Imagine taking a trip upward into the atmosphere in a hot-air balloon. You begin on a warm beach near the ocean, at an altitude of 0 kilometer above sea level. You near a roar as the balloon’s pilot turn up the burner to heat the air in the balloon. The balloon begins to rise, and Earth’s surface gets farther and farther away. As the balloon rises to an altitude of 3 kilometers, you realize that the air is getting colder. As you continue to rise, the air gets colder still. At 6 kilometers you begin to have trouble breathing. The air is becoming less dense. It’s time to go back down.
What if you could have continued your balloon ride up through the atmosphere? As you rose higher, the air pressure and temperature would change dramatically. Scientist divide Earth’s atmosphere into four main layers classified according to changes in temperature. These layers are the troposphere, the stratosphere, the mesosphere, and the thermosphere.
You live in the inner, or lowest, layer of Earth’s atmosphere, the troposphere (TROH puh sfeer). Tropo- means “turning” or “changing.” Conditions in the troposphere are more variable than in the other layers. The troposphere is the layer of the atmosphere in which Earth’s weather occurs.
The depth of the troposphere varies from 16 kilometers above the equator to less than 9 kilometers above the North and South poles. Although it is the shallowest layer, the troposphere contains almost all of the mass of the atmosphere.
The stratosphere extends from the top of the troposphere to about 50 kilometers above earth’s surface. Strato- means “layer” or “spread out.” The stratosphere is the second layer of the atmosphere and contains the ozone layer.
The lower stratosphere is cold, about -60oC. Surprisingly, the upper stratosphere is warmer than the lower stratosphere. Why is it? The middle portion of the stratosphere contains a layer of air where there is much more ozone than in the rest of the atmosphere. (Recall that ozone is the three-atom form of oxygen.) When the ozone absorbs energy from the sun, the energy is converted into heat, warming the air. The ozone layer is also important because it protects Earth’s living things from dangerous ultraviolet radiation from the sun.
Above the stratosphere, a drop in temperature marks the beginning of the next layer, the mesosphere. Meso- means “middle”, so the mesosphere is the middle layer of the atmosphere. The mesosphere begins 50 kilometers above Earth’s surface an ends at an altitude of 80 kilometers. In the outer mesosphere, temperatures approach -90oC.
The mesosphere is the layer of the atmosphere that protects Earth’s surface from being hit by most meteoroids. Meteoroids are chunks of stone and metal from space. What you see as a shooting star, or meteor, is the trail of hot, glowing gases the meteoroid leaves behind in the mesosphere.
Near top of the atmosphere, the air is very thin. At 80 kilometers above earth’s surface, the air is only about 0.001 percent as dense as the air at sea level. It’s as though you took a cubic meter of air at sea level and expanded it into 100,000 cubic meters at the top of the mesosphere. The outermost layer of Earth’s atmosphere is the thermosphere. The thermosphere extends from 80 kilometers above Earth’s surface outward into space. It has no definite outer limit, but blends gradually with outer space.
The thermo- in thermosphere means “heat.” Even though the air in the thermosphere is thin, it is very hot, up to 1,800oC. This is because sunlight strikes the thermosphere first. Nitrogen and oxygen molecules convert this energy into heat.
Despite the high temperature, you would not feel warm in the thermosphere. An ordinary thermosphere would show a temperature well below 0oC. Why is that? Temperature is the average amount of energy of motion of each molecule of a substance. The gas molecules in the thermosphere move very rapidly, so the temperature is very high. However, the molecules are spaced far apart in the thin air. There are not enough of them to collide with a thermosphere and warm it very much.
The thermosphere is divided into two layers. The lower layer, called ionosphere (eye AHN uh sfeer), begins about 80 kilometers above the surface and extends to about 400 kilometers. Energy from the sun causes gas molecules in the ionosphere to become electrically charged particles called ions. Radio waves bounced off ions in the ionosphere. In the Northern hemisphere, these displays are called the Northern Lights, or the aurora borealis. Auroras are caused by particles from the sun that enters the ionosphere near the poles. These particles strike atoms in the ionosphere, causing them to glow.
Exo- means ‘outer”, so the exosphere is the outer portion of the thermosphere. The exosphere extends from about 400 kilometers outward for thousands of kilometers.