There are many things that influence your accuracy when firing a weapon. The only one that is impossible to control is wind speed. Estimating wind speed is simple using a flag. Knowing the wind speed can help you compensate for any wind and get more accurate shots. There are a few different methods for using flags to estimate wind speed.
Find a flag hanging in the wind. The flag should be the size of an average American flag. The direction the flag flies will indicate the direction of the wind.
Observe the angle between the flag and the flag pole.
Estimate the angle. This can be eyeballed fairly easily and it doesn't have to be 100% accurate to give you a reasonable estimate for wind speed.
Divide the angle by four. The result will be your wind speed in mph. For example, if your flag is flying at 40 degrees, dividing it by four will give you the result of 10mph.
Compensate for the wind by aiming your gun upwind. The wind will push your bullet back towards your target. Aiming into the wind isn't an exact method, so this will take practice.
Find a flag blowing in the wind. This method requires no math but more of a subjective reading. The Beaufort Method uses a heavy flag, which are large flags found at military bases or weather stations.
Observe how it reacts to the wind. If the flag is not moving at all, the wind speed is below 11mph.
Observe the flag. If it flaps lightly and sporadically the wind is blowing at around 12 to 18mph.
Estimate the flapping of the flag. If the flag is flapping over the whole length of the flag, it is blowing at around 19 to 24mph.
Measure how far the flag extends in the wind. If it is partially extended and flaps quickly, the wind speed is 25 to 31mph. A fully extended flag flapping hard in the wind will be around 32 to 37mph. It's not possible to use a flag to measure any higher as the flag will react in the same way, even with harsher winds.