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Jet Stream 101

Jet Stream 101

What do changes in speed and direction in the jet stream mean for skiers?
By Joel Gratz
posted: 01/24/2011
The jet stream constantly changes location and speed. Each map above is centered on the U.S. with the colors showing the wind speed around 30,000ft (pink colors show higher winds). The black line shows the general position of the jet stream, and notice how much it changes in only a week.
Photo by: Joel Gratz

You hear the term “jet stream” quite often from meteorologists or anyone who claims some knowledge of weather. But what is the jet stream, and as a skier looking for powder, why should you care?

Oddly, the jet stream has everything and nothing to do with finding powder. That sounds like some strange quote from Groucho Marx, but it’s true. Local terrain and other factors in the weather that have nothing to do with the jet stream are critical to producing powder. However, even though the jet stream is miles above the mountains that we ski, it guides the overall development of the storms that bring us snow.

The jet stream is a fast-moving “river” of air around 30,000ft, or about the elevation that planes fly. In fact, this phenomenon was only discovered about 75 years as the aviation industry took off. Pilots realized that sometimes they were flying into a headwind of 100+mph, and at other times they experienced a tailwind of similar speeds. Eventually, a German meteorologist coined the term jet stream to explain these high winds.

The reasons for the jet stream are relatively simple. Throw together some physics equations, the earth’s rotation, and temperature differences between the warmer equator and the colder poles, and out pops the jet stream. You can generally find the jet stream flowing west-to-east as it separates warmer air to the south from colder air to the north.

In the summer months, the area of greatest temperature contrast from south to north is pretty far north up in Canada, and that’s where the jet stream is as well. Hence, most of the large-scale storminess is up in Canada and not in the U.S. However, the winter season allows the colder air around the pole to sag south into the U.S., and this is where the jet stream sets up shop. Luckily, the U.S. is at about the right latitude to have the jet stream overhead most of the winter, and this means that most wintery storms track either through the U.S. or southern Canada.

But like the double rainbow guy, you’re probably still asking ‘What does it all mean?’ In CliffsNotes form…

  • The jet stream controls the storm track, and as the jet stream CONSTANTLY changes speed and location, so does the storm track. See the image, which shows how the jet stream changes over just one week.

  • Dips in the jet stream are called “troughs”, and this is where storms and colder air generally exist. If you’re looking for powder, try to find the troughs. When the jet stream bulges north, this is called a ridge and usually signifies warm and dry weather.

  • The jet stream brings high winds, especially in the higher mountains of the west. If the jet stream is overhead, chances are that winds at mountain top level will be very strong and avalanche danger could increase markedly if there’s fresh snow that can be blown around.

  • Intense and narrow snow bands can form under the jet stream. Often times these bands can throw down 1-3” of snow per hour but will only be 20-30 miles wide. If your mountain is lucky enough to be under the snow band – expect loads of powder. If you’re on either side, however, chances are you could get much less snow. While meteorologists can forecast the general position of the jet stream, forecasting the exact position of the heavy snow bands created by the jet stream is impossible (at this point in time…who knows what the future holds).

  • Some areas of the jet stream help to create lift in the atmosphere, and this rising motion helps the air to expand, cool, and produce snow. Basically, if the jet stream is overhead, there’s a fair chance that you could see some heavier snow.

And there you have it. This dynamic river of air around 30,000ft can bring the storms, wind, and heavy snow right to your home hill, or it can throw up a ridge over your head and keep the weather warm and dry. Either way, now you know a little bit more about the jet stream and what it means for powder. If you remember only one thing from this article, remember this: Find the dips (troughs) in the jet stream to find the powder. Enjoy!

Meteorologist Joel Gratz is the creator of http://www.ColoradoPowderForecast.com and is based in Boulder, CO.

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