One of the interesting things about wind is that friction with the ground and contact with the objects along the ground slow it.
Thus, wind is significantly stronger at higher elevations than at ground level. This was evident last week when strong winds caused significant damage in the Blair area.
As a powerful storm swept through the area Tuesday morning, Aug. 6, a trained weather spotter reported to the National Weather Service that winds in Blair were blowing at about 70 mph. His report was based on his own educated estimate.
The nearest actual wind sensor, located at the Fort Calhoun Nuclear Station, clocked winds at 92.5 mph — extremely powerful for a straight-line wind.
Widespread damage was reported at Blair, while at the Fort Calhoun nuclear plant, no significant damage was reported.
In Blair, trees were knocked down, the city was a swirl of downed power lines, and east of town a warehouse roof was blown off.
At the nuclear plant, which is about 8 miles south of Blair, a small tree was toppled and skirting was blown off a modular building, according to Jeff Hanson, spokesman for the Omaha Public Power District.
Power outages were a problem in both locales. The nuclear plant’s administration building lost power that morning, as did Blair’s water treatment plant.
Ground-level winds of 90 mph would have caused measurably more damage at the nuclear plant than was reported.
So what happened?
The nuclear power plant’s wind sensors are mounted in the air, so they clocked winds that were stronger than what was occurring at ground level, whatever that might have been.
Hanson said the meteorological tower at Fort Calhoun is 60 meters tall and has wind sensors at 10 meters and 60 meters.
In U.S. measurements, that would mean the sensors are at 32 feet, 10 inches and 196 feet, 10 inches. Both sensors recorded a maximum wind speed of 92.5 mph, he said.
The wind didn’t damage the reactor building, Hanson said. Fort Calhoun is designed to withstand the force of 500-mph winds driving an object the size of a utility pole into the reactor building, he said.
Federal rules require nuclear plant owners to notify regulators when wind speeds exceed 90 mph. OPPD did that Tuesday at about 7:22 a.m. when the wind gusted to 90.2 miles per hour, Hanson said.