Changes in weather involve air movements, formation of clouds, and precipitation. Energy is needed to make all these things happen. That energy comes from the sun. Heat energy enters and moves through the atmosphere in three different ways.
One way that heat energy is transferred is radiation. Hot bodies such as the sun radiate their energy mainly in the form of short waves. These short waves are seen as visible light. Cooler bodies such as Earth radiate their energy as longer waves. These longer waves are called infrared waves. They are longer than the longest visible light waves, which are red.
Another way is conduction. An object receives heat when it comes into contact with a hotter object. A pan on a hot stove is heated mainly by conduction. So is the air that touches warm ground or a warm ocean.
A third way is convection. Convection is the most effective form of heat transfer in liquids and gases. A kettle of water on a hot stove is an example of heating by convection. The bottom of the kettle is heated by conduction. The water near the bottom then heats also by conduction. Heating makes the water expand and become less dense. The denser cold water above it sinks, forcing up the warm water. A steady flow called a convection current forms. Convection is very important in moving heat through the atmosphere. It transfers heat from one place to another. For example, convection removes heat from hot beaches. Winds from the tropics carry heat away and into middle latitudes. The transfer of warm or cold air by horizontal winds is called advection
Ideally just as much energy enters Earth as leaves. When this is true, Earth’s heat budget is in balance. If Earth’s heat budget was out of balance, Earth would gradually heat up or cool down. The only way energy can enter or leave Earth is by radiation. There are not enough molecules in space for conduction or convection. The sun radiates energy into space in all directions. Earth, tiny by comparison and far away from the sun, receives only about one two-billionth of the sun’s rays. This incoming solar radiation is called insolation. Suppose 100 units of solar radiation reach the atmosphere. Of these, 30 are reflected back to space, with only about 70 units absorbed. This heat is not absorbed evenly. In the atmosphere, 19 units of sunlight are absorbed by water vapor and clouds, ozone, and dust. Earth’s surface absorbs the remaining 51 units of sunlight.
In order to keep Earth’s heat budget balanced, 70 units of energy are radiated to space as infrared radiation. The atmosphere and surface also have heat budgets that balance. Of the 51 units of heat absorbed in the form of solar radiation, 21 are radiated back as infrared radiation and 30 are left over. The remaining 30 units are released through the two remaining means of heat transfer, conduction and convection. Conduction from the heated ground heats the very lowest layer of air. Convection currents carry the heat into the atmosphere. The 30 units released by conduction and convection plus the 19 from the atmosphere and the 21 released by infrared radiation add together to make the 70 units.
Earth’s surface radiates infrared waves. These infrared waves warm the atmosphere because they are absorbed mainly be the water vapor and carbon dioxide in the air. In a greenhouse, the glass roof acts like the carbon dioxide and water vapor in the air. It lets in the sun’s light to heat the soil. However, it does not allow the longer-wave infrared radiation from the warm soil to escape. Scientists call this trapping of the sun’s energy by the atmosphere the greenhouse effect. The gases that create this effect are known as greenhouse gases.
The greenhouse effect of Earth’s atmosphere is increasing. The burning of fossil fuels - coal, oil, and natural gas - is constantly adding carbon dioxide into the air. There is too much carbon dioxide to be used by green plants or dissolved in the ocean waters. Scientists estimate that if the present rate of burning continues, the percentage of carbon dioxide in the atmosphere will double in about 100 years. Industry is also producing more CFCs, nitrous oxide, and methane, which are also greenhouse gases. The resulting warming of the atmosphere could have serious effects. The glaciers of Greenland and Antarctica could melt enough to raise sea levels all over the world. Such melting would flood low coastal areas such as Miami and New York City. Rainfall patterns could change in ways that would shift desert areas. Obviously, this problem needs serious attention and study.
