As a result, the air temperature rises with altitude, which influences the stratification stability of the troposphere and especially all convective processes. The area where this inversion occurs is called the inversion layer.
The inversion shields the lower air layer from the upper one, which is referred to as stable stratification . This is due to the higher density of the colder air layer, which largely suppresses turbulent mixing with the warmer air layer above. The cold air bubbles caused by inversions or shielded by them are responsible for cold records worldwide. As a result of the shielding, air pollutants and other additions can accumulate in the cooler, lower layer, especially in the case of inversions over urban areas . A particularly strong manifestation of such air pollution , which occurs especially over urban areas, is smog . Above the inversion layer, on the other hand, the distance view is significantly increased, whereby the view of a large area of haze near the ground is usually revealed.
Inversion weather conditions also change the propagation conditions for radio waves , as these are reflected back into the denser medium, here the cold soil air, at the density transition ( total reflection ). Radio amateurs use this effect to increase the range of their signals, and VHF broadcasting leads to overreaching . On the same basis, an inversion weather situation favors the propagation of sound close to the ground because it is refracted towards the ground and can therefore spread over great distances. The speed of sound is greater in warm air than in cold air.
Species and their formation
A very stable inversion is formed by the tropopause and is explained by the slowly increasing ozone concentration at an altitude of 10 to 15 kilometers . The ozone absorbs the very short-wave UV-B part of the solar radiation and thus leads to a temperature increase contrary to the general trend of temperature decrease.
Radiation inversion / soil inversion
Usually the air temperature decreases with increasing altitude. Flue gases from heating systems and car exhausts lead to increased dust concentrations in the air , especially in winter . This dust filters sunlight and is heated at the same time. If there is still no wind for days (plumes of smoke rise vertically), a warm, stable layer of air forms above the (large) chimneys, which remains above the cold ground due to heat radiation and thus above the cold air layers ("inversion"). The heat radiation of the ground further heats the dusty warm air layer, at the same time the weaker sunlight in winter, which is additionally filtered, can no longer sufficiently warm the ground and indirectly the cold air masses close to the ground. As a result, a “ cold air lake ” forms below and a “cloud of vapor” above, and the air masses mix only slowly. Car exhaust fumes then accumulate near the ground and, together with ground fog, result in "thick air" ( smog ). But because car engines also give off a lot of waste heat and traffic creates air turbulence, car traffic can contribute to the fact that layers of air close to the ground heat up more quickly.
Such inversion weather conditions can also occur briefly in summer, but then the sun is stronger; the warming of the soil and the resulting thermal dissolve the temperature differences and the inversion faster in summer.
A radiation inversion usually only affects the immediate proximity to the ground and is therefore also referred to as a ground inversion . It is caused by the radiation and thus cooling of the earth's surface and occurs especially in autumn and winter high pressure weather conditions, as the temperature is then particularly low and the lack of cloud cover favors the nightly cooling.
Around the time of the daily maximum air temperature, i.e. between noon and three o'clock, the surface of the earth is very warm, which also heats up the air above it. Because of the adiabatic temperature gradient near the ground and the consequent unstable stratification of the atmosphere, the air layers near the ground are mixed through convective processes. However, as the time of day increases, the amount of solar radiation and thus the warming of the earth's surface decrease. Since the radiation balance finally becomes negative, the surface of the earth and with it the layers of air near the ground begin to cool down. This ultimately results in an initially weak inversion in the evening hours, which practically prevents the vertical exchange of air. The warmer layers of air generated during the day at higher altitudes cannot prevent the ground from cooling down, which continues to progress. The usually weaker wind also contributes to this and increases the tendency to cool. An inversion several hundred meters thick can then have developed into the early hours of the morning. It is then broken down again in the morning with increasing solar radiation and is completely gone again by noon at the latest. The fumigation layer, which inevitably occurs when the inversion is broken down, with an unstable stratification on the ground and an inversion above it, lasts longer, the thicker the inversion layer. This condition, also known as lifted soil inversions, usually only exists for short periods of time, so that no significant accumulation of pollutants occurs.
The weaker the wind and the better the radiation, the stronger the resulting radiation inversion will be. Certain valley and basin locations therefore have a particularly high tendency to inversion. Such an inversion forms practically every night, especially when there is little cloudiness. If the temperatures are below the freezing point of the water, frost will occur . Only a strong wind can prevent or at least weaken this and is therefore an important feature, especially for farmers, on cloudy autumn and especially spring nights.
