Wind Predictions: Deteriming Wind Speed and Direction
By Joan Whetzel
What is wind? Wind is brought about by changes in air pressure and temperature causing a horizontal movement of the air. Measuring wind speed and direction is important in many occupations like meteorology, flying airplanes, sailing or boating, and energy/electricity production. Wind speed and direction can be measured accurately with the proper instruments or estimated by eyeballing environmental factors (e.g.trees, flags, windsocks).
Wind Measurement Tools
Weather vanes, or wind vanes, measure wind direction most accurately when placed high enough above the ground that the wind is unobstructed by surrounding trees and buildings. They also must be able to swivel freely. Wind vanes are found at airports, marinas and in rural areas.
Wind socks, located at virtually all airports, help pilots estimate wind speed and wind direction. They must be placed free of all obstructions, be able to swivel freely, and be made of bright colored fabric so they can be spotted easily from the ground or the air.
Flags flap in the wind at varying angles from the flagpole, depending on the wind strength. They indicate the approximate wind speed and direction upon visual inspection. Trees can also be an indicator of wind speed, simply by listening to how much sound that the wind stirs up in the leaves and branches.
Anemometers - either the professional variety or the homemade type made from popsicle sticks, straws and cups - help determine the wind speed. They must be able to swivel freely and be placed free of obstructions. First the revolutions per minute (RPM) are counted. The RPMs are converted to a miles per hour reading (MPH) by using an RPM to MPH conversion chart that is usually supplied with most purchased anemometers.
Satellites, used by meteorologists, determine wind speed and direction by taking multiple samplings from weather stations both on land and at sea.
Estimating Wind Speeds
Professional instruments give more accurate readings. However to estimate the wind speeds use one of three methods. The first involves watching either flags or wind socks. If the flag or windsock are not flying outward at all, the air is still, If they are barely fluttering out from their poles, there's a light breeze around 5 MPH. A fresh breeze around 15 to 20 MPH will cause the flag or windsock to fly outward about halfway to vertical (350- 450 angle), while a Strong Gale around 32 to 38 MPH will have the windsock and flag flying straight out from the pole (900 angle).
The third way is to consult the Beaufort Wind Scale which describes wind speeds in several categories: force, knots, classification (wind name), wind effects on the water, and the wind effects on land. A downloadable copy of the Beaufort Wind Scale can be obtained from NOAA at the following web address: http://www.spc.noaa.gov/faq/tornado/beaufort.html
Estimating Wind Direction
Meteorologists report the wind direction as the direction from which the wind is blowing. So a northeasterly wind blows from the northeast to the southwest. Meteorologists also give wind direction in cardinal directions (N, S, E, W, NE, SE, SW, & NW). There are three ways to estimate wind direction. The first method entails observing a wind vane, and determining the wind direction by comparing the direction the arrow is pointing as compared to the attached cardinal directions indicator. The second requires observation of a wind sock. When the wind sock fills with air, it will swivel on its pole. The wide end points in the direction that wind is coming from, whereas the narrow end points in the direction the wind blows toward. Wind socks, like wind vanes sometimes have cardinal wind direction indicators on them or nearby. If not, it's helpful to have a compass or to have a general sense of North, South, East and West. The third method of wind direction estimation uses flags. If you have a good idea of which direction North, South, East and West are, simply observe which way the end of the flag is flapping and you'll get a pretty good idea of which direction the wind is blowing from .
It's hard to imagine being able to measure or even estimate the speed and direction of something you can't even see. But it's easier if you understand that you are measuring the wind's effects on land, sea and people or animals In other words, we're measuring what the wind's doing more than measuring the wind itself. And it doesn't even require the purchase of expensive gadgets. By simply using our eyes and ears, we can get a rough estimate that, if we compared it to the meteorologists' official readings, we'd find we were pretty close.