Atmospheric instability

The stratification stability of the Earth's atmosphere describes their thermodynamic stability or instability with respect to the vertical atmospheric temperature gradient based on different equilibrium states. A distinction is made between an unstable, stable and neutral atmospheric stratification.

The stratification of the atmosphere all vertical air movements and is thus of fundamental importance for all convective processes within the Earth's atmosphere as well as the associated processes of cloud formation and air pollution. About the thermals it forms also the basis of gliding and other non-motorized sports such as hang gliding and paragliding.

• 6.1 looping
• 6.2 coning
• 6.3 Fanning
• 6.4 Lofting
• 6.5 fumigation
• 6.6 combinations

Basics

Atmospheric temperature gradient

Basically, two types is between distinguished from atmospheric temperature gradient: the dynamic gradient of an air parcel and the static gradient of the atmosphere. The measurable air temperature increases in the atmosphere often very inconsistent with height, but usually with a clear tendency. Usually there is a decrease in temperature, the air is upward so getting colder. If the air temperature instead with altitude, it is called an inversion. Against this Umgebungsgradienten has a vertically moving parcel of air its own dynamic change in temperature.

The distinction between the two cases is now because the rising air in general hardly mixes with the ambient air and also an adaptation to the ambient temperature in an idealized observation can be neglected. The actual cooling of the air packet is independent of the environment and the prevailing temperature, ie adiabatic vonstatten. However, this has the result that the decrease in temperature does not have to be identical to the height with respect to the movement of an air packet with the static condition of the earth's atmosphere. Here, the air parcel change its temperature faster, more quickly or more slowly than its surroundings. Their relative temperature at a given height can therefore be arbitrary, but this also has consequences for the movement of air parcels themselves.

Vertical motion of an air parcel

As an air parcel model assumption is generally considered that behaves according to the trockenadiabatischen temperature gradient and at a certain height has the same temperature and density as the surrounding air. The air parcel undergoes So, starting from this initial temperature in raising a cooling and in lowering a warming of 9.8 degrees Celsius per kilometer respectively. This height changes are adiabatic reversible, so it will be the parcel of air is supplied or no heat is drawn, it shall not be mixed with one of the surrounding air and there is no condensation of water vapor contained in the air. However, the latter is at a feuchtadiabatischen rise of the air packet of the case, and it usually comes to such a following a first trockenadiabatischen ascent. By the resulting cooling of the relative humidity increases, and finally reached the level of condensation at the dew point. At this height, the condensation and thus cloud formation begins. The liberated latent heat reduces the trockenadiabatischen on the feuchtadiabatischen gradient. Since all observations of atmospheric stratification condition for both cases apply equally, so only the respective reference changes, is only generally speaking in more of an adiabatic gradient.

Archimedes' principle

Of importance is now warm air has lower density than cold air. According to Archimedes ' principle, it follows from the difference between the density of the observed air package and that of the surrounding medium, a static lift or downforce. If the air parcel looked at has the same density as the surrounding area, resulting in a state of suspension. The initial elevation or reduction must therefore be enforced, but this is provided in addition, for any reason. The mathematical relationship is detected by the following equation:

Here is the index for the U L and the ambient air for the air parcel. The symbol stands for the acceleration experienced by the air parcel in the vertical direction and the acceleration of gravity. The griechesche letter stands for the density and the temperature.

As you can see, the acceleration is equal to zero when the temperatures and densities of air parcel and the surrounding area are identical. It is the greater, the farther they are apart. The sign determines the direction. A positive acceleration leads to a rise and a negative acceleration to a decrease of the air package.

Neutral stratification

The simplest case is that of the neutral or indifferent atmosphere stratification. The vertical decrease in temperature of the atmosphere is equal to that of the air package. In reality, a very well-mixed atmosphere of this state would come closest. The correspondence of mechanics is the indifferent equilibrium.

Because of the neutral stratification of the atmosphere, a rising air parcel cools just as fast as the surrounding atmosphere. The points A, B, and C, which represent each a parcel of air cools trockenadiabatisch or heated (red line), that are identical to the conditions of the illustrated as black ambient air. Updraft and downdraft of air package from the same consequently, for there is no difference in density between the air parcel and the respective air layer. So there is no net force acts, the air parcel changes its vertical position not independently and as soon as the forced uplift from B to A and from B to C reduction stops, the air parcel remains stable height.

