Facts of the Matter
Earth's atmosphere a multilayered subject
EARTH'S atmosphere is a necessity for life, but we are familiar with only its bottom layer, the troposphere. Although we refer to the atmosphere as a "sea of air," it is much more complicated than a sea of water because of the freedom of motion of gas molecules and the solar radiation that they interact with.
The atmosphere becomes thinner and less dense at higher altitudes, but it never "stops." There is no universally accepted definition of how much air in a given volume of space it takes to constitute an atmosphere, so there is no way to establish an absolute upper boundary. It gradually merges with the sun's atmosphere, which pervades interplanetary space on a grander scale.
For convenience the "top" is thought of as 100 kilometers or 60 miles, which 99.99997 percent of the atmosphere lies below. Compared to Earth's 4,000 mile radius, the atmosphere is a thin gaseous skin, less than 2 percent of the radius, equivalent to two yards on a football field.
The composition of the atmosphere remains quite constant throughout the lower 50 miles, where nitrogen (78.08 percent) and oxygen (20.95 percent) dominate. Of the remaining 97 one-hundredths of one percent, the noble gases argon, neon, helium, krypton, and xenon predominate.
Hydrogen comprises only five one-hundred thousandths of one percent.
The variable gases, water vapor, carbon dioxide, ozone, methane and nitrous oxide are present in small amounts and affect the behavior of the atmosphere significantly in ways that are beyond the scope of this column, but we will get to them in the future.
Above 50 miles, hydrogen and helium become increasingly abundant. They are lightweight atoms and have higher thermal velocities than heavier atoms at any temperature. They slowly leak into space when their velocities exceed escape velocity.
The troposphere is the "mixing" layer. Its thickness averages 40,000 feet (seven to eight miles), depending on its average temperature, being thicker when it is warm. Because air is compressible, the troposphere contains 80 percent of the mass of the atmosphere despite being the thinnest of the four layers.
The troposphere is heated from the bottom by Earth's surface, which is heated by sunlight. The temperature decreases upward from about 59 degrees Fahrenheit at ground level (Earth's average global temperature) to about minus 71 degrees Fahrenheit at the top.
Convective movements within the troposphere mix the air and cause weather when water changes state via evaporation and condensation. The top of the troposphere is generally the highest altitude at which the air contains water vapor. One important exception is when powerful thunderstorm clouds extend into the stratosphere carried by violent updrafts.
ABOVE THE TROPOSPHERE is the stratosphere. There is very little mixing of the two layers except across the tropopause, a thin boundary layer between them where temperature is constant with height. This boundary layer represents a layer between heating from below by the ground in the troposphere and heating from above by ozone in the stratosphere.
The stratosphere is heated from the top down so its temperature increases with height. The "ozone layer" lies at the bottom of the stratosphere where the heavier ozone molecules accumulate and protect the surface from the damaging UV radiation.
Of the one-tenth of one percent of atmosphere that is not contained in the troposphere and stratosphere, the remaining 99.9 percent is in the mesosphere, the next layer above the stratosphere. Like the troposphere temperatures in the mesosphere decrease upward from its base at around 30 miles to top at around 55 miles. The mesosphere is heated from the bottom by the top of the stratosphere.
Beginning in the upper mesosphere and continuing into the thermosphere above, electrons are stripped from molecules of air. This is the ionosphere, and influences the distance that AM radio waves can travel. It is higher at night when there is no solar radiation to strip the electrons.
The thermosphere is where temperatures soar upward again, reaching in excess of 2,500 degrees Fahrenheit. The high temperatures do not mean that astronauts and rockets flying through it are "fried." The high temperatures are misleading when we forget the distinction between temperature and heat content.
Gas molecules in the thermosphere are moving very fast, but they are very sparse. A molecule at this height may typically move a kilometer or more before colliding with another molecule (compared with one ten-thousandths of a millimeter "mean free path" in the lower troposphere). With so few gas molecules, the air can not have a high heat content at any temperature.
A thermometer placed in the thermosphere would not measure the temperature correctly since it would have little contact with the surrounding air. The everyday concept of temperature has no meaning in the context of such low pressure.
The thermosphere expands and contracts with the seasons -- and daily to some extent. This interferes with the motion of satellites in near-Earth orbits, which are corrected to account for the changes.
Richard Brill, professor of science at Honolulu Community College, teaches earth and physical science and investigates life and the universe. E-mail questions and comments to firstname.lastname@example.org