Where is the Wind this Week?

2018-07-15 18:09:32.000 – Griffin Mooers, Summit Intern

 

The Mount Washington Observatory’s claim to fame has long centered on wind, with the observatory seeing the highest wind gust ever (recorded by man) at 231 mph on April 12th, 1934. And at an average annual wind speed of 35 mph, the observatory has the highest average wind of any station in North America. And it’s personally given me by far the strongest wind’s I’ve experience in my life, with gust’s over 90 miles an hour on previous shifts. But so far this shift, the mountain has not lived up to its reputation as the home of the world’s worst weather.

Average daily wind speed for the shift so far according to the RMYoung Wind Speed and Vane Instrument

Although these winds may not be the ‘extreme’ that people expect from Mount Washington, in truth, it’s not unprecedented at all. On Mount Washington, the northeast, and the northern hemisphere more broadly, winds tend to drop off in summer as compared to winter.

 

Mount Washington’s average monthly wind speeds in mph (mountwashington.org)

The summer the wind speed average, in miles per hour, hovers in the twenties, which is well below the average of 40s in the winter months. This substantial difference is largely due to a changing temperature gradient.

 

Image from the North Carolina Climate Office showing seasonal movement of the polar jet

The temperature gradient between the equator and the poles decreases significantly from winter to summer. This in turn causes the polar jet, which is located far to our south in the winter (Generally around 40° N) to weaken substantially and retreat far up into Canada.

 

Jet Stream separates cold and warm air masses (nasa.gov)

The polar jet stream normally acts to separate the cold air mass from the warm air mass. But as it weakens and moves north, the division between cold and warm, as well as between high and low pressures, decrease. Wind is simply the flow of air away from a high pressure or towards a low pressure. When the gradient of the pressure decreases, the speed air flows also decreases. So although our summers are still windy compared to other locations in the region, the weakening summer hemispheric temperature gradient makes them more mild, especially compared to the mountains winter’s, when the wind frequently gusts over 100 mph. These calmer conditions make summer an ideal time to come up to the summit for a hike or visit.

This is not to say that extreme winds never happen on Mount Washington in the summer. All visitors should always be prepared for harsh weather, even in summer. An example of strong summer winds was July 20, 1996, when the wind gusted up to 154 mph. However, these powerful winds normally need a powerful external trigger in the summer, such as a tropical system moving north into New England, or a line of powerful thunderstorms passing through the White Mountains.

 

Griffin Mooers, Summit Intern

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