Atmosphere, Weather and Climate

Roger G. Barry, Richard J Chorley

• Publisher: Routledge
• Year: 2009
• ISBN:
  9781135267483
• Edition: 1
• Topic: Ciencias biológicas
• Subtopic: Ciencia medioambiental

This book presents a comprehensive introduction to weather processes and climatic conditions around the world, their observed variability and changes, and projected future trends. Extensively revised and updated, this ninth edition retains its tried and tested structure while incorporating recent advances in the field. From clear explanations of the basic physical and chemical principles of the atmosphere, to descriptions of regional climates and their changes, the book presents a comprehensive coverage of global meteorology and climatology. In this new edition the latest scientific ideas are again expressed in a clear, non-mathematical matter.

New features include:

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• extended and updated treatment of atmospheric models

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• final chapter on climate variability and change has been completely rewritten to take account of the IPCC 2007 scientific assessment.

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• new four-colour text design featuring over 30 colour plates

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• over 360 diagrams have been redrawn in full colour to improve clarity and aid understanding.

Atmosphere, Weather and Climate continues to be an indispensable source for all those studying the earth's atmosphere and world climate, whether from environmental and earth sciences, geography, ecology, agriculture, hydrology, or related disciplinary perspectives. Its pedagogic value is enhanced by several features: learning points at the opening of each chapter and discussion topics at their ending, boxes on topical subjects and on twentieth century advances in the field.

CHAPTER ONE
Introduction and history of meteorology and climatology

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.

A THE ATMOSPHERE

The atmosphere, vital to terrestrial life, envelops the earth to a thickness of only 1 percent 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 land and ocean surface, the latter which, at present, covers some 71 percent of the surface of the globe. Although air and water share somewhat similar physical properties, they differ in one important respect – air is compressible, while water is largely incompressible. In other words, in contrast to water, if one were to ‘squeeze’ a given sample of air, its volume would decrease. Study of the atmosphere has a long history involving observations, theory, and, since the 1960s, numerical modeling. Like most scientific fields, incremental progress has been interspesed by moments of great insight and rapid advance.

Scientific measurements only became possible 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 1740s (Celsius). In 1643 Torricelli invented the barometer, and demonstrated that the weight of the atmosphere at sea level would support a 10m column of water or a 760mm column of liquid mercury. Pascal used a barometer of Torricelli to show that pressure decreases with altitude, by taking one up the Puy de Dome 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 made the discovery that air volume is also directly proportional to its temperature. Combining Boyle’s and Charles’ laws yields the ideal gas law relating pressure, volume and temperature, one of the most important relationships in atmospheric science. By the end of the nineteenth century the four major constituents of the dry atmosphere (nitrogen 78.08 percent, oxygen 20.98 percent, argon 0.93 percent and carbon dioxide 0.035 percent) had been identified. It had been long suspected that human activities could have the potential to alter climate. While the atmospheric ‘greenhouse effect’ was discovered in 1824 by Joseph Fourier, the first serious consideration of a link between climate change, the greenhouse effect and changes in atmospheric carbon dioxide also emerged in the late nineteenth century through the insights of Swedish scientist Svante Arthenius. His expectation that carbon dioxide levels and temperature would rise due to fossil fuel burning has sadly turned out to be correct.

The hair hygrograph, designed to measure relative humidity (the amount of water vapor in the atmosphere relative to how much it can hold at saturation, expressed as a percent), was 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–1860s.

The greater density of water compared with that of air (a factor of about 1000 at mean sea level pressure) gives water a higher specific heat. In other words, much more heat is required to raise the temperature of a cubic meter of water by 1°C than to raise the temperature of an equal volume of air by the same amount. It is interesting to note that just the top 10–15cm of ocean waters contain as much heat as does the total atmosphere; the total heat in the ocean in turn dwarfs that of the atmosphere. As is now known, this tremendous reservoir of heat in the upper ocean and its exchanges with the atmosphere is key to understanding climate variability. Another important feature of the behavior 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 vapor 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 vapor 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. While it is common experience to the aviator and mountain traveler that temperature tends to decrease with height, the reverse pattern of temperature increasing with height, known as an inversion, is also quite common, and in fact dominates in certain regions and atmospheric levels. A low-level (i.e., near-surface) temperature inversion was discovered in 1856 at a height of about 1km on a mountain in Tenerife. Later investigations revealed that 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 500m and 2km 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 10km at the equator and 8km 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. Observations from both kites and balloons also revealed that strong inversions extending up to about 1000m are a near ubiquitous feature of the Arctic in winter.

