An air current is a mass of moving air. They are caused by a number of conditions, but especially differences in pressure and temperature. Air currents both shape and are influenced by the Earth's climate and weather and are experienced as wind, both on the surface of the Earth and when aloft, by aircraft and flying animals. Air currents have also shaped the history of the world, as they have influenced the trade and exploration routes of oceangoing sailing ships since ancient times.
The Earth's climate and weather is an extremely complex system of many interrelated components, of which air currents are just one component. An air current is usually created by one of two conditions. A difference in air pressure or temperature between two air masses is by far the most common, but other types of air currents exist, such as the jet stream.
When two air masses have differing air pressure, an air current will form as air flows from the area of higher pressure to the area of lower pressure. This type of air current is common at or near the Earth's surface, and air pressure currents are the source of much of the planet's surface wind. On a weather map, differences in pressure are often marked by lines called isobars. Isobars themselves connect areas of equal atmospheric pressure. When they appear very close together, a large difference in air pressure between air masses is present over a relatively short distance, causing air currents to flow very quickly in the form of strong winds.
A temperature current, or rising air current, is formed when air masses of differing temperatures meet. Cold air is denser than warm air, and when a cold air mass meets a mass of warmer air, the warmer air tends to rise, forming a rising air current. This interaction between warm and cold air is the source of thunderstorms and, in extreme cases, tornadoes.
The jet stream is a type of air current, caused by great differences in temperature between air masses. It is sometimes characterized as a river of fast-moving air that often flows at over 200 miles per hour (320 km/h), giving it its name. At any one time, as many as four or more distinct jet streams may be flowing high above the Earth's surface, circling the globe at altitudes of around 4 miles (6.4 km). A pair of jet streams, the polar jet stream and the tropical jet stream, flow from west to east in both the northern and the southern hemisphere.