Wind effects have long been a primary weather concern. The 19h-century Beaufort Scale that mariners still rely on divides wind speed into 12 categories based on the effects of wind at different velocities. The Beaufort Scale uses knots (nautical miles) to measure wind speed. Today, most anemometers and other wind measuring devices are calibrated in miles or kilometers per hour. To calculate wind speed in knots, you'll need to convert the instrument readings into nautical miles per hour.
Understand what knots are. The Earth's equator is a circle divided into 360 degrees. Each degree can be subdivided into 60 "minutes of arc," for a total of 21,600 minutes of arc (60 X 360 = 21,600). A nautical mile is the distance represented by one minute of arc and is a little longer than a mile. To be exact, a knot is 6080 feet, while a mile is 5280 feet.
Determine wind velocity. To measure wind speed you need an anemometer or equivalent device. Anemometers look like small windmills with the arms horizontal. Each arm ends in a small cup to catch the wind. When wind strikes the cups, it causes the arms to spin around a central shaft (again, like a windmill). The rate of rotation can be converted into a measure of wind speed. Inexpensive anemometers are available at building supply stores like Home Depot and Lowe's. Follow manufacturer's instructions to setup the anemometer. Taking the measurement is as simple as looking at the wind speed indicator.
Convert wind speed in miles per hour to knots per hour. To do this, multiply the wind speed by 5280, and then divide by 6080. For example, a wind speed of 15 miles per hour equals 15*(5280/6080), which works out to 13.03 knots.
Calculate knots per hour from kilometers per hour. One nautical mile is equal to 1.852 kilometers. Divide the speed in kilometers per hour by 1.852 to find knots. If you measure a wind speed of 30 km/hr, you have 30/1.852 = 16.2 knots.
