fig. 13-1 The variation of atmospheric pressure with altitude. The millibar (mb) is the pressure unit used in meteorology, where 1 mb = 100 Pa = 1 hPa.

fig. 13-2 The variation of temperature with altitude. The altitudes of the boundaries between regions of the atmosphere are averages.

fig. 13-3 The world's water content and its daily cycle. Upward arrows indicate evaporation; downward arrows indicate precipitation. If all the water vapor in the atmosphere were condensed, it would form a layer only about 2.5 cm thick.

fig. 13-4 How the mass of water vapor per cubic meter of air varies with temperature for various relative humidities. Each curve corresponds to a different relative humidity. The curve for 100% relative humidity represents saturation, when the air can hold no more water vapor.

fig. 13-5 Why mountains often have cloud caps over them.

fig. 13-6 Every object gives off electromagnetic (em) radiation. These curves show how the intensity of the radiation varies with wavelength for objects at the temperatures indicated. The predominant wavelength decreases as temperature increases. Em radiation from the sun is mostly visible light; em radiation from the earth is mostly infrared light.

fig. 13-7 The greenhouse effect. Much of the energy in the short-wavelength visible light from the sun that is absorbed by the earth's surface is in turn radiated by the earth as long-wavelength infrared light that is absorbed by CO₂ and H₂O in the atmosphere. Some energy also reaches the atmosphere b contact with the earth and by means of water evaporated from the sea. Thus the atmosphere is heated mainly from below by the earth rather than from above by the sun. On the average, the total energy the earth and its atmosphere radiate into space equals the total energy they receive from the sun.

fig. 13-8 The equatorial regions of the earth are on the average warmer than the polar regions because at the equator the sun's rays are spread over a smaller surface.

fig. 13-9 The annual balance between incoming solar radiation and outgoing radiation from the earth. More energy is gained that lost in the tropical regions, and more energy is lost than gained in the polar regions. The latitude scale is spaced so that equal horizontal distances on the graph correspond to equal areas of the earth's surface.

fig. 13-10 The seasons are caused by the tilt of the earth's axis together with its annual orbit around the sun. As a result, the daylight side of the northern hemisphere is tilted away from the sun in January, which means that sunlight strikes this hemisphere at a glancing angle and delivers less energy to a given area than in June. The seasons are reversed in the southern hemisphere.

fig. 13-11 Convection currents are produced by unequal heating. The temperature of a land surface rises more rapidly in sunlight than the temperature of a water surface. The resulting convection produces the sea breeze found on sunny days near the shores of a body of water. At night, the land cools more than the sea, and the convection current is reversed to give a land breeze that blows offshore.

fig. 13-12 (a) Because of the coriolis effect, which is a consequence of the earth's rotation, winds in the northern hemisphere are deflected to the right. As a result, air does not flow directly toward the center of a low-pressure region but spirals inward in a counterclockwise direction. (b) Similarly, air flows away from the center of a high-pressure region in a clockwise spiral. In the southern hemisphere these directions are reversed. An isobar is a line of constant pressure on a weather map; it corresponds to a contour line of constant altitude on an ordinary map.

fig. 13-13 (a) The summer monsoon of India and south Asia. Heating of the land produces a low-pressure region (L) centered inland and a high-pressure region (H) centered in the Indian Ocean that together cause southwest winds to occur. Rice cultivation in this part of the world depends on the warm, moist air brought by the summer monsoon. (b) The winter monsoon. Now the land is cooler than the ocean, so the low- and high-pressure regions are reversed to give dry northeast winds.

fig. 13-14 The convectional circulation that would occur if the earth did not rotate. The arrows in the center of the diagram indicate surface winds.

fig. 13-15 Simplified pattern of horizontal and vertical circulation in the actual atmosphere. Regions of high and low pressure are indicated.

fig. 13-16 Average January sea-level pressures (in millibars) and wind patterns. High- and low-pressure systems are indicated. Isobars connecting points of equal pressure are shown in white.

fig. 13-17 The air masses that affect weather in North America. The importance of the various air masses depends upon the season. In winter, for instance, the continental tropical air mass disappears and the continental polar air mass exerts its greatest influence.

fig. 13-19 Weather maps show pressure patterns, winds, rain, and snow. This is a weather map of the eastern United States one April morning. A cold air mass on the west and north (polar continental air) is separated from a warm air mass (tropical maritime air) by a cold front extending from Louisiana to Michigan and by a warm front from Michigan to Virginia. Where the north end of the warm air mass lies between the two fronts a cyclone has formed, bringing rain (colored area) to the Great Lakes region. The unit of pressure in this map is the millibar. The small circles indicate clear skies; solid dots indicate cloudy skies. The small lines show wind direction, which is toward the circle or dot, and wind strength; the greater the number of tails, the faster the wind.

fig. 13-20 Cross-section diagrams of (a) a warm front and (b) a cold front. In each case the front is moving to the right. Photographs of the various cloud forms appear on P. 000.

fig. 13-21 Why mountains often have cloud caps over them.

fig. 13-22 The track of Hurricane Emily in September 1987, showing the positions of the storm center at 24-h intervals. Wind speeds of up to 200 km/h (124 mi/h) were recorded. This track is typical of North Atlantic hurricanes, which are born near the bulge of Africa and grow in strength by absorbing energy from the warm tropical water as they are swept westward by the trade winds. They then turn north to curve around the permanent high-pressure region of the mid-Atlantic, and finally are blown eastward by the westerlies of the higher latitudes until they lose energy and die out. When such a hurricane swings over land, its winds can do immense damage. Seawater surging ashore to flood low-lying coastal area is even more destructive and causes more hurricane deaths.

fig. 13-23 Worldwide average annual surface temperatures since 1900. The average in 1997 was 16.9° C. The figures combine land and ocean measurements. The sharp drop in 1991 was triggered by the eruption in June of that year of Mt. Pinatubo, a Philippine volcano. The eruption sent a huge amount of debris into the atmosphere where it spread around the world and acted as a partial shield for solar energy for two years.

fig. 13-24 Three variations in the earth's motion that may be responsible for causing ice ages. (a) The time of year when the earth is nearest the sun varies with a period of about 23,000 years. (b) The angle of tilt of the earth's axis of rotation varies with a period of about 41,000 years. (c) The shape of the earth's elliptical orbit varies with a period of about 100,000 years. These variations have relatively little effect on the total sunlight reaching the earth but a considerable effect on the sunlight reaching the polar regions in summer.

fig. 13-25 Carbon dioxide concentration in the atmosphere since 1860, in parts per million (ppm). Measurements made on bubbles of air trapped in Antarctic ice deposits show that there was little change in CO₂ concentration in the ten thousand years before 1860.

fig. 13-26 Profile of the earth's surface with heights and depths in km. The vertical scale is greatly exaggerated: the earth is actually quite smooth. Although the earth's circumference is about 40,000 km, the vertical distance from the deepest point in the oceans to the top of the highest mountain is less than 20 km. A car on a level road can easily go this far in 15 min.

fig. 13-27 Principal ocean currents of the world.