DK Earth: Atmosphere
Most life on Earth depends on the atmosphere, a bubble of gases around our planet. This bubble, extending about 430 miles (700 km) into space, protects us from meteorites and warms Earth?s surface. It includes the OZONE LAYER, which shields us from the Sun?s harmful rays.
Earth?s atmosphere contains five main layers?the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. The main gases in the atmosphere are nitrogen (78 percent) and oxygen (21 percent). There are also small amounts of argon, carbon dioxide, and water vapor.
Atmospheric (air) pressure is the force produced by air as it pushes against its surroundings. This force is over 14 lb per sq in (1 kg per sq cm). We cannot feel it, however, because the air presses evenly from all directions, and our body fluids press outward. Air pressure is greatest at sea level and decreases with altitude.
Extending about 7 miles (12 km) into space, the troposphere contains 75 percent of the air and water in the atmosphere.
Lying 7?30 miles (12?50 km) above Earth?s surface, the stratosphere is a calm layer above the winds and weather. The lower stratosphere contains the ozone layer.
The mesosphere is 30?50 miles (50?80 km) above Earth?s surface. Meteors (fragments of rock and dust from space) burn up here, creating shooting stars.
The thermosphere extends 50?280 miles (80?450 km) into space and contains the ionosphere, a layer of electrically charged particles from which radio waves for communications can be bounced back to Earth.
A layer of ozone gas in the stratosphere protects us from harmful ultraviolet (UV) rays in sunlight. UV rays can cause skin cancer, eye damage, and other health problems in humans and animals. In the 1980s, scientists discovered that the ozone layer is getting thinner and that so-called ?holes? (areas containing less ozone) were appearing over Antarctica and the Arctic each spring.
Chemicals called CFCs (chlorofluorocarbons), used in the manufacture of refrigerators and aerosol sprays, are causing the holes in the ozone layer. CFCs collect in the upper atmosphere, where they destroy ozone. During the 1990s, the holes steadily got bigger. Most countries have now stopped using CFCs, which should prevent the damage from getting worse.