What is the jet stream?

The jet stream is a band of swiftly moving air located high in the atmosphere and affects the movement of storms and air masses closer to the ground. The currents of air flow from west to east and are usually a few miles deep, up to 100 miles (160 kilometers) wide, and well over 1,000 miles (1,600 kilometers) in length. The speed at which the air travels in the jet stream is over 57.5 miles (92 kilometers) per hour, sometimes moving as fast as 230 miles (386 kilometers) per hour.

There are two polar jet streams, one in each hemisphere. The jet streams meander across the troposphere and stratosphere (up to 30 miles [48 kilometers] high) and between 30 and 70 degrees latitude. There are also two subtropical jet streams (one in each hemisphere) that range between 20 and 50 degrees latitude. The subtropical streams flow between altitudes of 30,000 to 45,000 feet (9,150 to 13,700 meters) and are even swifter than the polar streams, moving at speeds of over 345 miles (550 kilometers) per hour.

What is a Rossby wave?

Named after Swedish meteorologist Carl-Gustaf Rossby (1898-1957), Rossby waves refer to the large air mass waves that inhabit the middle layer of our atmosphere, including the jet stream and high- and low-pressure systems.

How was the jet stream discovered?

With the advent of airplanes that could cruise at elevations of over 30,000 feet (over 9,000 meters) high, pilots—such as World War II bomber pilots flying over Japan and the Mediterranean Sea—discovered the effects of the jet stream on their aircraft.

What is the low-level jet stream?

Seen in the central United States, low-level jet streams are air flows coming in from the Gulf of Mexico into the Central Plains, but they also occur as streams flowing from the Indian Ocean into Africa. Low-level jet streams occur at altitudes of only a couple thousand feet and can bring in moisture and warm air that creates severe thunderstorms and tornadoes. In the Central Plains, though, they only occur at night.

What is the arctic oscillation?

Arctic oscillation is a measure of the differences in air pressure between air within the Arctic Circle and air that is between the circle and about 55°N longitude. The arctic oscillation can be either positive (air pressure is lower over the arctic region) or negative (the air pressure is higher). In the case of the former, winds become stronger across mid-latitude regions, Eurasia becomes warmer, and drier conditions prevail in the American West and the Mediterranean. Also, storms move farther north into Alaska and northern Europe. When the arctic oscillation is negative, the opposite is mainly true, and the American West Coast and the Mediterranean experience wet weather, while Eurasia cools.

What is wind shear?

Wind shear refers to rapid changes in wind speed and/or direction over short distances and is usually associated with thunderstorms. Sometimes, though, wind shears can also occur as a result of a strong front moving through a region, or from abrupt changes in air mass near mountains. Wind shear is especially dangerous to aircraft, and Doppler radars at airports help to warn pilots of this threat.

What is the effect of a microburst on aircraft?

Microbursts are downbursts of air with a diameter of 2.5 miles (4 kilometers) or less. Often associated with thunderstorms, they can generate winds of hurricane force that change direction abruptly. Headwinds can become tailwinds in a matter of seconds, forcing aircraft to lose air speed and altitude. After microbursts caused several major air catastrophes in the 1970s and 1980s, the Federal Aviation Administration (FAA) installed warning and radar systems at airports to alert pilots when conditions were right for wind shears and microbursts.

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