Comparing Wind Speed at Different Locations

2015-11-14 20:38:32.000 – Michael Dorfman, Weather Observer/IT Specialist

The Mount Washington Observatory has a regional network of 19 different mountainous automated weather stations. This includes 6 sites every 1000 feet along the Mount Washington Auto Road from base to summit. We’ve recently installed anemometers at our 4000 and 5300 foot sites in hopes to better record wind speeds above tree line on Mount Washington. Our 4000 foot site is located near an old Signal Corps foundation block, and is rumored to occasionally have wind speeds rivaling the summit. Our 5300’ site is our highest site in our network of stations and is exposed to some impressive weather as well!

I wanted to do a very brief analysis of the data we’ve recorded so far from these anemometers (they’ve been installed since September). Don’t consider this to be a thorough analysis of this data-it’s more of a sneak-peak with more to come.

First, let’s take a look at the average wind speed from various directions. The following wind roses indicate the average wind speed from various directions. Winds are named for where they come from, so the wedge pointing to the northwest indicates the average wind traveling from the northwest.

We can see that lower elevations see dramatically lower wind speeds. As you travel further down the mountain, you are less exposed, lowering wind speeds. In additions, the Venturi Effect, which is responsible for giving Mount Washington its famous winds, has less of an effect the further down the mountain you travel.

Another big reason for these values is due to geography. Both 5300′ and 4000′ are surrounded by mountainous terrain, blocking and steering the air dependent on direction. Take a look at the map below to see the three sites along with their surrounding land features:

Another explanation for these values is the size of the sample set. We’ve only had a few months of data from these sites, giving us a very good idea of prevailing wind directions (typically west and northwest on the summit), but not a very good sense of not-so-common directions. Our previous world record wind speed of 231 mph came from the Southeast, a direction we haven’t seen too much of in the last few months.

One thing stands out to me, and that is the northwest wind direction at 4000 feet. You can easily see wind speed from the northwest is over twice as strong as any other direction. Let’s look a bit more into this. So, while wind speed is from the northwest at 4000 feet, what direction is it on the summit? The answer to this question will tell us the effect terrain has on steering the wind.

This shows that wind speed from the northwest at the 4000 foot site typically yields wind direction from the west on the summit. When looking at the map above, you can see a ridge immediately to the west of the site. It’s possible that the wind is steered around this ridge, approaching the site from a more northwesterly direction. However, more research would be required to confidently say that this is the cause.

How do speeds on the summit compare to speeds at 4000 feet when coming from the northwest? Does the ratio of summit-to-4000-foot speeds change depending on wind speed? Let’s take a look:

For this specific direction, wind speed at 4000 feet is about one quarter that of the summit wind speed at low winds and around half the speed of the summit wind speed at higher wind speeds. If we’re recording 48 mph from the Northwest at our 4000 foot site, we will, on average, record 105 mph on the summit. While this is what we would see in theory, reality may be completely different. We’re looking forward to getting more data so we can come to more solid conclusions!

Michael Dorfman, Weather Observer/IT Specialist

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