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What is the Jet Stream?

Now that we have moved out of summer into fall the jet stream plays a major role in our everyday weather.  One can imagine the jet stream as a river of air flowing high in the atmosphere steering our winter weather systems through the mid latitudes.  The jet stream is caused by the uneven heating of the earth.  Cold air is denser than warm air and along edges of the cold to warm air the atmosphere has a slope to it.  Try to imagine it as a bowl of cold air and a mound of warm air.  The sides of the bowl represent the temperature difference between the air masses.  The larger the temperature difference the steeper the sides of the bowl.  Now this may be difficult to imagine, but the temperature difference with respect to distance is the driving force of wind.  Where there is the largest change in temperature in distance there is a steeper side to the “bowl” and the strongest winds reside where the slope of the “bowl” is greatest.

The jet stream winds separate the large scale warm air form the cold air.  Some areas have larger temperature difference than others and the winds are stronger in these areas.  These areas of strongest winds are called jet streaks.  These jet streaks are responsible for movement and location of the comma clouds or storm systems we see on Satellite imagery.  The jet stream is a large feature and so are the storm systems associated with it, some storms can be thousands of miles across and affect more than half of the U.S.

In the summer months the jet stream weakens as the North Pole warms and the temperatures difference from north to south decreases.  Jet stream winds decrease and is pushed north into Canada rarely dropping into Colorado.  Daytime heating is the driving force in summer as warm moist air becomes unstable in the afternoon resulting in afternoon and evening thunderstorms.  As the sun sets daytime heat is lost and thunderstorms usually subside overnight only to redevelop the next day.

This winter the jet stream has remained north of Colorado making for a rather tranquil start to the fall season.  If the jet stream drops further south our weather pattern will become more active here in Colorado during the months of November and December.  November is usually one of the snowiest months in the mountains along and west of the Continental Divide due to the jet stream migrating south steering winter storms into our area.