Jet Stream and Horizontal Temperature Gradients
Today's weather reports often discuss the position of the jet stream. A jet stream is a narrow stream of relatively strong winds (i.e. wind speeds greater than 50 knots). The term jet stream is used to describe these winds because of how they were discovered.
The existence of a jet stream moving from west to east was suspected because of the movement of storms and cloud systems, but remained unproven until World War II, as the United State prepared for major air raids against Japan. The B-29s flew at an altitude of 30,000 to 35,000 feet where they encountered a strong stream of westerly winds, the speeds of which were comparable to the air speed of the jet planes of the time. This jet stream slowed the forward progress of the planes as they flew towards the west.
The region that separates warm tropical air from cold polar air is referred to as the polar front. Air pressure decreases more rapidly with altitude in the cold polar air mass than in the warmer tropical air. A pressure gradient force is thus generated between cold and warm masses of air, with relative lower pressure over the cold air. The difference between the air pressure in the warm and cold air increases with altitude above the surface. The horizontal pressure difference generates a pressure gradient force that accelerates the air from the region of higher pressure (the warm air) towards the lower pressure (the cold air). The Coriolis force eventually balances the horizontal pressure gradient force resulting in a strong stream of air that flows from the west towards the east in the upper troposphere in the vicinity of the polar front. This jet stream of air is referred to as the polar front jet.
The existence of the polar front jet streams is tied to the presence of horizontal temperature gradients. If temperature gradients exist through a deep layer of the troposphere, a pressure gradient force increases with height throughout the layer, and therefore so does the wind. The relationship between wind speed and temperature gradients is demonstrated with the following simple model. (Why use simple models?)
Use the thumbwheel at the top to set the surface temperature gradient between the equator and polar regions. The gradient is shown in the upper left hand figure. The resulting maximum wind speed is plotted as a function of latitude in the lower diagram. The right hand figure shows how wind speed varies with altitude at a given latitude. Drag the orange vertical bar to adjust the latitude to use for the vertical wind speed profile.
Use the simple model below to answer the following questions:
- As you change the temperature gradient what happens to the maximum wind speed?
- What does this simple model predict about the global winds if the temperature was uniform across the globe?
- Does this model predict stronger winds during summer or winter?
Equations from Stull: "Meteorology for Scientists and Engineers, 2nd Edition", Brooks/Cole 2000.