|Figure A: Jet Stream Wind Speeds|
|Image from NOAA|
Jet streams are the major means of transport for weather systems. A jet stream is an area of strong winds ranging from 120-250 mph that can be thousands of miles long, a couple of hundred miles across and a few miles deep. Jet streams usually sit at the boundary between the troposphere and the stratosphere at a level called the tropopause. This means most jet streams are about 6-9 miles off the ground. Figure A is a cross section of a jet stream.
|Figure B: Location of Jet Streams|
|Image from NOAA|
The dynamics of jet streams are actually quite complicated, so this is a very simplified version of what creates jets. The basic idea that drives jet formation is this: a strong horizontal temperature contrast, like the one between the North Pole and the equator, causes a dramatic increase in horizontal wind speed with height. Therefore, a jet stream forms directly over the center of the strongest area of horizontal temperature difference, or the front. As a general rule, a strong front has a jet stream directly above it that is parallel to it. Figure B shows that jet streams are positioned just below the tropopause (the red lines) and above the fronts, in this case, the boundaries between two circulation cells carrying air of different temperatures.
|Figure C: Typical Locations of Jet Streams Across North America|
|Image from NASA|
The two jet streams that directly affect our weather in the continental US are the polar jet and the subtropical jet. They are responsible for transporting the weather systems that affect us. The polar front is the boundary between the cold North Pole air and the warm equatorial air. The polar jet sits at roughly 60Â°N latitude because this is generally where the polar front sits. The subtropical jet is at roughly 30Â°N latitude. The subtropical jet is located at 30Â°N because of the temperature differences between air at mid-latitudes and the warmer equatorial air. The polar and subtropical jets are both westerly, meaning they come from the west and blow toward the east. Both jets move north and south with the seasons as the horizontal temperature fields across the globe shift with the areas of strongest sunlight.
|Figure D: Typical Locations of Polar Jet Stream Throughout Summer and Winter|
In the winter the polar jet moves south and becomes stronger because the North Pole gets colder but the equator stays about the same temperature. This increases the temperature contrast and moves the strengthened polar front jet farther south. As you probably have noticed, jet streams are not just straight across, but have a wavy pattern. The jets follow the contours of low and high pressure areas (troughs and ridges, respectively), which move like waves in the atmosphere across the earth. In winter when the polar jet dips into the US, the troughs and ridges affect what kind of weather an area will have. Figure D is a comparison of the strength and position of the polar jet in summer versus winter. If a trough is sitting over you, it is generally very cold and snowy or rainy. If a ridge is sitting over you, it is generally warm and dry. So if itâ€™s winter and the jet stream looks like it does in the picture, Utah would be warmer than average and Tennessee would be colder than average, possibly experiencing some snow.
The subtropical jet also moves and evolves over time. The strongest effect of the subtropical jet in the southeastern US is in winters during years when El NiÃ±o is occuring in the eastern Pacific Ocean. The subtropical jet is then positioned in such a way that southern Georgia and Florida are right underneath the main jet, and experience colder and wetter conditions than years when there is no El NiÃ±o.