Polar and subtropical jets

A jet is defined as any core of fast moving air, often indicated by a curved arrows on maps and by television meteorologists. The line is drawn in the location of the strongest winds. Jetstreams are typically wider and not as distinct, but more of a region where the wind increases toward a core of the strongest wind. One way of visualizing this is consider a river. The river's current is generally the strongest in the center with decreasing strength as one approaches the river's bank. It can be said that jetstreams are "rivers of air".

Observation of winds from rawinsondes show that there are two principle regions in the upper troposphere where winds are strongest. The first is in the 50°-60° N/S region and is called the polar jet and is typically associates with the polar front. The second is called the subtropical jet and is located around 30°N. Jet streams vary in height of four to eight miles (about 500 to 200 mb) and reach a maximum at the tropopause, reaching speeds of more than 275 mph.

The actual appearance of jet streams result from the complex interaction between many variables - such as the location of high and low pressure systems, warm and cold air, and seasonal changes. They meander around the globe, dipping and rising in altitude/latitude, splitting at times and forming eddies, and even disappearing altogether to appear somewhere else. Jet streams also "follow the sun" in that as the sun's elevation increases each day in the spring, the jet streams shifts north moving into Canada by Summer. As Autumn approaches and the sun's elevation decreases, the jet stream moves south into the United States helping to bring cooler air to the country.

Source: NWS