Beaufort's Scale
( How to Guess a Wind Speed, Beaufort's Number )

#Max. SpeedNameConditions noted Sea/Land
knotskm/hmi/h
0121CalmSea like a mirror.
Smoke rises vertically.
1354Light AirRipples only.
Smoke drifts and leaves rustle.
26117Light BreezeCrests, non-breaking smooth, 1/2 foot
Wind felt on face.
3101911Gentle BreezeWaves 2 feet, crests begin to break.
Flags extend
4162918Moderate BreezeWaves 3 feet, 10% whitecaps.
Dust moves about.
5213924Fresh BreezeWaves 4 1/2 foot, 40% whitecaps.
Small Trees begin to sway.
6275031Strong BreezeWaves 8 foot, some spray.
Large branches move, wires whistle, umbrellas are impossible.
7336138Near GaleMounting sea 12 foot with foam blown in streaks downwind.
Whole trees in motion, walking inconvenient.
8407446GaleWaves 18 foot, crests break, foam clumps.
Walking dangerous, small branches break off trees.
9478754Strong GaleWaves 20 feet, dense foam, visibility affected.
Minor structural damage occurs ( shingles blown off roofs ).
105510263StormWaves 30 feet, heavy sea roll, visibility impaired. Sea surface white.
Trees uprooted, structural damage siding of building torn away.
116311873Violent StormWaves 40 feet, visibility poor.
Widespread damage to structures.
1264+119+74+HurricaneWaves 50 feet, air filled with foam and spray, visibility 12 feet.
Severe structural damage to buildings.