Earth and the Environment

Earth The third planet in the solar system, outwards from the Sun. The mean distance of the Earth from the Sun is 149.6 × 106 km. This distance provides the standard ‘astronomical unit’ (AU) of measurement. The Earth has a mean radius of 6371 km, density of 5517 kg/m3, and a mass of 5.99 × 1027 g. The oceanic (5–7 km thick) and continental (40 km thick) crusts are separated by the Mohorovičić discontinuity from the silicate mantle, which extends to the Gutenberg discontinuity at 2900 km depth, and overlies a molten, iron-rich core. The oldest rocks are about 3980 million years old, and the Earth formed about 4600 million years ago.

atmosphere [Gr.,=sphere of air], the mixture of gases surrounding a celestial body with sufficient gravity to maintain it. Although some details about the atmospheres of other planets and satellites are known, only the earth's atmosphere has been well studied, the science of which is called meteorology . Components and Characteristics of the Earth's Atmosphere The first 40 to 50 mi (64-80 km) above the earth contains 99% of the total mass of the earth's atmosphere and is generally of a uniform composition, except for a high concentration of ozone, known as the ozone layer , at 12-30 mi (19-50 km). Calculated according to their relative volumes, the gaseous constituents of the atmosphere are nitrogen, 78.09%; oxygen, 20.95%; argon, 0.93%; carbon dioxide, 0.03%; and minute traces of neon, helium, methane, krypton, hydrogen, xenon, and ozone. The lower atmosphere contains varying amounts of water vapor, which determine its humidity . Condensation and sublimation within the atmosphere cause clouds or fog, and the resulting liquid water droplets or ice crystals may precipitate to the ground as rain, sleet, snow, hail, dew, or frost. The air also carries many kinds of dust , of meteoric as well as terrestrial origin, and microorganisms, pollen, salt particles, and various gaseous and solid impurities resulting from human activity (see pollution ). Because of the pull of gravity the density of the atmosphere and the pressure exerted by air molecules are greatest near the...

tornado dark, funnel-shaped cloud containing violently rotating air that develops below a heavy cumulonimbus cloud mass and extends toward the earth. The funnel twists about, rises and falls, and where it reaches the earth causes great destruction. The diameter of a tornado varies from a few feet to a mile; the rotating winds may attain velocities of 200 to 300 mi (320-480 km) per hr, and the updraft at the center may reach 200 mi per hr. The Enhanced Fujita scale is the standard scale for rating the severity of a tornado as measured by the damage it causes. A tornado is usually accompanied by thunder, lightning, heavy rain, and a loud "freight train" noise. In comparison with a cyclone or hurricane , a tornado covers a much smaller area but can be violent and destructive. The atmospheric conditions required for the formation of a tornado include great thermal instability, high humidity, and the convergence of warm, moist air at low levels with cooler, drier air aloft. Although tornadoes have occurred on every continent except Antarctica, they are most common in the continental United States, where tornadoes typically form over the central and southern plains, the Ohio valley, and the Gulf states. The area where the most violent storms commonly occur in the United States is known as Tornado Alley, which is usually understood to encompass the plains from N central Texas north to the Dakotas, with the peak frequency located in Oklahoma. A tornado typically travels...