In the case of the atmosphere, the jet stream is the force that pushes storm systems along. The jet stream, or polar jet stream, is a narrow band of fast-moving air located roughly between 30, and 45, feet in the air based on the time of year.
It separates the colder air to the north from the warmer air south of it. The jet stream is nicknamed the "storm track" since systems push from one end of the country to the other, courtesy of the jet stream. However, if a system gets detached from the jet stream, it can meander in place for days without that atmospheric push. So, there are times when systems move differently from the general west to east direction. Cold fronts tend to move in from the north, northwest, or west.
However, backdoor cold fronts move in from the northeast. 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. As the difference in temperature increases between the two locations the strength of the wind increases. The actual appearance of jet streams result from the complex interaction between many variables - such as the location of high and low pressure systems, warm and cold air, and seasonal changes. Jet streams also "follow the sun" in that as the sun's elevation increases each day in the spring, the average latitude of the jet stream shifts poleward.
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