In a neutral stratified atmosphere occurs due to the lack of convection hard to cloud formation. For the spread of substances and pollutants in particular provides a neutral stratification neither an obstacle nor does it constitute an advantage

Stable stratification

A stable atmosphere stratification denotes the state of the atmosphere in which the temperature decrease of the vertical atmosphere is less than the temperature decrease of the rising air packet, so there is a temperature gradient unteradiabatischen. If the temperature corresponding to the trockenadiabatischen gradient, then one speaks of a dry stable stratification. Accordingly, it is at a loss with the feuchtadiabatischen gradient to a moist stable stratification.

Since the environment of the air package cools slower than it yourself, decreases when the uplift from C to B, the temperature difference is always further and changes its sign at B. During the further advancement of B to A, the air pack is thus colder than the ambient air and thus has a higher density. Hear the forced uplift at A, so the air parcel decreases according to Archimedes' principle again downwards. The greater the temperature difference, the faster the air sinks - it temporarily creates a downdraft. However, the parcel of air decreases as a result not only from the simple to B, but moves due to the inertia of something beyond this point. Thus, the air packet in turn has a somewhat lower density than the surrounding air and the direction of movement is reversed, so the package will rise again. This process is repeated and the air package describes a harmonic oscillation in the vertical. Due to the air resistance, the amplitude of vibration over time decreases. Without further interference from the outside, the air parcel would therefore settle into a stable equilibrium state at B. The same consideration is valid also for a reduction from B to C, except that the air parcel will rise after the cessation of forced reduction, since its temperature is now higher than that of the environment. In a stable atmosphere vertical movements, therefore, be cushioned by a negative feedback, which impedes the mixing of the air.

This is also evident in the case of an inversion, ie an inverse gradient with a temperature increase in height increase. This is a particularly strong special case of stable stratification, because every ascent of an air parcel is blocked with a sufficient thickness of the inversion layer. This can be explained simply by the fact that warm with a slight inversion and thus air masses above colder air masses, the atmosphere is therefore in an ideal balance that requires no natural air exchange.

In the case of a feuchtadiabatischen rise in a stably stratified atmosphere, the clouds formed from the condensation level height are comparatively stable and usually very flat. Through horizontal pressure differences and the associated wind will spread out in the area, there is a typical layered clouds or cirrostratus cirrus type in larger, altostratus in the middle and Stratus at lower altitudes.

Unstable stratification

A labile or unstable atmospheric stratification refers to the state of the atmosphere in which the vertical temperature decrease is greater than the decrease in temperature of the rising air parcel, so it is a überadiabatischen temperature gradient. Decreases the temperature of the air packet according to the trockenadiabatischen gradient, one speaks of a dry unstable stratification. In feuchtadiabatischer cooling one speaks accordingly of a moist unstable stratification.

Since the temperature of the rising air package from B to A is always greater or its density in unstable atmosphere is lower than that of the environment, it undergoes an upward restoring force, the buoyancy. The air parcel is even without a forced elevation rise further and since this is still cools down more slowly than its surroundings, this climb is always accelerated. The air package to be removed thus applies a positive feedback and farther from the equilibrium point at B. The same for the reverse direction from B to C when the air parcel so falls and always colder than its surroundings, and thus has a greater density. It will then fall faster and faster until it eventually hits at high speed on the surface. Winds that one feels on the surface to be particularly strong and sudden, called gusts are usually nothing more than such accelerated and then deflected into the horizontal air packages.

A dry unstable stratification occurs with a strong local heating of the air over the broadcast, for example, near the ground. For example, while in the morning only a slight warming occurred and a überadiabatischer temperature gradient is established, it comes with increasing time of day to stronger sunlight, but can heat up very differently, the air near the ground, depending on the nature of the earth's surface. If the difference is large enough, dissolve in the episode thermal bubbles, which cause a mixing of the lower atmosphere. The result of such mixing, which can range in deserts and plateaus up to several kilometers altitude, is ultimately a neutral stratification. As an unstable stratification thus ultimately weakens the mixing itself, it is usually of short duration.

Also dynamic causes can cause an unstable stratification, especially when cold air after a cold front passage in the level moves faster than at ground level. The consequences are often severe thunderstorms in conjunction with snow, rain and hail and strong gusts, which are then referred to as the squalls.

In temperate latitudes, a labile stratification is usually limited to a certain height range and reached only in very exceptional cases a greater extent. As the lower or upper stratification usually lie before stable conditions. At high altitudes, this results in the formation of Cirrus in the case of a horizontal wind and Cirrocumuluswolken in the absence of such. In mid-levels on the other hand shows a Altocumulus clouds, at low altitudes rather Stratocumulus and Cumulus. Cumulonimbus and nimbostratus, however, are dependent on high -reaching instability and occur more frequently near the equator on.

