Atmosfera, Tempo e Clima

Atmosfera, Tempo e Clima

(Parte 3 de 9)

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

Learning objectives When you have read this chapter you will:

Be familiar with key concepts in meteorology and climatology, Know how these fields of study evolved and the contributions of leading individuals.

Introduction and history of meteorology and climatology 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.

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.

The first attempt to explain the global atmospheric circulation was based on a simple convectional concept. In 1686 Halley associated the easterly trade winds with low-level convergence on the equatorial belt of greatest heating (i.e. the thermal equator). These flows are compensated at high levels by return flows aloft. Poleward of these convectional regions, the air cools and subsides to feed the northeasterly and southeasterly trades at the surface. This simple mechanism, however, presented two significant problems – what mechanism produced high-pressure in the subtropics and what was responsible for the belts of dominantly westerly winds poleward of this high pressure zone? It is interesting to note that not until 1883 did Teisserenc de Bort produce the first global mean sea-level map showing the main zones of anticyclones and cyclones (i.e. high and low pressure). The climatic significance of Halley’s work rests also in his thermal convectional theory for the origin of the Asiatic monsoon which was based on the differential thermal behaviour of land and sea; i.e. the land reflects more and stores less of the incoming solar radiation and therefore heats and cools faster. This heating causes continental pressures to be generally lower than oceanic ones in summer and higher in winter, causing seasonal wind reversals. The role of seasonal movements of the thermal equator in monsoon systems was only recognized much later. Some of the difficulties faced by Halley’s simplistic large-scale circulation theory began to be addressed by Hadley in 1735. He was particularly concerned with the deflection of winds on a rotating globe, to the right (left) in the northern (southern) hemisphere. Like Halley, he advocated a thermal circulatory mechanism, but was perplexed by the existence of the westerlies. Following the mathematical analysis of moving bodies on a rotating earth by Coriolis (1831), Ferrel (1856) developed the first three-cell model of hemispherical atmospheric circulation by suggesting a mechanism for the production of high pressure in the subtropics (i.e. 35°N and S latitude). The tendency for cold upper air to subside in the subtropics, together with the increase in the deflective force applied by terrestrial rotation to upper air moving poleward above the Trade Wind Belt, would cause a build-up of air (and therefore of pressure) in the subtropics. Equatorward of these subtropical highs the thermally direct Hadley cells dominate the Trade Wind Belt but poleward of them air tends to flow towards higher latitudes at the surface. This airflow, increasingly deflected with latitude, constitutes the westerly winds in both hemispheres. In the northern hemisphere, the highly variable northern margin of the westerlies is situated where the westerlies are undercut by polar air moving equatorward. This margin was compared with a battlefield front by Bergeron who, in 1922, termed it the Polar Front. Thus Ferrel’s three cells consisted of two thermally direct Hadley cells (where warm air rises and cool air sinks), separated by a weak, indirect Ferrel cell in mid-latitudes. The relation between pressure distribution and wind speed and direction was demonstrated by Buys-Ballot in 1860.

During the nineteenth century it became possible to assemble a large body of global climatic data and to use it to make useful regional generalizations. In 1817 Alexander von Humboldt produced his valuable treatise on global temperatures containing a map of mean annual isotherms for the northern hemisphere but it was not until 1848 that Dove published the first world maps of monthly mean temperature. An early world map of precipitation was produced by Berghaus in 1845; in 1882 Loomis produced the first world map of precipitation employing mean annual isohyets; and in 1886 de Bort published the first world maps of annual and monthly cloudiness. These generalizations allowed, in the later decades of the century, attempts to be made to classify climates regionally. In the 1870s Wladimir Koeppen, a St Petersburg-trained biologist, began producing maps of climate based on plant geography, as did de Candolle (1875) and Drude (1887). In 1883 Hann’s massive three-volume Handbook of Climatologyappeared, which remained a standard until 1930–40 when the five-volume work of the same title by Koeppen and Geiger replaced it. At the end of the First World War Koeppen (1918) produced the first detailed classification of world climates based on terrestrial vegetation cover. This was followed by Thornthwaite’s (1931–3) classification of climates employing evaporation and precipitation amounts, which he made more widely applicable in 1948 by the use of the theoretical concept of potential evapo-transpiration. The inter-war period was particularly notable for the appearance of a number of climatic ideas which were not brought to fruition until the 1950s. These included the use of frequencies of various weather types (Federov, 1921), the concepts of variability of temperature and rainfall (Gorczynski, 1942, 1945) and microclimatology (Geiger, 1927).

