Discovery Of The Tropopause And Stratosphere

Early scientific exploration of the upper atmosphere began with manned balloon flights in the mid-nineteenth century. Notable among these was the ascent by J. Glaisher and H. T. Coxwell in 1862. Glaisher last consciousness due to lack of oxygen at about 8800-m altitude and they barely survived the hypoxia. In 1902 L. Teisserenc de Bort in France reported a totally unexpected finding: that temperatures ceased decreasing at altitudes of around 12 km. Indeed, at higher elevations temperatures were commonly observed to begin increasing with altitude. This mean structure is shown in Figure 2.15.

The terms troposphere (turbulent sphere) and stratosphere (stratified sphere) were proposed by Teisserenc de Bort in 1908; the use of tropopause to denote the inversion or isothermal layer separating them was introduced in Great Britain during the First World War. The distinctive features of the stratosphere are its stability compared with the troposphere, its dryness, and its high concentration of ozone.

2 Stratosphere

The stratosphere extends upward from the tropopause to about 50 km and accounts for about 10 per cent of the atmospheric mass. Although the stratosphere contains much of the total atmospheric ozone (it reaches a peak density at approximately 22 km), maximum temperatures associated with the absorption of the sun's ultraviolet radiation by ozone occur at the stratopause, where they may exceed 0°C (see Figure 2.15). The air density is much lower here, so even limited absorption produces a high temperature rise. 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. In the circumpolar low-pressure vortex over both polar regions, polar stratospheric clouds (PSCs) are sometimes present at 20 to 30 km altitude. These have a nacreous ('mother-of-pearl') appearance. They can absorb odd nitrogen and thereby cause catalytic destruction of ozone.

Marked seasonal changes of temperature affect the stratosphere. The cold 'polar night' winter stratosphere in the Arctic often undergoes dramatic sudden warmings associated with subsidence due to circulation changes in late winter or early spring, when temper atures 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 (twenty-six-month) wind regime, with easterlies in the layer 18 to 30 km for twelve to thirteen months, followed by westerlies for a similar period. The reversal begins first at high levels and takes approximately twelve months to descend from 30 to 18 km (10 to 60 mb).

How far events in the stratosphere are linked with temperature and circulation changes in the troposphere remains a topic of meteorological research. Any such interactions are undoubtedly complex.

3 Mesosphere

Above the stratopause, average temperatures decrease to a minimum of about -133°C (140 K) or around 90 km (Figure 2.15). This layer is commonly termed the mesosphere, although as yet there is no universal terminology for the upper atmospheric layers. Pressure is very low in the mesosphere, decreasing from about 1 mb at 50 km to 0.01 mb at 90 km. 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 on summer 'nights' over high latitudes. Their presence appears to be due to meteoric dust particles, which act as ice crystal nuclei when traces of water vapour are carried upward by high-level convection caused by the vertical decrease of temperature in the mesosphere. However, their formation may also 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. The layers between the tropopause and the lower thermosphere are commonly referred to as the middle atmosphere, with the upper atmosphere designating the regions above about 100 km altitude.

4 Thermosphere

Atmospheric densities are extremely low above the mesopause, 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 (N2 and N). Temperatures rise with height, owing to the absorption of extreme ultraviolet radiation (0.125 to 0.205 |m) by molecular and atomic oxygen, probably approaching 800 to 1200 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 exos-phere 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, cosmic radiation, solar X-rays and ultraviolet radiation increasingly affect the atmosphere, 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 term ionosphere is commonly applied to the layers above 80 km. 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 to 20° latitude from the earth's magnetic poles. On occasion, however, aurora may appear at heights up to 1000 km, demonstrating the immense extension of a rarefied atmosphere.

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 (see B, this chapter) 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 downward 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 - which is a plasma of electrically conducting gas.

SUMMARY

The atmosphere is a mixture of gases with constant proportions up to 80 km or more. The exceptions are ozone, which is concentrated in the lower stratosphere, and water vapour in the lower troposphere. The principal greenhouse gas is water vapour. Carbon dioxide, methane and other trace gases have increased since the Industrial Revolution, especially in the twentieth century due to the combustion of fossil fuels, industrial processes and other anthropogenic effects, but larger natural fluctuations occurred during the geologic past.

Reactive gases include nitrogen and sulphur and chlorine species. These play important roles in acid precipitation and ozone destruction. Acid precipitation (by wet or dry deposition) results from the reaction of cloud droplets with emissions of SO2 and NOx. There are large geographical variations in acid deposition. The processes leading to destruction of stratospheric ozone are complex, but the roles of nitrogen oxides and chlorine radicals are very important in causing polar ozone holes. Aerosols in the atmosphere originate from natural and anthropogenic sources and they play an important but complex role in climate.

Air is highly compressible, so that half of its mass occurs in the lowest 5 km, and pressure decreases logarithmically with height from an average sea-level value of 1013 mb. The vertical structure of the atmosphere comprises three relatively warm layers - the lower troposphere, the stratopause and the upper thermosphere - separated by a cold layer above the tropopause (in the lower stratosphere), and the mesopause. The temperature profile is determined by atmospheric absorption of solar radiation, and the decrease of density with height.

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    Who is discovered Tropopause?
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