Due to unstable stratification

A conditionally unstable atmosphere stratification refers to a situation in which a trockenadiabatisch rising air parcel results in a stable or neutral stratification, but in return feuchtadiabatisch rising air parcel would have an unstable atmosphere stratification result.

In this frequently occurring problem in the summer, the question arises whether it comes to cloud formation or not. When the water vapor of the air parcel is not condensed, and nothing else happens. At onset of condensation, however, the clouds grow rapidly approaching thunderstorms.

Stratification and air pollution

The stratification stability has a strong influence on air pollution, since the propagation of exhaust gases can be considered identical to the propagation of an air parcel. There are differentiated some specific stratification.

The basis for the descriptions of a factory that emits over a stack gases into the atmosphere. It dominates a western wind direction from left to right, so that the exhaust gases pass after a certain vertical propagation in horizontal. This initial phase slope is due to the usually high temperature of the exhaust gases. The height reached by the end is referred to as effective stack height. In the figures, the red line drawn the temperature profile of the atmosphere and the black line represents the adiabatic gradient of the air package

Looping the loop

There is a mild to medium-strong unstable stratification, the air contaminants spread looped eastward. Due to turbulence and convective processes the output can touch the ground after a short time, so that the pollution is quite large in relatively close proximity from the chimney. You can, however, it also quickly and is quite low at medium distances. The situation is typical for afternoons of sunny summer days.

Coning

There is a neutral to slightly stable stratification and the air contaminants spread out conically, with the vertical extent of the increasingly widening exhaust cone is fairly evenly. The dilution of the exhaust gases is quite low, however, the plume does not directly touch the ground. A situation in advance in cloudy weather.

Fanning

In type Fanning the stable stratification has been amplified to a massive ground inversion, which extends beyond the effective stack height. After initial slope on the effective stack height, it is practically no further vertical propagation and thus dilution of the plume more. The high exhaust gas concentrations are maintained even in a considerable distance to the chimney. Close to the ground, the load is low here high it is, however, upon the occurrence of terrain elevations in the propagation direction. Occurrence V.A. at night and at low pressure, which is common ( see below ) is the precursor to fumigation.

Lofting

At the bottom is here also an inversion, usually a nocturnal radiation inversion, but now is the upper limit at or even below the effective stack height. This is followed by an adiabatic temperature drop with neutral stratification. For the vertical spread of the column of smoke, the inversion, however, proves once again as a barrier layer, in this case down. Since the emissions is above the inversion (otherwise it would again be a Fanning - position ), the pollutants can sink up to the ceiling for the inversion. There, is also usually a higher exhaust gas concentration. Towards the top, the vertical propagation is not hindered. For the air pollution this is a very desirable situation: The gas plume dilutes but does not reach the earth's surface. Occurrence V.A. in the early evening with a cloudless sky, in most cases of shorter duration.

Fumigation

The type of fumigation is the most harmful from the perspective of air pollution situation dar. Here there is an unstable stratification at the bottom, followed by an inversion height above the effective stack height. Below the inversion, the exhaust gases can very well spread due to the unstable stratification, but are blocked up by the inversion. The mixing therefore takes place only near the bottom.

This situation is obtained for a long time, there may be a dramatic enrichment of the pollutants. If the mixing zone, for example, through a valley or even boiler situation quite small, as well as a resolution of the inversion is severely restricted by wind, which favors the accumulation of the exhaust accordingly. Such a valley and also a high emissions is characteristic just for agglomerations. These are, as urban climatology shows also important sources of heat, so tend inversions near the ground level inversions to " rebuild ", then what is the main cause for the onset of fumigation location.

Combinations

The five presented cases have been considered in isolation, but over a greater horizontal distance hardly reflects the real conditions. Stratification of the atmosphere can therefore change with distance from the stack, which is the case especially in surveys orographic and a change in the thermal characteristics of the earth's surface. If now we imagine several layers in a row, specific combinations can arise that are one of Contaminants help or hinder.

Swell

• T. R. Oke: Boundary Layer Climates. Methuen and Others, London 1978, ISBN 0-416-70530-8.
• Peter Fabian: atmosphere and environment. Chemical processes, human intervention. Ozone layer, air pollution, smog, acid rain. 4 expanded and updated edition. Springer, Berlin et al 1992, ISBN 3-540-55773-3.