The Earth did not always have the same atmosphere we are so fond of breathing today. The earliest atmosphere was rich in carbon dioxide and water vapor with smaller amounts of hydrogen, helium, methane and ammonia. As the oceans grew in size, more and more carbon dioxide was dissolved into the water. More carbon dioxide combined with compounds to form silicon dioxide, limestone, calcium carbonate and other mineral sediments. This began to leave the atmosphere rich in nitrogen.
The removal of all that carbon dioxide was an important step in Earth's history. Too much carbon dioxide in the atmosphere will trap heat by a process called the greenhouse effect. Sunlight coming in warms the surface. Heat then radiates from the surface as infrared radiation. But carbon dioxide will not allow infrared radiation to escape and so the planet continues to heat up with no way for the heat to escape. Ultraviolet radiation from the Sun removed the methane and ammonia.
Our current atmosphere consists of about 79% nitrogen, 20% oxygen and tiny amounts of water vapor, carbon dioxide, ozone and argon. The atmosphere reaches about 150 km above our planet. It also thins as elevation increases, humans have a difficult time breathing and functioning at high elevations without oxygen support.
Scientists have divided our atmosphere into layers determined by variation of temperature with height. In the lowest level, the Troposphere, temperature decreases with height up to about 10 km. The troposphere is the layer where our weather occurs
Above the troposphere is the Stratosphere and here, the temperature increases with height. The increase in temperature is caused by absorption of ultraviolet radiation by the ozone layer which is at the top of the stratosphere.
The mesosphere is the next layer up in the atmosphere. Again the temperature decreases with height. Above this is the ionosphere, the highest layer in the atmosphere and another layer where temperatures increase with height due to ultraviolet radiation. This layer is also filled with charged particles. The ionosphere was important in communications. Before satellites were in use, the only way to send radio signals beyond the horizon was to bounce them off the ionosphere.
There are a number of forces which drive atmospheric circulation. The main driving force is heat from the Sun that is re-radiated back into the atmosphere from the surface. There is uneven heating because continents are warmer than the oceans. It is because of this uneven heating that there are high and low pressure areas in the atmosphere.
As an example, consider air at the equator. It is hot and so rises. Because it is rising, the atmospheric pressure at the equator is low. Air from nearby high pressure areas will move toward the low pressure areas, creating winds. Now add the complication introduced by the Earth's rotation. The effect caused by rotation is the Coriolis effect. This causes winds in the northern hemisphere to veer toward the east, and winds in the southern hemisphere to veer towards the west.
Low pressure occurs when atmospheric cells rise and high pressure occurs when the cells fall. The upward and downward movements within the cells at latitudes near 30 and 60 degrees produce the high speed winds we call the jet stream.
Copyright © 1997 Kathy A. Miles and Charles F. Peters II