Stratification stability of the earth's atmosphere

This article covers the troposphere. For the higher layers see Earth's atmosphere # structure .

The stratification stability of the earth's atmosphere describes its thermodynamic stability or lability with regard to the vertical atmospheric temperature gradient based on various states of equilibrium . A distinction is made between unstable, stable and neutral atmospheric stratification.

The stratification of the atmosphere determines all vertical air movements and is therefore of elementary importance for all convective processes within the earth's atmosphere and the associated processes of cloud formation or air pollution . Unstable stratification is a prerequisite for the development of thermal updrafts. The stratification of the atmosphere determines an important aspect of gliding , hang gliding and paragliding .

Basics

Atmospheric temperature gradients

A basic distinction is made between two types of atmospheric temperature gradients: the dynamic gradients of an air parcel and the static gradients of the atmosphere. The measurable air temperature within the atmosphere often decreases very unevenly with altitude, but usually with a clear trend. Usually there is a decrease in temperature, so the air becomes colder and colder as it rises. If, instead, the air temperature increases with altitude, one speaks of an inversion . Compared to this ambient gradient, a vertically moving air parcel has its own dynamic temperature change.

The difference between the two cases is that the rising air usually hardly mixes with the ambient air and an adaptation to the ambient temperature can also be neglected in an idealized view. The actual cooling of the air parcel takes place independently of the environment and the temperature prevailing there, i.e. adiabatically . However, this has the consequence that the decrease in temperature with altitude in relation to the movement of an air parcel does not have to be identical to the static state of the earth's atmosphere. The air parcel can change its temperature faster, at the same speed or slower than its surroundings. Their relative temperature at a certain height can be arbitrary, but this also has consequences for the movement of the air parcel itself.

Vertical movement of an air package

As a model assumption, an air parcel is generally considered which behaves according to the dry adiabatic temperature gradient and has the same temperature or density as the surrounding air at a certain height . Based on this initial temperature, the air parcel cools down when it rises and when it sinks it is heated by 9.8 degrees Celsius per kilometer. These changes in height take place adiabatically and reversibly, so no heat is added to or withdrawn from the air package , there is no mixture with the surrounding air and the water vapor contained in the air does not condense . The latter, on the other hand, is the case with a wet adiabatic ascent of the air parcel, which usually occurs following an initially dry adiabatic ascent. Due to the associated cooling, the relative humidity rises and has finally reached the condensation level at the dew point . At this altitude, condensation and cloud formation begins . The latent heat released in the process reduces the dry adiabatic to the wet adiabatic gradient. Since all considerations of the atmospheric stratification state apply equally to both cases, i.e. only the respective reference changes, an adiabatic gradient will only be referred to in general below.

Archimedean principle

It is now important that warm air has a lower density than cold air. According to Archimedes' principle , the difference between the density of the air parcel in question and that of the surrounding medium results in a static lift or a downforce. If the air parcel in question has the same density as the surroundings, a state of suspension results. The initial rise or fall must therefore be forced for some reason, which is, however, assumed in the following. The mathematical relationship is captured by the following equation:

${\displaystyle a={\frac {\mathrm {d} v_{\mathrm {z} }}{\mathrm {d} t}}=g{\frac {T_{\mathrm {L} }-T_{\mathrm {U} }}{T_{\mathrm {U} }}}=g{\frac {\rho _{\mathrm {U} }-\rho _{\mathrm {L} }}{\rho _{\mathrm {L} }}}}$

The index U stands for the ambient air and L for the air package. The symbol stands for the acceleration that the air package experiences in the vertical direction and for the gravitational acceleration . The Greek letter stands for density and temperature. ${\displaystyle a}$ ${\displaystyle g}$ ${\displaystyle \rho }$ ${\displaystyle T}$

As you can see, the acceleration is zero if the temperatures or densities of the air parcel and the environment are identical. The further apart they are, the greater it is. The sign determines the direction. A positive acceleration leads to an ascent and a negative acceleration to a descent of the air package.

