Imagine you are an otherworldly explorer. As part of your survey of the Milky Way, you have come across an interesting stellar system with nine major planets. You have gotten permission from your superiors to investigate the lively third planet. The planet is called “Earth” in one of the many languages used there. In your last report you discussed the development of the planet as part of the entire system. Now you are going to begin a more detailed examination of Earth itself. The most logical place to start is the atmosphere, the envelope of gasses and vapors that gives this planet so much of its character. Since it is always best to begin at the beginning, you review your findings concerning the early history of the atmosphere.
How to Make an Atmosphere For Dummies
As the planet collected the rock and metal that formed it, it also gathered up gasses that made up most of the vast cloud from which the entire Solar System was formed. This primitive atmosphere was rich in hydrogen, helium, methane, and ammonia, but the hydrogen and helium rapidly escaped the Earth’s gravity.
In a second phase, pressure and radioactive decay created heat in the planet’s interior. This heat caused volcanic release of the gasses trapped in the rock. The main gasses released in this phase were hydrogen, hydrogen sulphide, and methane. After a while, most of the heavy, oxygen-trapping iron sank to the center of the Earth, and the volcanic outgassing released more oxygen in the forms of water vapor and carbon dioxide (CO2). The atmospheres of Venus and Mars are rich in CO2 today. What happened to Earth's?
Two processes contributed to Earth's oxygen-rich, carbon dioxide-poor atmosphere of today. First, most of the carbon dioxide dissolved in Earth's oceans, then later combined chemically with the rock of the planet's crust. Second, plant life “inhaled” carbon dioxide and “exhaled” oxygen. In addition, ultraviolet radiation from the Sun broke down water and CO2, freeing oxygen. This process also freed the nitrogen in methane and ammonia.
Layers of the Atmosphere
The atmosphere can be divided into two main sections, the lower homosphere and the heterosphere above it. The heterosphere is separated into layers of gasses (hetero is Greek for “different”). From top to bottom they are: hydrogen (the lightest gas), helium, oxygen, and nitrogen (and some compounds of nitrogen, heavier gasses).
The gasses of the homosphere (homo is Greek for “the same”) are well mixed into the blend of gasses humans call air. It is possible to distinguish three layers in the homosphere. The lowest layer is the troposphere, where most weather occurs. Above the troposphere is the stratosphere, where the protective ozone is. The ozone also extends into the mesosphere.
Above the homosphere is the lowest layer of the heterosphere, called the thermosphere or ionisphere. The absorption of ultraviolet rays causes gasses to lose electrons, or become ionized. This layer makes long-range radio communication possible on Earth, since it reflects radio waves back toward the surface. Without this layer, radio signals would drift out into space.
The atmosphere of the Earth has two basic roles to play (besides providing air for living creatures to breathe): it shields the surface from harmful radiation, and maintains a blanket of warmth for the planet.
All told, the incoming solar radiation is about 9 percent X-ray and ultraviolet, 41 percent visible light, and 50 percent infrared. The ultraviolet is absorbed by oxygen in the ionosphere and by ozone in the strato- and mesospheres. Infrared is absorbed by water vapor and CO2 in the troposphere. Visible light is allowed to pass through to heat the surface. The Earth's warm surface radiates energy in the form of longer wavelengths (mainly infrared). This outgoing infrared radiation is captured by the atmosphere (just as the incoming infrared is). Some is radiated out into space by the atmosphere, but much is radiated back to Earth. Thus the atmosphere lets through incoming visible light, but stops outgoing infrared radiation. Just as a blanket keeps sleeping humans warm by trapping heat they radiate, the atmosphere “blankets” Earth, maintaining stable, comfortable temperatures for the creatures that inhabit the planet.
Is Earth Too Hot to Handle?
Measurements of incoming and outgoing radiation point out an interesting feature of the planet. The surface absorbs more solar energy than it radiates, while the atmosphere radiates more than it absorbs. From this it would seem that the surface should get hotter and hotter, while the atmosphere gets colder and colder. Yet the temperatures of both are relatively stable. Why?
The answer to this question sheds a lot of light on the way weather is created on the planet. Besides radiation, the surface gives up heat in two other important ways: evaporation of water, and heating of the air near the surface. For liquid water to become vapor, it must absorb heat. On Earth, water from the oceans, ground, and plants takes heat energy and evaporates into the air. At the same time, air is being heated by the ground. The pressure (or density) of air decreases as it is heated. An air bubble rises to the surface of water because air is less dense than water. In the same way, heated air rises above the cooler, denser air above and around it.
In higher altitudes there is less air, and therefore less air pressure. Once the heated air rises to levels of lower pressure, it expands. The expansion requires heat, which is provided by the evaporated water. When this water gives up the heat it absorbed earlier, it becomes liquid again, or condenses. The water vapor becomes tiny droplets of water. Single droplets alone are too tiny to see, but in huge numbers they are visible as clouds.
If temperature and pressure conditions are right, some larger droplets will form and begin to fall, picking up smaller droplets on the way down. Often these drops evaporate before they fall very far. Sometimes, however, they fall to the surface as rain, sleet, or snow. Clouds can form rapidly, then disappear within an hour.
Sun—Star or Weather Machine?
One interesting feature of the Sun's influence is that the atmosphere is something of a vast solar engine, constantly in motion. Because the Earth is a sphere, different parts receive different amounts of energy from the Sun. For instance, the poles only receive about 40 percent as much energy as the equator. Thus the ground at the equator has more heat to transfer to the air above it.
Large areas of cold air move from the poles toward the equator, while warm air from the equator moves toward the poles. Such large areas are called air masses. Where cold and warm air masses meet, a front is formed. A warm front is formed where warm air is replacing cold air; a cold front is the opposite. At such fronts we find weather disturbances resulting in rain or snow, and sometimes severe weather such as thunderstorms and tornadoes. These fronts are usually found in the middle latitudes between the equator and the poles.
At fronts, the cold air mass slides under the warm air, forcing the warm air upward. At high altitudes, moisture in the air condenses into clouds and rain, snow, and sometimes hail. The southward motion of cold air combines with the northward motion of warm air to create a counterclockwise motion of air. This air moves toward the center of the motion and then upward, creating a center of low pressure. The development of low pressure systems is the source of much precipitation in the middle latitudes.
Less Tropical, More Storm
At lower latitudes, low pressure systems can develop into one of the most severe weather phenomena known on Earth, hurricanes. Hurricanes develop over oceans, where the air is moist and warm. A hurricane is an unusually intense and well-organized low-pressure system. Some unknown force triggers an upward motion of air. Vast quantities of moist air are then lifted upward. The heat released in evaporation releases huge amounts of energy to drive the hurricane winds. A hurricane covers thousands of square kilometers, and causes gale force winds in an area 10 times larger. A hurricane has more power output in one day than the electrical power generated by the United States in six months. A typical hurricane rains 10 to 20 tons of water each day. Though hurricanes are awful in their destructive power, they are also responsible for a large per cent of important annual rainfall in certain areas.
The Earth's atmosphere plays a large role in the lives of all the inhabitants. It warms them and brings water from oceans to continents. But it is also capable of unleashing violence and destruction as it releases the energy it absorbs from the Sun. With its atmosphere, the third planet teems with life and activity. Without it, Earth would never have been the inviting place your superiors have allowed you to explore.