The Atmosphere

The atmosphere, vital to terrestrial life, envelops the earth to a thickness of only 1 per cent of the earth's radius. It had evolved to its present form and composition at least 400 million years ago by which time a considerable vegetation cover had developed on land. At its base, the atmosphere rests on the ocean surface which, at present, covers some 70 per cent of the surface of the globe. Although air and water share somewhat similar physical properties, they differ in one important respect - air is compressible, water incompressible. Study of the atmosphere has a long history involving both observations and theory. Scientific measurements became possible only with the invention of appropriate instruments; most had a long and complex evolution. A thermometer was invented by Galileo in the early 1600s, but accurate liquid-in-glass thermometers with calibrated scales were not available until the early 1700s (Fahrenheit), or the 1740s (Celsius). In 1643 Torricelli demonstrated that the weight of the atmosphere would support a 10 m column of water or a 760 mm column of liquid mercury. Pascal used a barometer of Torricelli to show that pressure decreases with altitude, by taking one up the Puy de Dôme in France. This paved the way for Boyle (1660) to demonstrate the compressibility of air by propounding his law that volume is inversely proportional to pressure. It was not until 1802 that Charles showed that air volume is directly proportional to its temperature. By the end of the nineteenth century the four major constituents of the dry atmosphere (nitrogen 78.08 per cent, oxygen 20.98 per cent, argon 0.93 per cent and carbon dioxide 0.035 per cent) had been identified. In the twentieth century it became apparent that CO2, produced mainly by plant and animal respiration and since the Industrial Revolution by the breakdown of mineral carbon, had changed greatly in recent historic times, increasing by some 25 per cent since 1800 and by fully 7 per cent since 1950.

The hair hygrograph, designed to measure relative humidity, was only invented in 1780 by de Saussure. Rainfall records exist from the late seventeenth century in England, although early measurements are described from India in the fourth century bc, Palestine about ad 100 and Korea in the 1440s. A cloud classification scheme was devised by Luke Howard in 1803, but was not fully developed and implemented in observational practice until the 1920s. Equally vital was the establishment of networks of observing stations, following a standardized set of procedures for observing the weather and its elements, and a rapid means of exchanging the data (the telegraph). These two developments went hand-in-hand in Europe and North America in the 1850s to 1860s.

The greater density of water, compared with that of air, gives water a higher specific heat. In other words, much more heat is required to raise the temperature of a cubic metre of water by 1°C than to raise the temperature of a similar volume of air by the same amount. In terms of understanding the operations of the coupled earth-atmosphere-ocean system, it is interesting to note that the top 10-15 cm of ocean waters contain as much heat as does the total atmosphere. Another important feature of the behaviour of air and water appears during the process of evaporation or condensation. As Black showed in 1760, during evaporation, heat energy of water is translated into kinetic energy of water vapour molecules (i.e. latent heat), whereas subsequent condensation in a cloud or as fog releases kinetic energy which returns as heat energy. The amount of water which can be stored in water vapour depends on the temperature of the air. This is why the condensation of warm moist tropical air releases large amounts of latent heat, increasing the instability of tropical air masses. This may be considered as part of the process of convection in which heated air expands, decreases in density and rises, perhaps resulting in precipitation, whereas cooling air contracts, increases in density and subsides.

The combined use of the barometer and thermometer allowed the vertical structure of the atmosphere to be investigated. A low-level temperature inversion was discovered in 1856 at a height of about 1 km on a mountain in Tenerife where temperature ceased to decrease with height. This so-called Trade Wind Inversion is found over the eastern subtropical oceans where subsiding dry high-pressure air overlies cool moist maritime air close to the ocean surface. Such inversions inhibit vertical (convective) air movements, and consequently form a lid to some atmospheric activity. The Trade Wind Inversion was shown in the 1920s to differ in elevation between some 500 m and 2 km in different parts of the Atlantic Ocean in the belt 30°N to 30°S. Around 1900 a more important continuous and widespread temperature inversion was revealed by balloon flights to exist at about 10 km at the equator and 8 km at high latitudes. This inversion level (the tropopause) was recognized to mark the top of the so-called troposphere within which most weather systems form and decay. By 1930 balloons equipped with an array of instruments to measure pressure, temperature and humidity, and report them back to earth by radio (radiosonde), were routinely investigating the atmosphere.

B SOLAR ENERGY

The exchanges of potential (thermal) and kinetic energy also take place on a large scale in the atmosphere as potential energy gradients produce thermally forced motion. Indeed, the differential heating of low and high latitudes is the mechanism which drives both atmospheric and oceanic circulations. About half of the energy from the sun entering the atmosphere as short-wave radiation (or 'insolation') reaches the earth's surface. The land or oceanic parts are variously heated and subsequently re-radiate this heat as long-wave thermal radiation. Although the increased heating of the tropical regions compared with the higher latitudes had long been apparent, it was not until 1830 that Schmidt calculated heat gains and losses for each latitude by incoming solar radiation and by outgoing reradiation from the earth. This showed that equatorward of about latitudes 35° there is an excess of incoming over outgoing energy, while poleward of those latitudes there is a deficit. The result of the equator-pole thermal gradients is a poleward flow (or flux) of energy, interchangeably thermal and kinetic, reaching a maximum between latitudes 30° and 40°. It is this flux which ultimately powers the global scale movements of the atmosphere and of oceanic waters. The amount of solar energy being received and re-radiated from the earth's surface can be computed theoretically by mathematicians and astronomers. Following Schmidt, many such calculations were made, notably by Meech (1857), Wiener (1877), and Angot (1883) who calculated the amount of extraterrestrial insolation received at the outer limits of the atmosphere at all latitudes. Theoretical calculations of insolation in the past by Milankovitch (1920, 1930), and Simpson's (1928 to 1929) calculated values of the insolation balance over the earth's surface, were important contributions to understanding astronomic controls of climate. Nevertheless, the solar radiation received by the earth was only accurately determined by satellites in the 1990s.

Continue reading here: Midlatitude Disturbances

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    Is global warming directly proportional to cooler weather?
    2 years ago