Meteorologist Jim Woodmencey |
Spring is
the time of year that we can experience an increase in the wind around western
Wyoming, even here in the relatively wind-protected Jackson Hole Valley.
Usually when the wind picks up around here, I am asked one of two questions,
either, “When is it going to stop?” or “Why is it so windy?”
Answering
the first question correctly depends on how good a forecast I did that day.
Answering the second question usually requires a lengthier and more technical
explanation.
In this
week’s column, I will give you that more technical explanation, to save us both
some time when I see you on the street on the next “windy” day.
The Wind
Blows
Bob Dylan
says, “You don’t need to be a weatherman to know which way the wind blows”. I
say to Bob, “But, it doesn’t hurt!”
Wind is a
rather elusive meteorological variable, especially since we can’t really see
it, like we can clouds or precipitation. Wind, during a storm, is something we
expect. Wind can be an unpleasant nuisance though, especially on a bluebird
day, to cyclists, sailors, paragliders, climbers, etc.
The
atmosphere is constantly adjusting itself, trying to balance the changes in
temperature and humidity from one part of the planet to the other. This leads
to different areas of high and low pressure that encircle the globe, and the
bigger the difference in temperature, and/or humidity, from one area to another,
the bigger the difference in pressure, and the faster the wind blows.
That’s what
gets it started in motion, always moving from high pressure towards lower
pressure. Friction at the surface, mountains, buildings, etc. can slow the wind
down and alter its direction. In the upper levels of the atmosphere, the wind
starts moving from high to low, but it gets re-routed, and turned to the right
in the northern hemisphere, because the earth is rotating. This is known as the
Coriolis Effect.
When we
observe stronger winds, it means that there is a big difference in pressure
across the region, or sometimes across the entire country. A big low-pressure
center over the mid-western U.S. and a big area of high-pressure along the West
Coast, for instance, could result in strong winds in-between, over the Rockies.
That
difference in pressure from Point-A to Point-B is known as a pressure-gradient.
A strong pressure-gradient equals strong winds. You can track that each day by
looking at a surface weather map, and look for big highs and big lows, and lots
of pressure contour lines in-between, as well.
The other
thing that can cause strong winds at the surface is when the jet stream is
directly overhead.
The Jet
Stream
The jet
stream lives up at around 30,000-ft. in the atmosphere, up where jets fly. It
is defined as the region of the atmosphere where the fastest winds are
occurring. The jet stream is also a dividing line, of sorts, between the
coldest air to the north, and warmest air to the south. And once again,
wherever there is a big difference in temperature across a region of the world,
there will be a big difference in pressure, and thus faster wind speeds.
In the
springtime, the jet stream is trying to migrate from a more southern position
to a more northern position, following the line between the warmest and coldest
air, which is changing rapidly with the tilt of the earth, the angle of the
sun, and increased solar radiation over the northern Hemisphere.
Whenever
that jet is directly overhead, it has the potential to translate down to the
surface and give us a little more wind down here at the surface.
You can
track when the winds will be decreasing by following the progression of the jet
stream, and when it is moving away from your location.
Find
current, and forecasted, surface maps and jet stream maps here:
USA Forecast Maps
USA Forecast Maps
Jim is the
chief meteorologist at mountainweather.com and has been forecasting the weather
in Jackson Hole and the Teton Mountains for over 20 years.
Weather Map Examples of Northerly Wind over Rockies
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Source:
MeteoStar
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Source: NCAR
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Surface Map
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Jet Stream Map
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