From the gentle breath of a summer breeze to the 150-mph blast of a hurricane making landfall, wind speed and pressure can vary widely. Deriving a rough estimate of wind speed and pressure is fairly simple; all you need is some basic equipment you can find around your home. With some paper cups, a ruler and a few other supplies, you can build an instrument called an anemometer, use it to measure the wind speed, and then use that measurement to calculate wind pressure. The math, however, is a little bit trickier.
Using the scissors, cut or remove the tops from the cups. Staple the cardboard strips together to form a perfect cross. All four arms of the cross should be equal in length.
Staple a cup to each end of the four arms. The mouth of the cup should be perpendicular to the cardboard arm. All four cup openings should face the same direction (i.e., either clockwise or counterclockwise).
Drive the pushpin through the centre of the cardboard cross into the pencil eraser. The cardboard cross and cups should be able to turn easily; if they cannot, adjust them until they can.
Hold your anemometer up into the wind and count the number of rotations for one minute using the stopwatch.
Using your ruler, measure the diameter of your cardboard cross (the length from arm tip to arm tip) in inches, then divide by 12 to convert to feet. Since the circumference of a circle is pi times the diameter, multiply the diameter using your calculator; this will give you the distance around the circle. Record these values using your pen and paper.
Multiply the circumference of your anemometer by the number of revolutions you recorded in one minute; this will give you the wind speed in feet per minute. Multiply by 60 minutes per hour then divide by 5,280 feet per mile to calculate the wind speed in miles per hour.
Measure the air temperature outside with a thermometer and use the table in the resources section to find the air density for your elevation. Wind pressure is the pressure the wind exerts on a structure like a building. There are two components: one side of the building faces into the wind, so it experiences dynamic pressure, while the side facing away from the wind experiences a suction force. To determine the wind pressure, you need to know the air density, which varies with elevation and temperature.
Calculate the total wind pressure. Assuming the wind is blowing perpendicular to the building surface, the formula for dynamic pressure is P = (.5) (density of air) (wind speed squared). The formula for wind suction is P = (.5) (c) (wind speed squared), where c is a constant specific to the structure. The total wind pressure is the sum of these two. The structural constant varies from -0.3 for a cylinder to 1 for a long flat plate. You can also use the calculator provided on Cornell University's website (see the resources section).
The above formula and process will give you only a rough estimate of wind speed and an extremely rough estimate of wind pressure. For buildings and structures that do not present a flat surface to the wind, the calculations become considerably more complex and are beyond the scope of this article. If you need to be sure that a structure can resist wind pressure in a given location, you're best off consulting a professional engineer.
Tips and warnings
- The above formula and process will give you only a rough estimate of wind speed and an extremely rough estimate of wind pressure. For buildings and structures that do not present a flat surface to the wind, the calculations become considerably more complex and are beyond the scope of this article. If you need to be sure that a structure can resist wind pressure in a given location, you're best off consulting a professional engineer.