Boaters, shooters and archers can all benefit from knowing the wind speed on a given day. A flag is a useful aid for estimating wind speed up to a certain point. A very gentle breeze might have no effect, and once the flag is horizontal and flapping, it will remain that way, no matter how much stronger the wind blows. If the wind is breezy, though, you can gauge its speed by looking at a flag.
Look at any flag on a pole. If you don't see any motion, the wind is blowing less than about 5 kilometers per hour (3 miles per hour).
Estimate the angle between the bottom of the flag and the pole if the flag is extended.
Divide the estimated angle by 4 to get the wind speed in miles per hour. For example, if you estimate the angle to be 45 degrees, the wind speed is approximately 11 miles per hour (18 kilometers per hour). This is the method used by United States Army sharpshooters.
The maximum wind speed you can estimate by using a single flag and the army's technique is 22.5 miles per hour (36 kilometers per hour). You can make more accurate estimations by deploying several flags on the same pole, each of a different weight.