Air currents are caused by pressure and temperature differences, shaping Earth’s climate and weather. Pressure differences create surface winds, while temperature differences create rising drafts and thunderstorms. The jet stream is a fast-moving air current caused by large temperature differences.
A stream of air is a moving mass of air. They are caused by a number of conditions, but most notably by pressure and temperature differences. Air currents shape and are influenced by the Earth’s climate and weather conditions, and are felt as wind, both at the Earth’s surface and when overhead, by aircraft and flying animals. Air currents have also shaped world history, having influenced the trade and exploration routes of ocean-going sailing vessels since ancient times.
Earth’s climate and weather are an extremely complex system of many interconnected components, of which air currents are only one component. A draft 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 there are other types of air currents, such as the jet stream.
When two air masses have different air pressure, an air current will form as air flows from the higher pressure area to the lower pressure area. This type of air current is common on 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, pressure differences are often marked by lines called isobars. The same isobars connect areas of equal atmospheric pressure. When they appear very close together, there is a large difference in air pressure between the air masses over a relatively short distance, causing very rapid airflow in the form of strong winds.
A temperature current, or rising draft, is formed when air masses of different temperatures meet. Cold air is denser than warm air, and when a cold air mass meets a warmer air mass, 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 large temperature differences between air masses. It is sometimes characterized as a fast-moving river of air often flowing at over 200 miles per hour (320 km/h), hence its name. At any one time, up to four or more distinct jet streams can flow high above the Earth’s surface, circling the globe at altitudes of about 4 miles (6.4 km). A pair of jet streams, the polar jet stream and the tropical jet stream, flow west to east in both the northern and southern hemispheres.
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