The polar jet stream is a narrow band of high-speed winds at tropopause altitude that move in a wave pattern from west to east above the boundary of the polar front, between the Arctic and the midlatitudes.

The position of the peaks and troughs of the wave determine how far southward cold polar air reaches in a region at any given time.

To a great extent, the jet stream steers the daily weather that we experience. In recent years, there has been a noticeable uptick in the amount of extreme weather in the northern hemisphere, in part due to the interaction between climate change and the jet stream.

Explaining the Polar Jet Stream

Since cold air is more dense than warm air, it takes up less space (i.e. is less thick) and pressure decreases more with elevation gain than warm air – think of cold air as being packed more tightly at the surface of the earth.

As a result, although the cold Arctic air has higher pressure than the warmer midlatitude air at earth’s surface, higher up in the atmosphere the reverse is true – the pressure is higher in the midlatitudes.

Since air wants to flow from higher to lower pressure, air begins to flow northward but is then deflected to the right by the Coriolis force to flow from west to east, parallel to the polar front. The greater the difference in temperatures between the Arctic and midlatitudes, the greater the pressure difference and resulting westerly winds in the jet stream.