If a radiation fog is also created, the increased albedo can also lead to a longer-lasting radiation inversion, which then usually lasts for several days. This also explains a somewhat rarer case of radiation inversion at the top of haze layers. Since the albedo is very high here and the water droplets radiate strongly, the air temperature can drop so far that an inversion also occurs. These height inversions caused by radiation are closely linked to the stability of the haze or fog layer and consequently disappear with it. As a rule, however, such inversions initially sink to ground level, since the earth's surface is no longer heated by solar radiation and cools down accordingly.
Radiation inversion positions favor the formation of industrial snow .
An example of soil inversion is the phenomenon of the formation of a layer of clouds between the valley floor and the mountain peaks, which is documented in the Upper and Ostallgäu and Kleinwalsertal with the expression Obheiter (= "above bright "), whereby it is cool and cloudy under the clouds, but much warmer above the clouds and is sunny. This weather situation is particularly common in autumn and is very popular with mountain tourists because of the wide mountain views associated with it.
If air layers of great thickness are closed and offset in their height, the effect of the different path lengths for the individual air parcels and thus their different cooling according to the respective temperature gradient becomes apparent . A sinking , shrinking or subsidence inversion occurs , which is also known as height inversion due to its great height compared to other inversion layers .
When the air pressure is lowered, the air pressure rises and since the air is compressible, the layer thickness consequently decreases, which is equivalent to increasing the air density . Each air parcel within this air layer is lowered independently and therefore experiences a specific increase in temperature. The greater the difference in altitude that the air parcel covers, the greater this increase. But since an air parcel at the upper edge of the air layer under consideration travels a longer distance than an air parcel at the bottom of the layer, its temperature also increases more rapidly. This changes the temperature gradient within the then deeper layer compared to the formerly higher layer, which is intended to illustrate an example.
If you consider a dry adiabatic stratified atmosphere with a temperature of ten degrees Celsius on the ground, there is a temperature decrease as shown in the right figure by the black line. It shows a layer of air that has been lowered from a height of six to eight kilometers to a height of one to two kilometers. The layer thickness and depression are not realistic and halving the thickness does not correspond to the actual reduction in air pressure, so it was determined arbitrarily for the sake of simplicity. Four points were particularly emphasized in the diagram, each forming the upper and lower edge of the air layer. Before lowering, the air layer had a temperature of -75 ° C (A) on its upper side and -70 ° C (B) on the lower side. This corresponds to an extraordinarily sub-adiabatic temperature gradient of only two and a half degrees Celsius per kilometer, which is, however, at least the tendency, a prerequisite for the formation of a sinking inversion. This is followed by the lowering of the air layer, whereby the changes from A to C and from B to D should be considered. The lowered air layer then has a temperature of −20 ° C (D) on its bottom and −15 ° C (C) on its top. The temperature rises here by five degrees Celsius per kilometer.
Such a temperature reversal only occurs in pronounced high pressure weather conditions, especially in late autumn and winter. But even if the lowering should not be sufficient to generate an inversion, it at least weakens the temperature gradient and thus contributes to a further stabilization of the atmosphere. This often leads to several sink inversions lying on top of one another, which cause a rather complex stratification of the atmosphere. An important and comparatively stable special case of sinking inversion is the Passatin version . In the opposite case of an increase in the air layer, an inversion, regardless of its origin, can be reduced, but at least the gradient increases and the inversion is weakened.
Descent inversions become visible through their effect as a cloud barrier, because the vertical spread of a cloud stops abruptly at its bottom. The air humidity is also greatest there, while it has a minimum due to the adiabatic heating on the upper side of the inversion layer. It is also particularly noticeable that if the height of the inversion layer is sufficiently low, it can be observed that it is often much warmer in the mountains than in the valleys. For example, an increase in altitude of one kilometer can often result in a temperature increase of 15 ° C.
A slip or turbulence inversion is caused by advection , i.e. the approach of air masses in the horizontal.
A strong wind causes a thorough mixing of the initially subadiabatic stratified atmosphere. This instability with strong vertical movement of the air leads to an increasing approach of the temperature gradient to an adiabatic stratification within the mixing zone. However, the temperature gradient above this zone has not changed and is still sub-adiabatic, which causes an inversion relative to the mixing zone. The phenomenon usually occurs when, when a warm front is approaching , only the upper layers of air initially register warm air, while it has not yet reached the ground. This is especially the case in high pressure areas above the sea.
In contrast to a sinking inversion, the air humidity is highest here on the upper side of the inversion layer, since the air masses brought in usually contain more moisture than the cold air previously stored there and the convection phenomena caused a constant transport of moisture upwards. Below the inversion, stratus or stratocumulus clouds often form in the case of strong turbulence and cumulus clouds in the case of weak turbulence. In the case of foehn , too, slip inversions often occur, combined with the foehn clouds that are typical for this .
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