Neutral stratification

The simplest case is that of the neutral or indifferent stratification of the atmosphere . The vertical temperature decrease of the atmosphere is the same as that of the air parcel. In reality, a very well mixed atmosphere would come closest to this. The equivalent of mechanics is indifferent equilibrium .

Because of the neutral stratification of the atmosphere, a rising air parcel cools down just as quickly as the surrounding atmosphere. Points A, B and C, which each stand for an air parcel that cools down or heats up dry adiabatically (red line), are therefore identical to the conditions of the ambient air drawn in black. The upward and downward movement of the air parcel therefore balance each other out because there is no density difference between the air parcel and the respective air layer . Since there is no resulting force, the air parcel does not change its vertical position automatically and as soon as the forced lifting from B to A or lowering from B to C stops, the air parcel remains stable in height.

In a neutral stratified atmosphere, there is hardly any cloud formation due to the lack of convection. A neutral stratification is neither an obstacle nor an advantage for the spread of substances and especially pollutants.

Stable layering

Masses of steam from cooling towers break through the cloud cover and then “swing in”. Small waves can be seen in the cloud surface.

A stable atmospheric stratification describes the state of the earth's atmosphere in which the vertical temperature decrease of the atmosphere is smaller than the temperature decrease of the rising air parcel, i.e. it is a subadiabatic temperature gradient. If its temperature decreases according to the dry adiabatic gradient, one speaks of a drought stable stratification. Accordingly, a decrease with the moisture adiabatic gradient is a moisture-stable stratification.

Since the area around the air parcel cools down more slowly than it does itself, the temperature difference continues to decrease when it rises from C to B and changes its sign at B. During the further ascent from B to A, the air parcel is colder than the ambient air and therefore also has a higher density. If the forced lifting stops at A, the air parcel sinks down again according to the Archimedean principle. The greater the temperature difference, the faster the air sinks - there is temporarily a downward wind . As a result, the air parcel not only sinks to B, but also moves slightly beyond this point due to the inertia. The air parcel then has a slightly lower density than the ambient air and the direction of movement is reversed, so the parcel rises again. This process repeats itself and the air packet describes a harmonic oscillation in the vertical. Due to air friction, the amplitude of this oscillation decreases over time. Without further external disturbances, the air parcel would settle into a stable state of equilibrium at B. The same consideration is also valid for a lowering from B to C, only that the air parcel will rise after the forced lowering has been set because its temperature is now higher than that of the surroundings. In a stable atmosphere, vertical movements are cushioned by negative feedback , which prevents the air from mixing.

This can also be seen in the case of an inversion , i.e. a reversed gradient with a rise in temperature as the altitude increases. This is a particularly strong special case of stable stratification, because every ascent of an air parcel is blocked if the inversion layer is sufficiently thick. This is simply explained by the fact that during an inversion, warm and therefore light air masses lie above colder air masses, i.e. the atmosphere is in an ideal equilibrium that does not require any natural air exchange.

In the case of a moist adiabatic ascent in a stable stratified atmosphere, the clouds formed from the condensation level onwards are relatively stable in height and mostly very flat. Due to horizontal pressure differences and the associated wind, they spread over the surface, resulting in a typical layered cloud of the type Cirrus or Cirrostratus at higher, Altostratus at medium and Stratus at lower altitudes.

Unstable stratification

A labile or unstable atmospheric stratification describes the state of the earth's atmosphere in which the vertical temperature decrease in the atmosphere is greater than the temperature decrease in the rising air parcel, i.e. it is an overadiabatic temperature gradient. If the temperature of the air parcel decreases according to the dry adiabatic gradient, one speaks of a dry unstable stratification. In the case of moist adiabatic cooling, one speaks of a moisture-labile stratification.

