Jet Setting…

One of the most frequently asked questions I get deals with the jet stream. Okay… I’m lying. Nobody has ever asked that question, but I needed a way to get into today’s blog about jet streams. 

One of the tools most meteorologists look at when developing a forecast is the jet stream. Jet streams are segments of fast moving air that are found in the upper troposphere. The troposphere is the lowest level of the earth’s atmosphere. It is also where most of the earth’s weather occurs. (Extra credit points for anyone who can name the various layers of the earth’s atmosphere!) The two most common jet streams that affect weather in the U.S. are the Polar Jet and the Subtropical Jet.

Jet streams form when two air masses of different temperatures come together.  For example, in the United States, the Polar Jet is a west to east band of winds that forms along the boundary between the cold, polar air of Canada and the relatively warmer air over the continental U.S. Farther south, the subtropical jet forms along the boundary of tropical air from near the equator meets the more temperate air over the continental U.S. It also flows west to east. Typically, air on the poleward side of the jet is cooler than air to the south. Jet streams are not a nice continuous line of strong winds like depicted on weather maps. They can be broken into segments, they can split then come back together, and can meander wildly across the globe. It is these meanderings that develop into troughs and ridges aloft that ultimately affect weather at the surface. If that wasn’t enough, jet streams can have areas of faster or slower wind speeds within the stream itself. Whew!    

Jet streams were not discovered until World War II. Bomber pilots discovered that when traveling from west to east across the mid-latitudes, they would encounter bands of winds that would carry their plane along faster than the surrounding air. This, in turn, used less fuel and got them to their destinations faster. When returning west, these same winds slowed the forward speed of the plane, causing them to use more fuel and take longer to complete the trip. Some smart apple figured out it was these fast moving ribbons of air that were the culprits. In fact, this is where the word “jet stream” comes from.

Jet streams are important forecast tools for a couple of reasons. First… because they seperate air masses of different temperature, they can give an indication of where storm systems will develop. Low pressure systems like to form where cold air and warm air meet. Since jet streams are the boundaries between these air masses, they are a good place to look. Once a low pressure system forms, it will usually follow the path of the jet stream. Knowing where the jet stream lies also gives the forecaster a general idea of what the overall weather pattern will do. When I’m making a forecast, I will use the position of the jet stream over the next few days to get a “rough guess” as to what the overall forecast will bring. For example… when I look at a chart of upper level winds and see the jet stream arcing from the west coast to southern Canada, then diving south across the Great Lakes and Ohio Valley, I know right away that we are in for cool and somewhat wet pattern because we are on the north side of the jet stream, meaning we are on the cool air side. Of course, that is only a guide. We use other forecasting tools to fine tune the forecast. 

Here’s a good rule of thumb for you to remember… when the jet stream arcs north of the Great Lakes, we are said to be under a ridge. This usually leads to warm and dry weather. (The weather we’ve seen over the past two weeks is a good example of an upper level ridge). If the jet stream dives south of the Great Lakes, we are said to be in a trough and cloudy, cool and somewhat rainy weather will be the rule.

Take care and talk to you soon!

Cory