What is the Jet Stream?

A jet stream forms high in the upper troposphere over the boundary between two air masses of different temperature. The greater the temperature difference between the air masses, the greater the air pressure difference, and the faster the wind blows in the jet stream. This river of air has wind speeds which often exceed 100 mph, and sometimes peak over 200 mph. Jet streams usually form in the winter, when there is a greater contrast in temperature between cold continental air masses and warm oceanic air masses.

But how does the temperature difference between two air masses cause the jet stream? To answer this question, it is first helpful to understand what causes pressure variations (and thus wind) at the Earth's surface. The difference in pressure that cause winds at the surface of the Earth is ultimately due to the horizontal difference in air temperature summed up (integrated) over the entire depth of the atmosphere. Because warm air is less dense (it weighs less), and cold air is more dense (it weighs more), high and low pressure areas represent, in a vertical average sense, cooler and warmer air, respectively.

Jet streams, then, occur at altitudes where the vertically integrated horizontal temperature difference between two air masses is at a maximum. This causes a maximum in the pressure 'gradient' (horizontal difference in air pressure) at that altitude. In the absence of surface friction, the turning of the earth causes the wind (which wants to flow from high pressure to low pressure) to turn to the right (in the Northern Hemisphere), so the wind flows around high and low pressure areas, rather than from high to low pressure.

Get more information about the many factors that influence weather

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