Jet Streams

References

• Jet Streams Wikipedia Article

Related

• tropopause
• • The tropopause is the atmospheric boundary that demarcates the troposphere from the stratosphere, which are the lowest two of the five layers of the atmosphere of Earth. The tropopause is a thermodynamic gradient-stratification layer that marks the end of the troposphere, and is approximately 17 kilometers above the equatorial regions, and approximately 9 kilometers above the polar regions.

• polar vortices
• • A circumpolar vortex, or simply polar vortex, is a large region of cold, rotating air; polar vortices encircle both of Earth's polar regions. Polar vortices also exist on other rotating, low-obliquity planetary bodies. The term polar vortex can be used to describe two distinct phenomena; the stratospheric polar vortex, and the tropospheric polar vortex. The stratospheric and tropospheric polar vortices both rotate in the direction of Earth's spin, but they are distinct phenomena that have different sizes, structures, seasonal cycles, and impacts on weather.
• Atmospheric Circulation
• • Atmospheric circulation is the large-scale movement of air and together with ocean circulation is the means by which thermal energy is redistributed on the surface of the Earth. Earth's atmospheric circulation varies from year to year, but the large-scale structure of its circulation remains fairly constant. The smaller-scale weather systems - mid-latitude depressions, or tropical convective cells - occur chaotically, and long-range weather predictions of those cannot be made beyond ten days in practice, or a month in theory. The Earth's weather is a consequence of its illumination by the Sun and the laws of thermodynamics. The atmospheric circulation can be viewed as a heat engine driven by the Sun's energy and whose energy sink, ultimately, is the blackness of space.

• Coriolis Force
• • In Physics, the Coriolis Force is an inertial (or fictitious) force that acts on objects in motion within a frame of reference that rotates with respect to an inertial frame. In a reference frame with clockwise rotation, the force acts to the left of the motion of the object. In one with counterclockwise rotation, the force acts to the right. Deflection of an object due to the Coriolis force is called the Coriolis effect.

Though recognized previously by others, the mathematical expression for the Coriolis force appeared in an 1835 paper by French scientist Gaspard-Gustave de Coriolis, in connection with the theory of water wheels

• Internal Heat
• • Internal heat is the heat source from the interior of celestial objects, such as stars, brown dwarfs, planets, moons, dwarf planets, and even asteroids such as Vesta, resulting from contraction caused by gravity, nuclear fusion, tidal heating, core solidification, and radioactive decay. The amount of internal heating depends on mass; the more massive the object, the more internal heat it has; also, for a given density, the more massive the object, the greater the ratio of mass to surface area, and thus the greater the retention of internal heat. The internal heating keeps celestial objects warm and active.
• Thermosphere
• • The thermosphere is the layer of Earth's atmosphere directly above the mesosphere and below the exosphere. Within this layer of the atmosphere, ultraviolet radiation causes photoionization/photodissociation of molecules, creating ions; the thermosphere thus constitutes the larger part of the ionosphere.
• Dynamic North Atlantic Tracks
• • The North Atlantic Tracks, officially titles the North Atlantic Organized Track System (NAT-OTS), are a structured set of transatlantic flight routes that stretch from eastern North America to western Europe across the Atlantic Ocean, within the North Atlantic airspace region. They ensure that aircraft are separated over the ocean, where there is little radar coverage.

Notes

Jet streams are fast flowing, narrow, meandering air currents in the atmospheres of the Earth, Venus, Saturn, Uranus, and Neptune. On Earth, the main jet streams are located neared the altitude of the tropopause and are westerly winds (flowing west to east). Jet streams may start, stop, split into two or more parts, combine into one stream, or flow in various directions including opposite to the direction of the remainder of the jet.

The strongest jet streams are the polar jets around the polar vortices, at 9-12 km above sea level, and the higher altitude and somewhat weaker subtropical jets at 10-16 km. The Northern Hemisphere and the Southern Hemisphere each have a polar jet and a subtropical jet.

Jet streams are the products of two factors:

1. The atmospheric heating by solar radiation that produces the large-scale polar, Ferrel, and Hadley circulation cells.
2. 1. On other planets, internal heat rather than solar heating drives the jet streams.
2. The action of the Coriolis force acting on those moving masses.

There are also jet streams in the thermosphere.

Meteorologists use the location of some of the jet streams as an aid in weather forecasting. The main commercial relevance of the jet streams is in air travel, as flight times can be dramatically affected by either flying with the flow or against. Often, airlines work to fly with the jet stream to obtain significant fuel cost and time savings.

Dynamic North Atlantic Tracks are one example of how airlines and air traffic control work together to accommodate the jet stream and winds aloft that results in the maximum benefit for airlines and other users. Clear-air turbulence, a potential hazard to aircraft passenger safety, is often found in a jet stream's vicinity, but it does not create a substantial alteration of flight times.

Many sources credit real understanding of the nature of jet streams to regular and repeated flight-path traversals during World War ||.

Polar jet streams are located near the 250hPa pressure level, or seven to twelve kilometers above sea level, while the weaker subtropical jet streams are much higher, between 10 and 16 kilometers. Jet streams wander laterally dramatically, and change in altitude. The jet streams from near breaks in the tropopause, at the transition between the polar, Ferrel, and Hadley circulation cells, and whose circulation, with the Coriolis force acting on those masses, drives the jet streams.

The polar jet stream is most commonly found between latitudes 30 deg and 60 deg, while the subtropical jet streams are located close to latitude 30 deg. These two jets merge at some locations and times, while at other times they are well separated. The northern polar jet stream is said to follow the sub as it slowly migrates northward as that hemisphere warms, and southward again as it cools. The width of a jet stream is typically a few hundred kilometers or miles and its vertical thickness often less than give kilometers.

Jet streams are typically continuous over long distances, but discontinuities are also common. Each large meander, or wave, within the jet stream is known as a Rossby wave (planetary wave). Rossby waves are caused by changes in the Coriolis effect with latitude. The wind speeds are greatest where the temperature differences between air masses are greatest, and often exceed 92 km/h. Meteorologists now understand that the path of jet streams affects cyclonic storm systems at lower levels in the atmosphere, and so knowledge of their course has become an important part of weather forecasting.

In general, winds are strongest immediately under the tropopause (except locally, during tornadoes, typical cyclones or other anomalous situations). If two air masses of different temperatures or densities meet, the resulting pressure difference caused by the density difference (which ultimately causes wind) is highest within the transition zone. The wind does not flow directly from the hot to the cold area, but is deflected by the Coriolis effect and flows along the boundary of the two air masses.

All of these facts are consequences of the thermal wind relation. The balance of forces on wind in the vertical direction is referred to as hydrostatic. The dominant forces in the horizontal direction, the struggle between the Coriolis force and the pressure gradient force, is referred to as geostrophic.

On Earth, the northern polar jet stream is the most important one for aviation and weather forecasting, as it is much stronger and at a much lower altitude than the subtropical jet streams and also covers many countries in the Northern Hemisphere, while the southern polar jet stream mostly circles Antarctica and sometimes the southern tip of South America.

The location of the jet stream is extremely important for aviation. Commercial use of the jet stream began on 18 November 1952, when Pan Am flew from Tokyo to Honolulu at an altitude of 7,600 metres (24,900 ft). It cut the trip time by over one-third, from 18 to 11.5 hours.