The Polar Front Part 2: The Jet Stream

The Polar Front Part 1: Highs and Lows (http://cloudburst.areavoices.com/2016/11/14/the-polar-front-part-1-warm-and-cold-fronts/)

The jet stream: a big river of wind at high altitudes flying along at almost 200 miles per hour. It brings cold air from the north and warm air from the south. It pushes high and low pressure systems around, not just north or south, but also from west to east across the US. That’s all pretty common knowledge, right? Well, it’s not quite right. Let’s take a look at how the jet stream really works.

The jet stream is sometimes called the polar jet, because it sits over the top of the polar front. This isn’t just by coincidence. The polar front has a large temperature change over a relatively short distance across the ground. At the surface, this won’t cause any abnormal wind, but it is important aloft. There’s a few key facts you need to know to put this together. Wind speed is determined mostly by the pressure gradient – how much pressure is changing in any direction. Pressure is determined by the weight of the air above a point. Colder air is denser (the same weight of air takes up less space) than warm air. This means that if there are two areas with the same pressure at the surface but one is colder than the other, the colder air would take up less space, so would be shorter. If that air is shorter, that means that the pressure is lower at the same height over the cold air compared to the warm air. Put all this together with the polar front, and you have a big change in pressure over a short distance aloft. A strong pressure gradient means really strong winds. (If you’re interested in what determines the direction the wind blows, click here: https://cloudburst.areavoices.com/2016/11/22/curving-wind-the-coriolis-force/ ) What this all means is that the polar front essentially causes the jet stream.

jet1

Okay, so the jet stream sits on top of the polar front because of the temperature change, but does it push the high and low pressure systems around? Not so much. Remember that lows form along fronts, so tend to be imbedded in the polar front, sitting under the jet stream. Where the low moves, the front moves. Where the front moves, the jet stream moves. The low isn’t being pushed by the jet stream, it’s pulling it along for the ride. The jet stream doesn’t even really move warm or cold air around. Most movement of warm and cold air (known as temperature advection) takes place below 5,000 ft, with the jet stream at four times that height. The jet stream may help you recognize what’s going on with the weather, but it’s not causing it any more than you are!

Written by Scott Wolff

Image courtesy of http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cyc/upa/jet.rxml