Since in an unstable atmosphere the temperature of the air parcel rising from B to A is always higher or its density is lower than that of the surroundings, it experiences an upward restoring force, the buoyancy. The air parcel will continue to rise even without a forced lift and since it continues to cool down more slowly than its surroundings, this ascent is accelerated further and further. The air pack away thus a positive feedback further and further from the equilibrium point at B. The same applies to the reverse direction from B to C, so if the air packet drops and thereby is always colder thus has a greater density than its surroundings and. It will then sink faster and faster until it hits the earth's surface at high speed. Winds that are felt to be particularly strong and sudden on the earth's surface, so-called gusts , are usually nothing more than air packets that are accelerated in this way and then deflected horizontally.

A dry unstable stratification occurs, for example, close to the ground when the air is heated up locally through the radiation . For example, while there was only slight warming in the morning and an over-adiabatic temperature gradient has set in, the increasing time of day leads to stronger solar radiation, which, however, can heat the air close to the ground very differently depending on the type of earth's surface. If this difference is large enough, thermal bubbles are released which cause the lower layers of air to mix. The result of such a mixture, which in deserts and plateaus can reach a height of a few kilometers, is ultimately a neutral stratification. Since an unstable stratification ultimately weakens itself through mixing, it is usually only of short duration.

Dynamic causes can also lead to an unstable stratification, especially if cold air moves faster at heights when a cold front passes through than near the ground. The result is often strong thunderstorms in connection with snow , rain and hail showers as well as strong gusts, which are then referred to as gusts of wind.

In the moderate latitudes, unstable stratification is usually limited to a certain height range and only in rare exceptional cases reaches a greater extent. Stable conditions are usually present as an under- or over-stratification. At high altitudes this leads to the formation of cirrus clouds in the case of a horizontal wind and to cirrocumulus clouds in the absence of such. At medium altitudes, on the other hand, there is altocumulus cloud cover , while at low altitudes there is more of a stratocumulus and cumulus . Cumulonimbus and Nimbostratus , on the other hand, are dependent on extensive instability and therefore occur more frequently near the equator.

Conditionally unstable stratification

A conditionally unstable atmosphere stratification describes a situation in which a dry adiabatically rising air parcel results in a stable or neutral stratification, while an air parcel rising in a moist adiabatic manner would result in an unstable atmospheric stratification.

This problem, which often occurs in summer, raises the question of whether or not clouds will form. If the water vapor in the air package does not condense, nothing else happens. With the onset of condensation, however, the clouds quickly grow into thunderstorms.

Influences on the stratification

Extreme stratifications, i.e. unstable and stable stratifications, are favored by low wind speeds, whereas high wind speeds lead to more neutral stratification. Incidentally, cloudiness, i.e. low global radiation during the day and strong atmospheric counter-radiation at night, leads to a more neutral stratification, while a cloudless sky during the day with strong global radiation causes unstable stratification, while at night it causes stable stratification.

Stratification and air pollution

Air pollution in winter, clearly visible is the layer of haze above the surface of the earth, above it is clear layers of air

The stratification stability has a high influence on air pollution , since the propagation of exhaust gases can be regarded as identical to the propagation of an air packet. A distinction is made between some specific stratification conditions.

The basis for the descriptions is a factory that emits exhaust gases into the atmosphere through a chimney. There is a westerly wind direction from left to right, so that the exhaust gases pass into the horizontal after a certain vertical spread. This initial increase phase is due to the mostly higher temperature of the exhaust gases. The height ultimately reached is called the effective chimney height . In the figures, the red line shows the temperature curve of the atmosphere and the black line the adiabatic gradient of the air parcel.

Looping

There is a slightly to moderately unstable stratification, the air pollutants spread out in loops to the east . Due to turbulence and convective processes, the discharge can touch the ground after a short time, so that the pollutant load in relatively close proximity to the chimney is quite high. However, it also decreases quickly and is quite small at medium distances. The situation is typical for afternoons on sunny summer days.

