The Weather Guys

Wind has both speed and direction. Anemometers measure wind speed and wind vanes measure wind direction.

A typical wind vane has a pointer in front and fins in back. When the wind is blowing, the wind vane points into the wind. For example, in a north wind, the wind vane points northward.

A cup anemometer is a common tool to measure wind speed. The cups catch the wind and produce pressure difference inside and outside the cup. The pressure difference, along with the force of the wind, causes the cups to rotate. Electric switches measure the speed of the rotation, which is proportional to the wind speed.

At wind speeds below about 3 mph, the cup anemometer is prone to error because friction keeps the cups from turning. At wind speeds above 100 mph, cup anemometers often blow away or give unreliable measurements. In freezing rain, the anemometer can literally freeze up and stop turning.

Propellers also can measure wind speed. The propeller blades rotate at a rate proportional to the wind speed.

A windsock often is used at airports. A windsock is a cone-shaped bag with an opening at both ends. When it is limp, winds are light; when it is stretched out, winds are strong. Pilots can quickly determine the wind direction and speed along a runway just by observing the shape and direction of a windsock.

Sonic anemometers use sound waves humans cannot hear to measure wind speed and direction. The instrument determines the wind velocity by measuring the time between when the instrument sends a sonic pulse and when it is received.

Our exceptionally cold winter has been the subject of this column a couple of times in the past few months. Some readers have asked how the jet stream might be related to cold air outbreaks.

As we have mentioned before, the jet stream is a ribbon of strong west-to-east winds located approximately 6 miles above the ground. The jet exists as a result of a pole-to-equator temperature difference throughout the entire depth of the lowest 6 or so miles of the atmosphere.

Naturally, when the polar regions get colder in winter, this temperature difference increases and the jet stream intensifies. A strong, mostly west-east oriented jet can act like a dam to the southward progress of cold air produced in the polar regions.

However, the Northern Hemisphere winter jet is usually quite wavy, and this waviness allows excursions of warm air poleward or cold air equatorward. This winter, the waviness has led to repeated equatorward excursions of polar air over North America.

A recent study, co-authored by one of our colleagues at UW-Madison, has suggested that reductions in Arctic sea ice, which have made the Arctic warmer, have effectively reduced the pole-to-equator temperature difference. The suggestion is that this has weakened the wintertime jet and increased the likelihood that it will be wavier than normal. Such increased waviness, coupled with a related tendency for the waves to move more slowly, might underlie an increased frequency of such cold winters.

This theory – though plausible — has not gained wide acceptance and is being challenged from a number of different perspectives. But that is the nature of science. Ideas are constantly compared with each other, and skepticism prevails among colleagues as to what is the best answer. Only the most comprehensive explanations of nature emerge from this relentless and intense intellectual competition.