Chapter One

An Ocean of Air

Regardless of age or profession, location or wealth, all humans breathe the air. One can survive more than a month without food and even several days without water, but four short minutes without oxygen brings death. People even bottle the atmosphere to carry along when they dive into the oceans, climb the highest mountain peaks, or venture into outer space.

All life on Earth—people, plants, and animals—lives at the bottom of a vast ocean of air called our atmosphere. Although composed of gases, the atmosphere has distinct layers, like the ocean. It even has currents: jet streams and other prevailing winds. The atmosphere also interacts intricately with both the ocean and land masses.

Compared with the radius of the Earth (about 3,963 miles or 6,378 km), the atmosphere is quite shallow. Yet this thin layer of gases shields Earth from high-speed celestial rocks (meteorites) and harmful radiation, and provides oxygen for breathing—making life possible on this planet.

The atmosphere is also home to Earth's weather. Weather wild and majestic shapes our planet. Although we can understand or predict the weather, we still can neither tame nor control it. Farmers still eagerly beseech the sky for rain, and coastal residents in hurricane areas still tape their windows and evacuate their homes. And all humans pause to stand in awe before a brilliant rainbow or glorious sunset.

Layer-Cake Atmosphere

With all the winds that sweep across land and sea, you'd think that the atmosphere would be all mixed up and of fairly homogenous composition.

But you'd be wrong. Earth's atmosphere is highly structured. In fact, it can be viewed as a five-layer cake hundreds of miles thick. Each layer has its own distinct chemical composition, movement, density, and changes in temperature. Each layer of the atmosphere is bounded by "pauses" where the changes in its physical properties are most abrupt.

How thick is the atmosphere? There seem to be several answers. Fully 99 percent of the atmosphere's mass and moisture are in the two lowest layers; together, these layers are only some 30 miles (48 km) thick. Scientists differ on where the nominal edge of outer space begins, placing it between 50 and 62 miles' (about 80–100 km) altitude—a region where meteors are still incinerated by outlying atmospheric molecules. Many spacecraft in low Earth orbit 150 to 300 miles (240–480 km) high are slowed by the friction of air molecules rubbing against them as they pass through the tenuous uppermost layers of the atmosphere where auroras (northern and southern lights) flicker and dance. In fact, atmospheric friction is so significant that some spacecraft have spiraled down to Earth unless they are periodically reboosted to the desired altitude.

Temperature vs. Heat

Most people equate temperature with the heat or cold they feel—a winter's day with temperatures below freezing is frigid, whereas a summer's day with temperatures topping 100°F (38°C) is sweltering. But in meteorology and other sciences, the definition of "temperature" differs from common usage. That difference is essential to understanding discussions about the atmosphere, the oceans, and climate.

Temperature and heat are related, but they are not the same thing. A burning match has a much higher temperature than a steam radiator, but the match can't heat a living room. Boiling water just poured from a teakettle to make a cup of tea has the same temperature as the water in the kettle, but more ice cubes could be melted in the kettle than in the teacup because the kettle is so much bigger than a cup. Thus, something small at a high temperature may not give off much heat. Conversely, something massive at a lower temperature may give off a great amount of heat.

Generally speaking, temperature is a measure of a body's ability to either give up its heat to, or to absorb heat from, other bodies. Drop an ice cube into hot tea, and two things happen: The tea gives up heat to the ice, and the ice absorbs heat from the tea. There is a temperature difference between the two bodies. After the ice melts and the tea cools, the mixture reaches thermal equilibrium, which will be at an intermediate temperature. The word "temperature" actually has different specific definitions, depending on methods used for measuring it.

What is felt as heat or cold from a body, however, is actually the body's thermal energy: specifically, the kinetic energy of its constituent atoms and molecules resulting from their constant motion. Even when fixed in a crystal lattice of solid ice, water molecules are jiggling in place—and that movement grows faster and more furious if the ice is heated. Eventually the kinetic energy of the water molecules is so high that it breaks the molecular bonds holding them in a crystal lattice; that's when the ice begins to melt. When the liquid water is heated, its molecules move ever faster until—at water's boiling point—they begin to escape as a gas, or steam, in the case of water.

Thermal energy, or heat felt, is related to a body's density (mass and volume), whereas temperature is not. The Sun's beautiful outer corona—the silvery halo seen during a total solar eclipse—has a much higher temperature (1,800,000°F, or 1,000,000°C) than the surface of the Sun (about 10,000°F, or 5,500°C). But the corona's density is so low, maybe only 1 atom per cubic centimeter—scarcely greater than that of interplanetary space—that if you could put your hand into it, it would not even feel warm. Perception of heat depends not only on the kinetic energy of individual atoms, but also on the density of molecules hitting your skin.

Expanding and Contracting

Two other temperature-related concepts are vital to understanding weather and climate.

First, with few exceptions, solids and liquids will expand when heated, and contract when cooled, the amount of expansion and contraction varying with the substance. Such thermal expansion happens because the molecular bonds holding the constituent atoms and molecules behave rather like . . .

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Excerpted from Weather by Trudy E. Bell
All rights reserved by the original copyright owners. Excerpts are provided for display purposes only and may not be reproduced, reprinted or distributed without the written permission of the publisher.