Despite the problems of obtaining detailed measurements over the large ocean areas, the later nineteenth century saw much climatic research which was concerned with pressure and wind distributions. In 1868 Buchan produced the first world maps of monthly mean pressure; eight years later Coffin composed the first world wind charts for land and sea areas, and in 1883 Teisserenc de Bort produced the first mean global pressure maps showing the cyclonic and anticyclonic ‘centres of action’ on which the general circulation is based. In 1887 de Bort began producing maps of upperair pressure distributions and in 1889 his world map of January mean pressures in the lowest 4 km of the atmosphere was particularly effective in depicting the great belt of the westerlies between 30°and 50°north latitudes.

Theoretical ideas about the atmosphere and its weather systems evolved in part through the needs of nineteenthcentury mariners for information about winds and storms, especially predictions of future behaviour. At low levels in the westerly belt (approximately 40°to 70° latitude) there is a complex pattern of moving high and low pressure systems, while between 6000 m and 20,0 m there is a coherent westerly airflow. Dove (1827 and 1828) and Fitz Roy (1863) supported the ‘opposing current’ theory of cyclone (i.e. depression) formation, where the energy for the systems was produced by converging airflow. Espy (1841) set out more clearly a convection theory of energy production in cyclones with the release of latent heat as the main source. In 1861, Jinman held that storms develop where opposing air currents form lines of confluence (later termed ‘fronts’). Ley (1878) gave a three-dimensial picture of a low-pressure system with a cold air wedge behind a sharp temperature discontinuity cutting into warmer air, and Abercromby (1883) described storm systems in terms of a pattern of closed isobars with typical associated weather types. By this time, although the energetics were far from clear, a picture was emerging of mid-latitude storms being generated by the mixing of warm tropical and cool polar air as a fundamental result of the latitudinal gradients created by the patterns of incoming solar radiation and of outgoing terrestrial radiation. Towards the end of the nineteenth century two important European research groups were dealing with storm formation: the Vienna group under Margules, including Exner and Schmidt; and the Swedish group led by Vilhelm Bjerknes. The former workers were concerned with the origins of cyclone kinetic energy which was thought to be due to differences in the potential energy of opposing air masses of different temperature. This was set forth in the work of Margules (1901), who showed that the potential energy of a typical depression is less than 10 per cent of the kinetic energy of its constituent winds. In Stockholm V. Bjerknes’ group concentrated on frontal development (Bjerknes, 1897, 1902) but its researches were particularly important during the period 1917 to 1929 after J. Bjerknes moved to Bergen and worked with Bergeron. In 1918 the warm front was identified, the occlusion process was described in 1919, and the full Polar Front Theory of cyclone development was presented in 1922 (J. Bjerknes and Solberg). After about 1930, meteorological research concentrated increasingly on the importance of mid- and upper-tropospheric influences for global weather phenomena. This was led by Sir Napier Shaw in Britain and by Rossby, with Namias and others, in the USA. The airflow in the 3–10 km high layer of the polar vortex of the northern hemisphere westerlies was shown to form large-scale horizontal (Rossby) waves due to terrestrial rotation, the influence of which was simulated by rotation ‘dish pan’ experiments in the 1940s and 1950s. The number and amplitude of these waves appears to depend on the hemispheric energy gradient, or ‘index’. At times of high index, especially in winter, there may be as few as three Rossby waves of small amplitude giving a strong zonal (i.e. west to east) flow. A weaker hemispheric energy gradient (i.e. low index) is characterized by four to six Rossby waves of larger amplitude. As with most broad fluid-like flows in nature, the upper westerlies were shown by observations in the 1920s and 1930s, and particularly by aircraft observations in the Second World War, to possess narrow high-velocity threads, termed ‘jet streams’ by Seilkopf in 1939. The higher and more important jet streams approximately lie along the Rossby waves. The most important jet stream, located at 10 km, clearly affects surface weather by guiding the low pressure systems which tend to form beneath it. In addition, air subsiding beneath the jet streams strengthens the subtropical high pressure cells.

(Parte 3 de 9)