a wet meadow, a tilted towards the sun mountain flank is more heated than the lowlands. This is due to the different thermal storage capacity and moisture and evaporation of the subsoil. The perfect thermic soil should

As little sunlight back radiation (small albedo ),
Evaporate a little water and
Little heat into the soil derived, but are hot to heat the air above it.

Directs the ground heat down further (eg, clay ), he warmed himself very little. A poor conductor of heat, such as dry sand or a plowed field, on the other hand heats up. A wet soil conducts heat faster into the depths. Much of the sun's energy is depleted by evaporation also, correspondingly weaker ground-level air is heated. Plants may reduce depending on their type, growth state and the thermal density. The forest plays a special case: the day he reduced by the thermal evaporation, towards evening, the crown area but warmer than the environment and donates a weak thermals. In contrast, forest clearings and forest edges good thermal sources and tear edges. Saves the floor a lot of heat, such as Forests or cities, he can staggered back to the air and deliver results in thermals in the late afternoon to evening.

On the other plays for the intensity of a thermal updraft, the temperature gradient ( vertical temperature decrease) in ambient air an essential role, which can be between 0.65 ° C and 1.35 ° C per 100 m altitude. As air cools as it rises constant at 1 ° C per 100 m to reach the condensation level, would at a gradient of less than 1 ° C rising air soon be colder than the surrounding air ( stable stratification ). In a gradient of 1 ° C ( indifferent stratification ) remains equal to the temperature difference with increasing height and leads to mediocre to good thermals with constant ascent rate. If the gradient is about 1 ° C ( unstable stratification ), the temperature difference increases with altitude according to - as well as the climb rate and thermal strength.

Accordingly, it can significantly enhance the thermals for cold advection. It occurs when cooler air masses are brought into higher layers of the atmosphere from any other source such as after the passage of a cold front, the so-called back weather. This is already sufficient low heating of the soil to give a temperature of the heated air ahead of the ambient air and to bring about a separation and rapid ascent. Such weather conditions are often used by pilots thermals for long -haul flights.

Other effects support the rise:

With the emergence of cloud condensation also releases heat, which can lead to a further temperature projection for environment and thus to a further upgrade of the air parcels - the thermals strengthened.

In the marginal zones of the updrafts dry and cooler air is mixed by entrainment. Especially with Feuchtekonvektion, ie thermal clouds, may enhance the thermal even further by resulting evaporative cooling, since a thin cold air jacket sets to the cloud.

Measurement

In aviation, the strength of the thermals is measured as the speed of the rising air. This is between 0.1 up to 10 meters / second, with cumulonimbus clouds also much more. The measuring instrument used in an aircraft stuff the variometer used.

The spatial distribution of thermals in the atmosphere can also be measured as follows:

Measurement of the wind field ( by Doppler shift) by means of various radar techniques, RADAR, LIDAR, SODAR
Indirect measurement of the temperature distribution of air volume by measuring the thermal radiation ( infrared). From the temperature distribution can be concluded that the thermal

Thermal updrafts are moved might ions, thereby changing the electric field of the atmosphere. The measurement of the electric field, respectively. its gradient within the volume of air ( from an airplane ) to draw conclusions about the presence of thermals possible.

Use

In the motorless flying, for example, when gliding, hang-gliding and paragliding, thermalling is used to gain altitude ( 1,000 to 3,000 meters in the plains, in the mountains or higher). The upper limit of the usable thermals is the cloud base. Depending on the national legislation may continue to rise within a cloud gliding clouds with a pilot's license, which may be required to release the air traffic control. Flying in a cloud, however, is associated with risks and is rarely practiced. For the aviation engine, however, the thermal is rather disturbing, because it can cause unpleasant turbulence. For hot air balloons they can even be dangerous, since thermals due to the smaller temperature difference results ( balloon envelope to ambient) to sink the balloon.

Thermal power plants try the energy contained in the thermal to electrical energy converting.

Thermal

Formation

Factors of the thermal intensity

Measurement

Use

Related Topics