Layers of the Atmosphere
The lowest level, the troposphere, is where we live and where weather events happen. Seventy-five percent of the total mass of air is located in this layer, as well as almost all the water vapor and pollution. The name “troposphere” comes from the Greek “tropos,” or “turn.” This refers to the convective flow or mixing that occurs in this layer.
Air temperature in the troposphere decreases uniformly as altitude increases, except for an inversion over the winter pole. Generally, wind speed in the troposphere increases with altitude. However, near the surface the topography may interfere with this pattern.
At approximately 12 km above the earth (the exact altitude depends on the latitude and the season) lies the tropopause. This layer marks the upper limit of convection and therefore the “ceiling” of weather phenomena. There are a few cloud types which can occur in the next layer up, but they are very rare.
Just below the tropopause are the jet streams, paths of increased winds that move over the surface. Examples of these are the tropical jet stream and the subtropical jet which is further north. The jet streams influence the movement of air masses. For instance, a southern dip of the subtropical jet in the central United States will allow continental polar (cP) air to move southward, causing temperatures to drop significantly.
The stratosphere is the atmospheric layer above the tropopause. This layer contains ozone or O3, which is considered pollution when it is in the troposphere but is essential to human life in the stratosphere. Ozone has the ability to absorb ultraviolet rays from the sun, rays which can damage human skin and destroy life. The overall amount of ozone in the stratosphere has been steadily decreasing since the 1970’s, and “holes” in the ozone layer were discovered over the polar regions in the 1980’s. The loss of ozone has significant implications for human life, and as a result, governments have made laws to help protect the ozone that is left.
In the stratosphere the pattern of temperature change is opposite that of the troposphere. From the top of the tropopause to the bottom of the stratopause the temperature of the air may rise 60˚ C. This inversion is due to warming of the ozone as it absorbs UV radiation.
The stratopause is approximately 50 km above the earth, and the next layer is the mesosphere. In this layer the early pattern of decreasing temperature is resumed until the mesopause is reached at 80 km.
The thermosphere contains atomic oxygen, or O, which absorbs radiation of smaller wavelengths than the radiation absorbed by the ozone layer. This includes X-rays and cosmic rays, which have so much energy that they are more dangerous to humans than ultraviolet radiation. In the thermosphere there is once again a temperature inversion, as the oxygen is warmed by radiation.
Beyond the thermosphere is the ionosphere, the highest and most rarefied layer of the atmosphere. It stretches to approximately 300 km; beyond that, air is imperceptible and we speak of the “vacuum of space.” The ionosphere is the layer where the aurora borealis and aurora australis occur. The auroras, or polar lights, result from charged particles or ions from the sun colliding with the earth’s magnetosphere, the magnetic field around the planet. Most of the light produced is red or green, giving evidence of atomic oxygen. Occasionally, nitrogen ions will cause the aurora to appear blue or violet.
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