By Summer in the Northern Hemisphere, it is typically found near the U. Canadian border. As Autumn approaches and the sun's elevation decreases, the jet stream's average latitude moves toward the equator. Jet streams are often indicated by a line on a weather map indicating the location of the strongest wind. However, jet streams are wider and not as distinct as a single line but are actually regions where the wind speed increases toward a central core of greatest strength.
One way of visualizing this is to consider a river. The river's current is generally the strongest in the center with decreasing strength as one approaches the river's bank. Therefore, it is said that jet streams are "rivers of air". However, air moving toward the poles retains its eastward momentum while the earth's rotational velocity decreases beneath it.
The result is the wind moves faster than the earth rotates so it moves from west to east relative to us at the surface. The eddies modify the distribution of temperature and kinetic energy within the atmosphere, a process that has a pronounced effect on the location and movement of the jet stream. And the jet stream itself interacts with these waves, not only as a transport or steering mechanism but also in the transfer of momentum and energy back to the waves.
The topography of the land also influences the jet stream's location. Mountain ranges and plains on large continents, for example, significantly affect the distribution of atmospheric temperature.
And since the jet stream is a thermally driven phenomenon, the more complicated the three-dimensional temperature structure of the earth's atmosphere, the more 'wandering' will take place in the course of the jet stream. The first step in the forecast process is observation. Many thousands of atmospheric observations are made each day by civilian and military aircraft, land and maritime weather reporting stations and ships and weather balloons.
These observations help to define the present state and location of atmospheric circulations and weather systems, including the jet stream. These models assimilate this snapshot of the present state of the atmosphere and mathematically diagnose how the jet stream and other circulations and weather systems will change with time.
The output from the models is then made available to meteorologists, who apply their knowledge, experience and expertise in making the final forecast. The African jet stream is considered to play a crucial role in the southwest monsoon of Africa, and helps form the tropical waves which march across the tropical Atlantic and eastern Pacific oceans during the warm season.
A southerly low-level jet in the Great Plains helps fuel overnight thunderstorm activity during the warm season, normally in the form of mesoscale convective systems which form during the overnight hours.
A similar phenomenon develops across Australia, which pulls moisture poleward from the Coral Sea towards cut-off lows which form mainly across southwestern portions of the continent. Why does the jet stream winds blow from west to east? The warm air rising at the equator moves towards both poles. The earth's rotation divides this circulation into three cells. The earth's rotation is responsible for the jet stream as well. The motion of the air is not directly north and south but is affected by the momentum the air has as it moves away from the equator.
The reason has to do with momentum and how fast a location on or above the Earth moves relative to the Earth's axis. Your speed relative to the Earth's axis depends on your location. In the graphic above right the person at the position on the equator arrives at the yellow line sooner than the other two. Someone standing on a pole is not moving at all except that he or she would be slowly spinning. The speed of the rotation is great enough to cause you to weigh one pound less at the equator than you would at the north or south pole.
The momentum the air has as it travels around the earth is conserved, which means as the air that's over the equator starts moving toward one of the poles, it keeps its eastward motion constant. The Earth below the air, however, moves slower as that air travels toward the poles. The result is that the air moves faster and faster in an easterly direction relative to the Earth's surface below the farther it moves from the equator.
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