What is the Jet Stream and Why does it Matter?

One of the most important things we measure in the atmosphere is the wind, specifically where and how fast it’s moving. The jet stream is the current of strong wind speeds around 250 millibars or near the top of the troposphere. These winds act as both a source of weather systems to begin as well as a steering flow to direct weather systems once they have formed. There could be multiple jet streams with the most persistent one being in the mid-latitudes between about 30 and 60 degrees north in the Northern Hemisphere, and vice versa in the Southern Hemisphere. The jet stream almost always moves from west to east with the rotation of the Earth. Wind speeds in the jet stream can reach between 100 and 200 miles per hour.

How the Jet Stream Creates Weather Patterns

The jet stream doesn’t usually travel in a straight line; it usually has peaks and valleys. These peaks and valleys often create weather systems which lead to impactful weather events. The peaks are referred to as “ridges” and the valleys are referred to as “troughs.” These “waves” in the jet stream are referred to as Rossby waves, which are usually between 1,000 and 2,500 miles long. When there is a ridge in the jet stream in the Northern Hemisphere (where the jet stream is closer to the North Pole), warmer air is generally moving further north. When there is a trough in the jet stream in the Northern Hemisphere (where the jet stream is closer to the equator), colder air is generally moving further south. This movement of warmer and colder air can help lead to weather fronts, which are responsible for most weather phenomena.