You experience weather at the Earth's surface, but there is weather high in the atmosphere, too. Have you ever taken a ride in a hot-air balloon or climbed a high mountain? You would know that the air temperature usually decreases with altitude. The basic reason has to do with where the atmosphere receives its heat and where it loses its heat. The Earth's surface is heated by the sun at some times and places. It loses heat to outer space at other times and places. On balance, however, the Earth's surface gains more heat than it loses.

Large masses of air, as much as a thousand miles across, take on certain weather characteristics when they stay at high latitudes (near the poles) or at low latitudes (near the equator) for several days at a time. They may be very cold or very warm, or they may be very humid or very dry. Then, as they move into other areas, they can affect the weather there very strongly.

Why is the weather in high-pressure areas usually fair? Why is the weather in low-pressures areas usually cloudy and stormy? Most weather maps show areas, labeled with an H, where the atmospheric pressure is relatively high, and areas labeled with an L where the atmospheric pressure is relatively low. The isobars around such areas are closed curves with the approximate shape of circles. High-pressure areas are places where the atmosphere is relatively thick. Winds blow outward from these areas, although in a spiraling way.

Air has weight. That idea might seem strange to you, because air seems very thin, even at sea level. Remember however, that the atmosphere extends to great altitudes. The pressure of the air is equal to the weight of a column of air above a unit area on the land surface. The column of air above a square area that is one foot on a side is about 2,116 lb., at sea level. In the metric system, that's about 10,332 kg per square meter. If you try to pump the air out of a closed container, the container will collapse inward from the outside air pressure, unless it is very strong.

Most people are interested in what the weather will be tomorrow or in the next few days. Predictions of the weather for up to a week in the future are called short-term forecasts. Meteorologists also try to make long-term forecasts of the weather for a month, a season, or a whole year.

Weather reports vary a lot in how much information they contain. The simplest and shortest weather report contains only one piece of information: the present temperature. This is the type of report you often hear on the radio. More detailed weather reports also contain information about precipitation, wind speed and direction, relative humidity, atmospheric pressure, and other things as well.

Raindrops are formed when the cloud droplets grow big enough to fall out of the clouds. Most of the rain that falls in the winter, and even a lot of it that falls in the summer, is from melting of snowflakes as they fall through warmer air.

Clouds are formed when moist air rises upward. As the air rises, it becomes colder. Eventually the air can't hold all of the water vapor in it, and some of the water vapor condenses to form tiny water droplets. When moist air is cooled at the ground, fog is formed in the same way.

The wind blows because air pressure is higher in one place than in another place. The air moves from areas of higher pressure to areas of lower pressure. Objects like buildings, trees, and hills affect the direction of the wind near the surface. To get the best idea of the wind direction, try to stand far away from such objects. A park or a playing field is the best place to observe the wind.

Air consists of gas molecules, which are combinations of two or more atoms. Although you cannot see them with your eyes, the molecules are constantly moving this way and that at very high speeds. As they move, they collide with one another and with solid surfaces. The temperature of the air is a measure of how quickly the molecules are moving. The more energy of motion the molecules have, the higher the temperature you feel in the air.