Weather Science

Wicked windy weather: the jet stream, “jet streaks,” and severe weather.

What makes all that nasty weather whip around us in spring and fall? Weather Wonk Ben Ayres continues his exploration of the jet stream with a discussion of pressure systems and “jet streaks.”

Last week we discussed the role jet streams have on temperature. Northward movement of the jet allows for warmer air from the south to penetrate into the higher latitudes. Conversely, as the jet moves south toward the equator, chilly air from our Canadian friends permeates southward into the heart of the U.S. These drastic temperature variations can happen quickly and become quite an annoyance. However, the jet is associated with much more than temperature differences, as any area in the path of a strong jet stream can be subject to severe weather and significant precipitation.

Pressure systems and the Jet

Regardless if you’re a weather novice or expert, most people have heard the terminology “low and high pressure systems”. Discussing the development and formation of pressure systems is a looooong conversation for another day, but there is an obvious correlation between these pressure systems and the mid-latitude jet.

As cold air pushes southward, the jet is thus forced in the same direction and a trough in the upper atmosphere (5–8 miles above the surface) digs southward. In the opposite direction, warm air forces the jet northward, resulting in a bump in the jet also known as a ridge.

Large surface low-pressure systems form immediately to the east of an upper-level jet in the trough. Most lows have fronts attached known as warm and cold fronts, and these fronts give us much of our severe and rainy weather. Often, the most intense weather is associated with cold fronts as cold air violently lifts warmer air upwards, triggering precipitation. The greater the temperature difference, the stronger the cold front which is then able to produce more lifting. The stronger the jet is aloft, the greater the temperature difference at the surface, which can result in more precipitation.

Streaking in the Jet

Although a jet stream is defined as a thin current of rapidly moving air, flowing west to east in the upper part of the Earth’s atmosphere, there are sections within the jet that are faster than its surroundings. These sections are known as jet streaks and are usually located between the trough and ridge in a jet. Since jet streaks are faster than their surroundings, the air aloft diverges faster, which creates lower pressure at the surface and consequently enhances the amount of precipitation.

Think of a jet streak as a bottle of soda. The regular jet stream is a gently shaken bottle of soda and when opened the soda might fizz to the top or barely fizz over, removing only a little soda from the bottle (creating a weak low pressure in the atmosphere). On the other hand a jet streak is like a violently shaken bottle of soda, when it’s opened the soda explodes out of it (creating a strong low pressure in the atmosphere).

The evolution of the jet stream is one of if not the most important weather phenomena to understand. If stormy weather is coming your way, the amplified jet is probably to blame.