Liquid water consists of water molecules that are held near each other by attractive forces but are still free to move around among one another. Also, water molecules have thermal energy, in the form of extremely fast vibrations. Water molecules at a water surface occasionally vibrate so strongly that they fly out into the air to become vapor. At the same time, water molecules in the air occasionally crash back onto the water surface to join the liquid water. These motions are occurring all the time at the water surface.

A "closed system" consists of a container that allows energy, but not matter, to pass back and forth across the walls of the container. The Earth's atmosphere, oceans, and land surface act as an almost closed system. Water moves along a variety of pathways in this closed system. This system of movement is called the "water cycle."

Radar (radio detection and ranging) has become an important tool for observing and predicting the weather. Radar was invented and developed in Britain and the U.S. at the beginning of the Second World War. It was used to detect the approach of enemy airplanes. An antenna sends out radio waves. The waves are reflected from solids or liquids in the air and received back by the antenna. The radar equipment shows the position and distance of the objects. The results are shown on a screen. The screen is similar to the screen of a television or a computer monitor.

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.