Set up an ordinary flag in an open area. Do not place the flag near areas where wind is likely to swirl around and create unnatural wind pockets such as valleys. If you are on water, set up the boat’s flag in either the bow or stern of the boat.
Observe the flag’s movement. If the flag is not stirring, the Beaufort number is zero. Winds are calm and less than 1 mph.
Notice how the flag moves when it flaps in the wind. Note in particular the pattern of movement and how the lower edge hangs. If the lower edge hangs low, the Beaufort number is 1. The wind is considered light air and 3 miles or less per hour.
Notice if the flag is extended in the wind. Observe how the top corner behaves in the wind. If the top of the flag is flapping back and forth with the flag mainly extended, winds are likely 7 mph or less. This is Beaufort number 2.
Note the pattern of waves of the flag if it is extended. At Beaufort number 3, waves are smaller. Wind speeds approach 12 mph.
Watch how rapidly the flag flaps in the wind. Wind speeds edge toward 18 mph at Beaufort number 4. The flag is likely in constant motion.
Note if the flag is rippling. At Beaufort number 5, the flag flaps and ripples in the wind. Wind speeds up to 24 mph may be observed.
Listen for a popping sound as the flag waves in the breeze. As wind speeds climb to 31 mph, the flag will continue to ripple and flap. The higher force of the wind causes the flag to beat loudly. Damage to the flag is likely to occur at this point at Beaufort number 6.