B SOLAR ENERGY

Differential solar heating of low and high latitudes is the mechanism which drives the earth’s large-scale atmospheric and oceanic circulations. Most of the energy from the sun entering the atmosphere as short-wave radiation (or insolation) reaches the earth’s surface. Some is reflected back to space. The remainder is absorbed by the surface which then warms the atmosphere above it. The atmosphere and surface together radiate long-wave (thermal) radiation back to space. Although the land and ocean parts of the surface absorb different amounts of solar radiation and have different thermal characteristics, the differential solar heating between low and high latitudes dominates, fostering an equator-to-pole gradient in atmospheric and upper ocean temperatures.

Although increased solar heating of the tropical regions compared with the higher latitudes had long been apparent, it was not until 1830 that Schmidt made a key calculation, namely heat gains and losses for each latitude by incoming solar radiation and by outgoing longwave radiation from the earth. This showed that equatorward of about latitudes 35° there is an excess of incoming solar over outgoing longwave energy, while poleward of those latitudes the longwave loss exceeds solar input. If, at each latitude, the longwave loss to space equaled the solar radiation input (termed radiative equilibrium), this pattern would not be seen. That it exists is direct evidence that there must be an overall tranasfer of energy from lower to higher latitudes via the atmospheric and oceanic circulations. Put differently, while the differential solar heating gives rise to the equator-to-pole temperature gradient, the poleward energy transports work to reduce this gradient. Later and more refined calculations showed that the poleward flow (or flux) of atmospheric energy reaches a maximum around latitudes 30° and 40°, with the maximum ocean transport occurring at lower latitudes. The total poleward transport in both hemispheres is in turn dominated by the atmosphere. 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 (1928–1929) 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.

C GLOBAL CIRCULATION

While differential solar heating of the surface and the atmospheric temperature gradient that it generates fosters the large-scale transport of energy from equatorial to polar regions, what are the mechanisms by which this atmospheric transport is accomplished? While we now know that the transport is accomplished by the Hadley circulation in lower latitudes and in higher latitudes through disturbances in the basic westerly (west to east) flow in the form of transient cyclones and anticyclones, it is fascinating to briefly outline how our modern view of the global circulation emerged.

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 Trade Winds at the surface. This simple mechanism, however, presented two significant problems: what mechanism produced the observed 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 it was not until 1883 that Teisserenc de Bort produced the first global mean sea-level map showing the main zones of 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 behavior 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 surface 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, who 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 a 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 latitudinal increase in the deflective force (the Coriolis force, the product of wind speed and the the Coriolis parameter which increases with latitude) applied by terrestrial rotation to upper air moving poleward above the Trade Wind Belt, would cause a buildup 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.

D CLIMATOLOGY

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 (lines of equal temperature) 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 (lines of equal precipitation); 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 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 Climatology appeared, which remained a standard until 1930–1940 when the five-volume work of the same title by Koeppen and Geiger replaced it. At the end of World War I Koeppen (1918) produced the first detailed classification of world climates based on terrestrial vegetation cover. This was followed by Thornthwaite’s (1931–1933) 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 and 1945) and micro-climatology, the study of the fine climate structure near the surface (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 L. Teisserenc de Bort produced the first mean global pressure maps showing the cyclonic and anticyclonic ‘centers of action’ on which the general circulation is based. In 1887 de Bort began producing maps of upper-air pressure distributions and in 1889 his world map of January mean pressures in the lowest 4km of the atmosphere was particularly effective in depicting the great belt of the westerlies between 30° and 50° north latitudes.

E MID-LATITUDE DISTURBANCES

Theoretical ideas about the atmosphere and its weather systems evolved in part through the needs of nineteenth-century mariners for inform...