The Weather Notebook: Anemometer

Anemometer
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Windspeed is measured with a device called an anemometer. You have probably already seen the one with 3 cups that spin around. Meteorologists call this a 3-cup anemometer.

Hi, I'm Bryan Yeaton for The Weather Notebook.

The 3-cup works because as it spins, it measures a specific distance of wind. The faster it spins, the more wind is passing by, and we can measure that in miles (or kilometers) per hour, nautical miles, or meters per second.

Sonic Anemometer at the helipad testing site on Mount Washington.
There are other types of anemometer, though. Often, at the Automated Surface Observing stations (or ASOS), they will use an impeller: basically just a propeller pushed by the wind.

On top of Mount Washington, where high winds would shred most instruments, a pitot tube is used. It looks like a wingless airplane, with a hole in the nose. Wind squeezes into the hole, and the pressure registers on a chart inside the building. Then we have another chart to tell us what that chart is telling us, and voila‹windspeed.

The wind instruments atop Mount Washington are heated to 400 degrees Fahrenheit, which helps keep rime ice from forming and interfering with the readings. Still, observers often have to go out in winds over 100 miles per hour to break the ice off with a special instrument called a crowbar.

A fairly new type of anemometer may help measure winds in similarly nasty conditions: a sonic anemometer. Bursts of sound energy jet between sending and receiving probes, but wind will slow the sound at a predictable rate, and from that we can figure out the windspeed. The advantage of the sonic device is it has no moving parts to freeze up in the icy conditions.

The Weather Notebook is produced by The Mount Washington Observatory with Support from Subaru, and the National Science Foundation.