Troposphere to Outer Space

The depth of the atmosphere is difficult to define because it has no clear outer boundary; the top­most layer - the exosphere - becomes increasingly rarefied, gradually merging into outer space. The troposphere, the lowest layer, contains about 80 per cent of the total mass of the atmosphere. It extends up to about 8km over the poles, 10 to 11km over the middle latitudes, and 18km over the equator, where the greatest heating takes place.

The troposphere is the atmospheric zone of most interest to meteorologists, because it con­tains almost all of the water vapor and because most of the various weather phenomena occur in it. In the troposphere temperatures generally fall with increasing altitude; near the upper boundary (the tropopause), however, they stabilize at about -57°C, although this may vary by as much as 20°C. Above the tropopause is the stratosphere.

In the middle latitudes, strong winds rotate around the Earth in shifting bands from west to east. These winds are concentrated in the upper troposphere and lower stratosphere. Called the circumpolar vortex, or jet streams, these winds blow between the permanent areas of low press­ure over the poles (caused by the sinking of cold air) and permanent areas of high pressure over the tropics (caused by ascending warm air).

These high-level jet streams are fairly regular, because they are not subject to friction with land surfaces nor are they affected by various other fac­tors that complicate wind flows near the Earth's surface. The jet streams often move as fast as 290km/h and so they are of great importance to aircraft. On long flights a subsonic jet plane can save up to one hour and 10 tons of fuel if it uses routes with the strongest tail winds or the weakest headwinds. First discovered by high-flying aircraft during World War II, the jet streams are now known to have a considerable influence on the weather at ground level in middle latitudes.

The stratosphere extends from the tropopause to about 50km above ground level. In this zone is the important ozone layer, where heat is generated by the absorption of ultraviolet radiation. Hence, although temperatures are stable in the lower stratosphere, they increase steadily at higher levels, reaching about -10°C at the stratopause.

Between 50 and 500km above the surface is the rarefied ionosphere, which is often divided into the mesosphere (50 to 80km) and the thermosphere (80 to 500km). In the mesosphere tempe­ratures fall again, reaching about -80°C at the mesopause (the boundary between the mesophere and the thermosphere). But in the thermosphere, they increase steadily with height. This pheno­menon occurs because at an altitude of about 200km a layer of atomic oxygen (0) absorbs ultraviolet radiation.

In addition to ultraviolet radiation, the ionosphere is also bombarded by cosmic radiation and solar X-rays, which cause the gases in the ionosphere to ionize (that is, the gas molecules are changed into electrically-charged particles called ions). Brilliant displays of colored lights in the sky - called the Aurora Borealis in the Northern Hemisphere and the Aurora Australis in the Southern Hemisphere - occur when streams of electrically-charged particles from the Sun (the solar wind) ionize the atmos­pheric gases. Aurorae are normally visible only relatively near the poles and are usually accom­panied by magnetic storms.

Beyond 500km above the Earth's surface is the very rarefied exosphere, which consists only of scattered atoms of oxygen, hydrogen and helium.