Coning

There is a neutral to slightly stable stratification and the air pollutants spread conically, whereby the vertical expansion of the increasingly wider exhaust cone is quite even. The dilution of the exhaust gases is quite low, but the plume of smoke does not touch the ground directly. A situation in advance in cloudy weather.

Fanning

In the Fanning type, the stable stratification has been reinforced to form a massive floor inversion that extends above the effective chimney height. After the initial incline to the effective chimney height, there is practically no further vertical expansion and thus thinning of the smoke plume. The high exhaust gas concentrations are retained even at a considerable distance from the chimney. The pollution is low here near the ground, but it is high when the terrain increases in the direction of propagation. Occurrence of a. at night and in gravure, which is often the preliminary stage to fumigation (see below).

Lofting

There is also an inversion on the ground here, usually a nocturnal radiation inversion, but its upper limit is now at or even below the effective chimney height. This is followed by an adiabatic temperature drop with neutral stratification. For the vertical spread of the column of smoke, the inversion again proves to be a barrier, but in this case downwards. Since the emissions occur above the inversion (otherwise it would be a Fanning location again), the pollutants can drop at most to the upper limit of the inversion. A higher exhaust gas concentration is usually found there. Upwards, however, the vertical spread is not hindered. This is a very desirable situation for air pollution: the smoke plume is thinning but does not reach the surface of the earth. Occurrence of a. in the early evening under a cloudless sky, in most cases of short duration.

Fumigation

The type of fumigation represents the most harmful location from the point of view of air pollution. Here there is an unstable stratification on the ground, followed by a height inversion above the effective chimney height. Below the inversion, the exhaust gases can spread very well due to the unstable stratification, but are blocked upwards by the inversion. Mixing takes place only near the ground.

If this situation is maintained over a longer period of time, there can be a drastic accumulation of pollutants. If the mixing zone is very small, for example due to a valley or even a basin, the resolution of the inversion by wind is also severely restricted, which accordingly favors the enrichment of the exhaust gases. Such a valley location and also high pollutant emissions are characteristic of metropolitan areas. As urban climatology shows, these are also important sources of heat, so they tend to "convert" inversions near the ground into height inversions, which is then the main cause of fumigation.

Combinations

The five cases shown have only been considered in isolation, but this hardly corresponds to the real conditions over a greater horizontal distance. The stratification of the atmosphere can therefore change with distance from the chimney, which is particularly the case with orographic elevations and a change in the thermal properties of the earth's surface. If you now think of several layers in a row, specific combinations can arise that are conducive or hindering the spread of pollutants.

Designation of the stability classes

In addition to the designation of the stability classes as unstable, neutral and stable, there are various systematics for classifying the stability class. The systematics of the Pasquill classes according to Frank Pasquill is known , which designates the stability classes from very unstable to stable with the letters A to F. The technical instructions for keeping the air clean (TA Luft) uses dispersion classes according to Klug and Manier with designations from I (very stable) to V (very unstable). Other stability classes were defined by Vogt as well as by Nester and Thomas. A measure of stability is the Obukhov length introduced by Alexander Michailowitsch Obuchow .

swell

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

Web links

• WEBGEO module: Stratification states in the atmosphere - WEBGEO - E-learning portal for geography and related sciences

Individual evidence

1. ↑ ^{a b c d} Thomas Foken: Applied meteorology, micrometeorological methods. Springer, Berlin and Heidelberg 2003. ISBN 978-3-540-00322-9 . Book on Google Books.
2. ↑ Frank Pasquill: Atmospheric diffusion: The dispersion of windborne material from industrial and other sources. 2nd Edition. Halsted Press, New York 1974.
3. ↑ W. Klug: A method for determining the propagation conditions from synoptic observations. Dust - cleanliness of the air 29 (1969) pp 143-147.
4. ↑ G. Manier: Comparison between dispersion classes and temperature gradients. Meteorologische Rundschau 28 (1975), p. 6.