Clear Skies And Stormy Weather

Look up! tte clear air turns into the blue sky! Ms is because air molecules scatter blue sunlight more strongly than other colors, tte night sky is black because there is no sunlight to illuminate the air. And the sky on the Moon is black even when the Sun shines on it, for the Moon has no atmosphere to scatter sunlight.

FIG. 9.1 The water planet Almost three-quarters of the Earth's surface is covered with water, as seen in this view of the North Pacific Ocean. Earth is the only planet in the Solar System where substantial amounts of water exist in all three possible forms - gas (water vapor), liquid and solid (ice). Here white clouds of water ice swirljust below Alaska; the predominantly white ground area, consisting of snow and ice, is the Kamchatka Peninsula of Siberia. Japan appears near the horizon. From this orientation in space, we also see both the day and night sides of our home planet. (Courtesy of NASA.)

FIG. 9.1 The water planet Almost three-quarters of the Earth's surface is covered with water, as seen in this view of the North Pacific Ocean. Earth is the only planet in the Solar System where substantial amounts of water exist in all three possible forms - gas (water vapor), liquid and solid (ice). Here white clouds of water ice swirljust below Alaska; the predominantly white ground area, consisting of snow and ice, is the Kamchatka Peninsula of Siberia. Japan appears near the horizon. From this orientation in space, we also see both the day and night sides of our home planet. (Courtesy of NASA.)

FIG. 9.2 Global biosphere This global view of the Earth's biosphere portrays life on land and sea. The ocean portion shows the chlorophyll concentration arising from phytoplankton, microscopic plants that grow in the upper sunlit portions of the ocean and are the ultimate source of food for most marine life. At sea, the red and orange colors denote the highest concentration of plankton; yellow and green represent areas of moderate plankton concentration, and blue and violet describe the lowest concentrations. The land vegetation image shows rain forests (darkgreen areas), tropical and sub-tropical forests (lightgreen) and areas of low vegetation and deserts (yellow). This image combines ocean measurements from the Coastal Zone Color Scanner, abbreviated CZCS, aboard the Nimbus-7 satellite, collected from 1978 to 1986, and three years of land measurements from the Advanced Very High Resolution Radiometer, or AVHRR for short, flown on NOAA-7, which was launched on 7 June 1981. (Courtesy of G.C. Feldman and C.J. Tucker, NASA.)

FIG. 9.2 Global biosphere This global view of the Earth's biosphere portrays life on land and sea. The ocean portion shows the chlorophyll concentration arising from phytoplankton, microscopic plants that grow in the upper sunlit portions of the ocean and are the ultimate source of food for most marine life. At sea, the red and orange colors denote the highest concentration of plankton; yellow and green represent areas of moderate plankton concentration, and blue and violet describe the lowest concentrations. The land vegetation image shows rain forests (darkgreen areas), tropical and sub-tropical forests (lightgreen) and areas of low vegetation and deserts (yellow). This image combines ocean measurements from the Coastal Zone Color Scanner, abbreviated CZCS, aboard the Nimbus-7 satellite, collected from 1978 to 1986, and three years of land measurements from the Advanced Very High Resolution Radiometer, or AVHRR for short, flown on NOAA-7, which was launched on 7 June 1981. (Courtesy of G.C. Feldman and C.J. Tucker, NASA.)

When high in the sky, the Sun is colored yellow. At sunset the Sun's rays pass through a maximum amount of atmosphere; most of the blue light is then scattered out of our viewing direction, and the setting Sun is colored red. Dust in the air also helps redden sunsets.

Over the eons, our planet has been shaped and re-shaped by nature's powerful forces, and "air-conditioned" by several types of cycles. In the oxygen cycle, animals fill their lungs with oxygen, breathing it in from the air, while plants replenish the oxygen in our atmosphere.

In another global cycle, atmospheric carbon dioxide is dissolved in the ocean waters and taken up by plants; animals, volcanoes and burning coal, oil and natural gas return carbon dioxide to the air, completing the cycle. Between 350 million and 65 million years ago, great quantities of carbon were stored deep underground, when plants and animals died and decayed and their remains were compressed into what eventually became coal, oil and natural gas. ttis carbon was removed from the carbon cycle, until about 200 years ago when humans started using the fossil fuels and releasing carbon dioxide into the atmosphere again.

tte most obvious of these grand circulations is the water cycle, which is powered by the Sun's energy and involves interactions between the oceans, atmosphere and

FIG. 9.3 Altamaha river delta, Georgia The history of the Sea Islands in the Altamaha River delta on the coast of Georgia is revealed in this radar image. The outlines of long-lost plantation rice fields, canals, dikes and other inlets are defined. Salt marshes are shown in red, while dense cypress and live oak tree canopies are seen in yellow-greens. Agricultural development of the Altamaha delta began soon after the founding of the Georgia Colony in 1733. The first major crop was indigo, followed by rice and cotton. A major storm in 1824 destroyed much of the town of Darien (upper right). The Civil War (1861-1865) ended the plantation system, and many of the island plantations disappeared under heavy brush and new growth pine forests. The Butler Island (center left:) plantation became a wildlife conservation site growing wild sea rice for migrating ducks and other waterfowl. (AIRborne Synthetic Aperture Radar, abbreviated AIRSAR, image courtesy of NASA, JPL and the University of Edinburgh.)

