The layering of the atmosphere

  1. Composition of the atmosphere
  2. Effect of the atmosphere
  3. Energy transfer within the earth-atmosphere system

The atmosphere can be divided conveniently into a number of rather well-marked horizontal layers, mainly on the basis of temperature. The evidence for this structure comes from regular rawinsonde (radar wind-sounding) balloons, radio-wave investigations, and, more recently, from rocket flights and satellite sounding systems. Broadly, the pattern consists of three relatively warm layers (near the surface; between 50 and 60 km; and above about 120 km) separated by two relatively cold layers (between 10 and 30 km; and 80-100 km).

1 Troposphere

The lowest layer of the atmosphere is called the troposphere. It is the zone where weather phenomena and atmospheric turbulence are most marked, and it contains 75 per cent of the total molecular or gaseous mass of the atmosphere and virtually all the water vapour and aerosols. Throughout this layer, there is a general decrease of temperature with height at a mean rate of about 6.5 C / km. The decrease occurs because air is compressible and its density decreases with height, allowing rising air to expand and thereby cool. Additionally, the atmosphere is heated mainly by turbulent heat transfer from the surface, not by absorption of radiation. The troposphere is capped in most places by a temperature inversion level (i.e. a layer of relatively warm air above a colder one) and in others by a zone that is isothermal with height. The troposphere thus remains to a large extent self-contained, because the inversion acts as a 'lid' that effectively limits convection. This inversion level or weather ceiling is called the tropopause. Its height is not constant in either space or time. It seems that the height of the tropopause at any point is correlated with sea level temperature and pressure, which are in turn related to the factors of latitude, season and daily changes in surface pressure. There are marked variations in the altitude of the tropopause with latitude, from about 16 km at the equator, where there is great heating and vertical convective turbulence, to only 8 km at the poles.

The meridional temperature gradients in the troposphere in summer and winter are roughly parallel, as are the tropopauses, and the strong lower mid-latitude temperature gradient in the troposphere is reflected in the tropopause breaks. In these zones, important interchanges can occur between the troposphere and stratosphere, and vice versa. Traces of water vapour probably penetrate into the stratosphere by this means, while dry, ozone-rich stratospheric air may be brought down into the mid- latitude troposphere. For example, above-average concentrations of ozone are observed in the rear of mid-latitude low-pressure systems, where the tropopause elevation tends to be low. Both facts are probably the result of stratospheric subsidence, which warms the lower stratosphere and causes downward transfer of the ozone.

2 Stratosphere

The second major atmospheric layer is the stratosphere, which extends upwards from the tropopause to about 50 km. Although the stratosphere contains much of the total atmospheric ozone (it reaches a peak density at approximately 22 km), the maximum temperatures associated with the absorption of the sun's ultraviolet radiation by ozone occur at the stratopause, where temperature may exceed 0 C. The air density is much less here, so even limited absorption produces a large temperature increase. Temperatures increase fairly generally with height in summer, with the coldest air at the equatorial tropopause. In winter, the structure is more complex with very low temperatures, averaging -80 C, at the equatorial tropopause, which is highest at this season. Similar low temperatures are found in the middle stratosphere at high latitudes, whereas over 50-60 N there is a marked warm region with nearly isothermal conditions at about -45 to -50 C. Marked seasonal changes of temperature affect the stratosphere. The cold 'polar night' winter stratosphere often undergoes dramatic sudden warmings associated with subsidence due to circulation changes in late winter or early spring, when temperatures at about 25 km may jump from -80 to -40 C over a two-day period. The autumn cooling is a more gradual process. In the tropical stratosphere, there is a quasi-biennial (26-month) wind regime, with easterlies in the layer 18 to 30 km for 12 to 13 months, followed by westerlies for a similar period. The reversal begins first at high levels and takes approximately 12 months to descend from 30 to 18 km (10 to 60 mb).

