As reports of unusually high winds aloft became more and more frequent, investigations were initiated. These investigations uncovered an important weather phenomenon known today as a jet stream. Jet streams can be thought of as a narrow bands of high speed wind, the speed being greater than 70 knots, that are usually embedded in the mean flow pattern that moves through the atmosphere around the entire globe. The length of the area of high speed winds has to be greater than 300 nautical miles to be considered a jet stream. There is not a band or tube which has a identifiable boundary but the winds increase from the external regions to reach a maximum in the center or core of the jet stream.
Wind blows as a result of a pressure difference and the rotation of the earth.
The pressure difference is related to the temperature difference between hot and cold air masses. For the northern hemisphere a person can think of the equator as being warmer than the north pole. Therefore, the temperature differences between cold air masses that form over the northern parts of the northern hemisphere and the warm air masses that form over equatorial regions cause pressure differences which in turn start the wind blowing. The direction of the wind is related to the rotation of the earth. Because the difference between cold and warm air masses is greater in the northern hemisphere winter than in the northern hemisphere summer, the jet streams will be more frequent and stronger in the winter.