Air moves over the Earth because of uneven heating. While rays of light from the sun hit the flat surface near the equator directly, they reach the rest of the globe at an angle. This causes more heat to be absorbed in the atmosphere and areas of ocean near the tropics. This hot air expands, becoming lighter and less dense. As it rises above cooler, heavier air, it moves towards the northern and southern hemispheres in the form of wind.
In the tropics, air above the ocean is heated by the sun and becomes moist with water vapor. As it rises and expands it can no longer hold onto its moisture. This moisture condenses into drops of liquid water and is redistributed over the Earth as precipitation. Free of its moisture, the dry air can move in rapid currents towards the north and south poles. As air currents move over the ocean, the drag they create generates ocean currents. These currents also affect the uneven distribution of global heat.
When masses of dry air leave the equator and head north and south, they eventually run into each other. The pressure created as the winds press against one another increases the density of the air. This denser, dry air begins to sink, settling into desert areas. Other climates are created as low-pressure warm, moist air masses rise above high-pressure cold air masses and sink back toward the tropics.
Many technologies have been developed with air currents in mind. Airplanes and gliders were originally modeled after the flight of gliding birds. Air currents flow both underneath and over the top of wings. If the top of the wing is curved, giving it greater surface area than the bottom, a lifting effect can be achieved. In recent years, wind turbines have been developed to harness air currents and convert the energy into electric power for homes and businesses. Green advocates encourage use of wind power because it creates no pollution and will always be readily available.