In the United States, the jet stream is a wobbly line across the states which is blamed for just about every weather problem that arises. But more than a wind stream, it is called “jet” because of how fast its winds are.
Actually, there are four major jet streams – one 30–60 degrees from each pole (the polar jets), and two subtropical jets, which are at a higher altitude and weaker. And all four are affected by the same elements. The west-to-east winds are caused by the earth’s rotation and solar heating. This would simply explain the polar jet’s height, which is about 7–12 kilometers above Earth (23,000 to 39,000 feet). It is positioned between the troposphere, where temperature decreases with altitude, and the stratosphere, where the temperature increases with altitude. So there were temperature differences above and below the air stream, as well as the differences between polar regions and equatorial regions, causing a stream of air.
Such jet streams also exist on other planets. And sometimes new jet streams appear on Earth. For instance, in northern hemisphere summers, easterly (east-to-west) jets can form in a region where dry air meets humid air at high altitudes. In the central United States, low-level jet streams can occur. Sometimes the polar and subtropical streams merge, and at other times they are well separated.
When two air masses of different temperatures or densities meet, this causes wind, which is highest within the transitional tropopause. Rather than flowing directly from hot to cold areas, the air is deflected by the Coriolis Effect, and instead flows along the boundary of the two air masses. The effect, or force, is the deflection of objects (planes, wind, ocean currents) moving in a straight path relative to the Earth's surface. The closer to the poles the object is, the stronger the deflection.
Because the hot and cold boundaries are most pronounced in winter, this is the season wherein the jet stream is strongest. Because it is the strongest and affects the most countries, the northern polar stream is usually the one referred to as “the jet stream”.
The jet stream is usually a few hundred kilometers wide and vertically usually less than five kilometers (three miles). All of the measurements are general, as a jet stream can change shape and path and even break up for a spot of land.
As the tilt of the Earth’s axis causes the north pole to be closer to the sun, the northern hemisphere experiences summer, and the jet stream is said to follow “the sun” by shifting northward. Following the solar heat, it moves more southward for autumn and winter.
The speed of the wind in a jet stream varies, again according to the temperature differences, and are usually at least 57 miles per hour – but have been measured as much as 247 miles per hour.
El Nino, more technically known as the El Nino Southern Oscillation, or ENSO, adds another dimension to the jet stream behavior. This change in pressure and temperature causes variations in the location of the jet, thereby causing precipitation in California, cold season rainfall in Europe and precipitation in South America. La Nina causes the reverse – the jet stream moves northward and California dries out.
Other factors affecting the jet streams are nights, during the polar winter, which form a second stream up at an altitude of about 30 miles because the air is so much colder. Mountain chains cause a jet, as the jet stream runs parallel to the mountains, which increases the wind speed by as much as 45 percent. The low-level jet crossing the North American Plains forms overnight thunderstorms during warm seasons. During the last Ice Age, the jet stream was deflected southward, causing precipitation and huge lakes in what is now the dry Great Basin. In the 1930s it is believed that the jet did not move back north after going south into the Gulf of Mexico, and therefore did not bring precipitation to the Midwest, resulting in a drought called the Dust Bowl.
The jet streams keep weather from just sitting over an area and instead move weather masses around the globe. Now that the jet stream is explained, one has to ask why this is of any importance. There are three main reasons for learning about the jet stream.
The reason most people are familiar with the jet stream is because they see it on weather maps. Meteorologists follow the jet and its anomalies to help them predict weather. The path of the jet steers cyclonic storm systems, so meteorologist can tell where a cyclone is headed. In 2012, Britain had severe flooding because the polar jet stayed south for the summer. Although there is a long list of Hawaii hurricanes, the Hawaiian Islands seem protected by a jet stream vertical wind shear. The polar jets appear to be maintaining a position more polar than historically, which could lead to drier conditions across the southern United States, and more intense and frequent tropical cyclones in the tropic areas.
The aviation industry is also very interested in the pattern of the jet stream because by "riding" it in the west-to-east direction they can save fuel and up to five hours in travel time from Tokyo to Honolulu. Conversely, flying east to west can cost fuel and time, so airliners would want to avoid the jet stream in that direction. Also associated with jet streams are wind shears known as clear-air turbulence, or CAT. The CAT is strongest on the cold air side of the stream and can cause aircraft to plunge unexpectedly.
In these eco-conscious times, the jet is also being considered for an energy source. A mere one percent of the wind in the jet would meet the energy needs of the entire world. The technology is still 10 to 20 years in the future, and the climatic impact is still in question.
In conclusion, the more one knows about the jet streams of all types and places, the more one can take the opportunity to benefit from them.