Atmospheric Structure

The atmosphere has 4 distinct layers: 1) Troposphere, 2) Stratosphere, 3) Mesosphere, and 4) the Thermosphere. These layers are distinguished from one another by the a) mass budget [total amounts and types of elemental and molecular species], and b) changes in air motion that occur within each layer c) changes in temperature with change in altitude. These changes in the physical characteristics of air exhibit how Earth interacts with the Sun to produce our atmosphere, control our climate, distribute elements and molecules, and protect the biosphere of Earth.

The troposphere contains approximately 75% of the total mass of the atmosphere including essentially 100% of the water vapor and particulate matter. The average thickness of the troposphere is 7 miles, ranging from 5 miles thick at the poles to 11 miles at the equator. The air constantly overturns in the troposphere in regular patterns as a response to the amount of energy received from the sun; thus distributing heat, moisture, and particulate matter (pollution, volcanic debris) about Earth’s’ surface. Temperature decreases uniformly with positive changes in altitude at a rate of ~ 16 C per mile. Therefore the temperature at the top of the troposphere is, on average, 112 C colder than the surface.

Above the troposphere is a layer of constant temperature, the lower stratosphere. The stratosphere has two temperature regimes, in which the upper stratosphere differs by showing a slight temperature increase with positive changes in altitude. This temperature inversion is the result of two factors: 1) the air has little input of cool moisture from the troposphere, and 2) a combination of warming from the sun with the low air overturn rates create an insulating affect where less energy penetrates to the lower stratosphere. Solar energy also creates ozone in the stratosphere. Ozone production is concentrated near the lower and upper stratosphere boundary at about 15 miles above Earth’s surface. The upper boundary the stratosphere is nearly 28 miles above Earth’s surface, and therefore is a 15-20 mile thick lid of heated air that acts as a barrier to space and the troposphere. The stratosphere is a significant barrier because its static air masses help contain the troposphere’s moisture and buffers solar energy.

Above the stratosphere is the mesosphere. The mesosphere is rather unremarkable except that air temperatures reach a chilling -93 Celsius at the top. It is a 28-mile thick column of air that extends to a distance of 55 miles above Earth. The physical characteristics of air change dramatically above mesosphere that result from the extremely low atmospheric pressures acting on the mixture of gases we call air.

The thermosphere extends from the mesosphere to the outer edge of the atmosphere. Here low pressures result in expanded gases that do not interact, otherwise known as thin air. The thermosphere is so called because it was predicted that temperatures would be very high here do to solar inputs, but because there is few molecules available to trap energy it is like heating a large room with a handful of sparks spread about that room. Two popular physical phenomena occur in the thermosphere: Aurora Borealis, and aurora Austrialis. These phenomenon occur at the north and south poles, respectively; and are a result of the input of solar particles cascading through the thermosphere. Solar particles are charged ejecta from the sun, which can be concentrated in showers during solar magnetic storms. In northern latitudes we call this glowing solar rain the northern lights.

The four inner spheres of the atmosphere connect at zones of transition that are about a mile thick. A transition zone is called a pause, because a pause in normal behavior occurs there. Therefore, the tropopause marks the border between the tropopause and the stratosphere and so on for the stratopause and the mesopause. The outer border of the thermosphere is marked by a fringe contact of our atmosphere and space, the ionosphere. Unfiltered x-rays, that is not interfered with by air or mass, and ultraviolet energy from the sun ionize the atoms of gases in this fringe zone. The resultant charged atoms bounce back all radio signals having a shorter wavelength than 30MHz. Thus short wave communications may occur over long distances, and the rest like television signals pass into space.

Illustration by Kim Gardner, 2002