How far these events in the stratosphere are linked with temperature and circulation changes in the troposphere, remains a topic of meteorological research. Any interactions that do exist, however, are likely to be complex, otherwise they would already have become evident.

3 Mesosphere

Above the stratopause, average temperatures decrease to a minimum of about -133 C (140 K) or around 90 km. This layer is commonly termed the mesosphere, although it must be noted that as yet there is no universal acceptance of terminology for the upper atmospheric layers. The layers between the tropopause and the lower thermosphere are now commonly referred to as the middle atmosphere, with the upper atmosphere designating the regions above about 100 km altitude. Above 80 km, temperatures again begin rising with height and this inversion is referred to as the 'mesopause'. Molecular oxygen and ozone absorption bands contribute to heating around 85 km altitude. It is in this region that 'noctilucent clouds' are observed over high latitudes in summer. Their presence appears to be due to meteoric dust particles, which act as ice crystal nuclei when traces of water vapour are carried upwards by high-level convection caused by the vertical decrease of temperature in the mesosphere. However, their formation is also thought to be related to the production of water vapour through the oxidation of atmospheric methane, since apparently they were not observed prior to the Industrial Revolution.

Pressure is very low in the mesosphere, decreasing from about 1 mb at 50 km to 0.01 mb at 90 km.

4 Thermosphere

Above the mesopause, atmospheric densities are extremely low, although the tenuous atmosphere still effects drag on space vehicles above 250 km. The lower portion of the thermosphere is composed mainly of nitrogen (N2) And oxygen in molecular (O2) And atomic (O), forms, whereas above 200 km atomic oxygen predominates over nitrogen (N2and N). Temperatures rise with height, owing to the absorption of extreme ultraviolet radiation (0.125-0.205 m) by molecular and atomic oxygen, probably approaching 800-1,200 K at 350 km, but these temperatures are essentially theoretical. For example, artificial satellites do not acquire such temperatures because of the rarefied air. 'Temperatures' in the upper thermosphere and exosphere undergo wide diurnal and seasonal variations. They are higher by day and are also higher during a sunspot maximum, although the changes are only represented in varying velocities of the sparse air molecules.

Above 100 km, the atmosphere is increasingly affected by cosmic radiation, solar X-rays and ultraviolet radiation, which cause ionization, or electrical charging, by separating negatively charged electrons from neutral oxygen atoms and nitrogen molecules, leaving the atom or molecule with a net positive charge (an ion). The Aurora Borealis and Aurora Australis are produced by the penetration of ionizing particles through the atmosphere from about 300 km to 80 km, particularly in zones about 10-20 latitude from the earth's magnetic poles. On occasion, however, the aurorae may appear at heights up to 1,000 km, demonstrating the immense extension of a rarefied atmosphere. The term ionosphere is commonly applied to the layers above 80 km.

5 Exosphere and magnetosphere

The base of the exosphere is between about 500 km and 750 km. Here atoms of oxygen, hydrogen and helium (about 1 per cent of which are ionized) form the tenuous atmosphere, and the gas laws cease to be valid. Neutral helium and hydrogen atoms, which have low atomic weights, can escape into space since the chance of molecular collisions deflecting them downwards becomes less with increasing height. Hydrogen is replaced by the breakdown of water vapour and methane (CH4) Near the mesopause, while helium is produced by the action of cosmic radiation on nitrogen and from the slow but steady breakdown of radioactive elements in the earth's crust.

Ionized particles increase in frequency through the exosphere and, beyond about 200 km, in the magnetosphere there are only electrons (negative) and protons (positive) derived from the solar wind - a plasma of electrically conducting gas.

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ISBN 978-5 -86813-306-0 | ISBN 978-5-86813-306-0 | Solar radiation | Altitude of the sun | Distance from the sun | Length of day | Energy transfer within the earth-atmosphere system | Effect of the atmosphere | Composition of the atmosphere | Variation with height |

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