FIG. 9.3 Altamaha river delta, Georgia The history of the Sea Islands in the Altamaha River delta on the coast of Georgia is revealed in this radar image. The outlines of long-lost plantation rice fields, canals, dikes and other inlets are defined. Salt marshes are shown in red, while dense cypress and live oak tree canopies are seen in yellow-greens. Agricultural development of the Altamaha delta began soon after the founding of the Georgia Colony in 1733. The first major crop was indigo, followed by rice and cotton. A major storm in 1824 destroyed much of the town of Darien (upper right). The Civil War (1861-1865) ended the plantation system, and many of the island plantations disappeared under heavy brush and new growth pine forests. The Butler Island (center left:) plantation became a wildlife conservation site growing wild sea rice for migrating ducks and other waterfowl. (AIRborne Synthetic Aperture Radar, abbreviated AIRSAR, image courtesy of NASA, JPL and the University of Edinburgh.)

land. It brings us our daily weather and produces the climatic differences that starkly distinguish one part of the globe from another.

About one-third of the solar energy reaching the Earth's surface as sunlight is expended on the evaporation of seawater. ttis evaporation releases warm fresh-water moisture into the air and cools the surface of the ocean, tte moisture rises high into the cold atmosphere, where clouds are formed. Winds drive the clouds for great distances over sea and land. Rain or snow from the clouds can then fall to land, refreshing streams, lakes and underground reservoirs, tte water then runs down to the sea, where the cycle begins once more.

All of the water in the oceans passes through this water cycle once in 2 million years. Yet, the ocean waters are at least 3.5 billion years old, so they have, on average, completed more than a thousand such cycles.

Sunlight illuminates and warms one side of our rotating globe at a time. And because the Earth is spherical, the tropical regions near the equator face the Sun more directly, receiving the greatest amount of heat. At these low latitudes, the Sun's almost vertical rays travel to the ground through the least amount of intervening air. At higher latitudes nearer

FIG. 9.4 New Guinea This Synthetic Imaging Radar, abbreviated SIR, image shows rivers emptying into the sea on the southern coast of the Indonesian half of New Guinea. It was taken from SIR-A aboard the Space Shuttle Columbia in 1981, demonstrating the use of imaging radar to acquire and transmit data of different geologic regions. (Courtesy of NASA and JPL.)

FIG. 9.4 New Guinea This Synthetic Imaging Radar, abbreviated SIR, image shows rivers emptying into the sea on the southern coast of the Indonesian half of New Guinea. It was taken from SIR-A aboard the Space Shuttle Columbia in 1981, demonstrating the use of imaging radar to acquire and transmit data of different geologic regions. (Courtesy of NASA and JPL.)

the poles, sunlight strikes the ground at a glancing angle and it must also penetrate a greater thickness of absorbing atmosphere, so the ground is heated less than at the equator.

Winds and ocean currents attempt to correct the temperature imbalance, carrying heat from the equator to the poles, or from the warmer to colder places, in both northern and southern directions. Warm air rises at the equator, circulates toward the poles, eventually cools and sinks, and then flows back toward the equator at lower levels near the ground. Two loops of flowing air are formed, one in each hemisphere; and they are named Hadley cells after the English meteorologist George Hadley (1685-1768) who first proposed them in 1735. tte two Hadley cells circulate north or south, but they are deflected sideways by the Earth's rotation, forming the low-latitude trade winds that blow westward almost every day of the year.

But climate isn't just determined by how the air circulates, tte land and oceans also play a role. Warm water, for example, flows from the tropics to the Arctic and Antarctic, helping move heat around the globe and determining world climate.

And in another of nature's grand transformations, the great landmasses on Earth are continually reorganizing, growing in places and eroding away or breaking apart in others. Over long periods of time, measured in millions of years, entire continents and oceans can be destroyed or created anew, remodeling the entire surface of the Earth.

FIG. 9.5 A thin colored line Brilliant red and blue mark the thin atmosphere that warms and protects us, as viewed from space at sunrise over the Pacific Ocean. Without this atmospheric membrane we could not breathe and our lake and ocean waters would freeze. (Courtesy of NASA.)

ttis results in a changing flow of air and ocean currents that create entirely different global climate patterns.

Nowadays, and in all former times, it is the Sun-driven seasons that dominate our weather, ttey are created by the tilt of the Earth's rotational axis, together with our planet's annual circuit around the Sun. In each hemisphere, the greatest sunward tilt defines the summer part of its orbit when the Sun is more nearly overhead and its rays strike the surface more directly than in the winter part when that hemisphere is tilted away from the Sun (Fig. 9.6). In fact, the word climate comes from the Greek word klima, for "tilt."

Ms annual change in solar radiation produces large seasonal temperature fluctuations in the Northern Hemisphere where most of the world's land is now found, tte difference in surface air temperature, averaged over the northern hemisphere, between winter and summer, is 15 kelvin. Temperatures measured on the more familiar Celsius scale, denoted C, are given by C = K—273, where K denotes the temperature in kelvin units. So a temperature difference of 15 kelvin is also a temperature difference of 15 degrees Centigrade, and it is this difference that brings the climatic seasons, which mark the passage of time.

Without both sunlight and our thin atmosphere, there would be no blue skies or red sunsets, no clouds or rain, and no climates or seasons. And without our atmosphere, the oceans would be frozen solid.

FIG. 9.6 The seasons As the Earth orbits the Sun, the Earth's rotational axis in a given hemisphere is tilted toward or away from the Sun. This variable tilt produces the seasons by changing the angle at which the Sun's rays strike different parts of the Earth's surface. The greatest sunward tilt occurs in the summer when the Sun's rays strike the surface most directly. In the winter, the relevant hemisphere is tilted away from the Sun and the Sun's rays obliquely strike the surface. When it is summer in the northern hemisphere, it is winter in the southern hemisphere and vice versa. (Notice that the radius of the Earth and Sun and the Earth's orbit are not drawn to scale.)

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