Force

Wind
(Knots)

WMO
Classification

Appearance of Wind Effects

0

Less than 1

Calm

Sea surface smooth and mirror-like

Calm, smoke rises vertically

1

1-3

Light Air

Scaly ripples, no foam crests

Smoke drift indicates wind direction, still wind vanes

2

4-6

Light Breeze

Small wavelets, crests glassy, no breaking

Wind felt on face, leaves rustle, vanes begin to move

3

7-10

Gentle Breeze

Large wavelets, crests begin to break, scattered whitecaps

Leaves and small twigs constantly moving, light flags extended

4

11-16

Moderate Breeze

Small waves 1-4 ft. becoming longer, numerous whitecaps

Dust, leaves, and loose paper lifted, small tree branches move

5

17-21

Fresh Breeze

Moderate waves 4-8 ft taking longer form, many whitecaps, some spray

Small trees in leaf begin to sway

6

22-27

Strong Breeze

Larger waves 8-13 ft, whitecaps common, more spray

Larger tree branches moving, whistling in wires

7

28-33

Near Gale

Sea heaps up, waves 13-20 ft, white foam streaks off breakers

Whole trees moving, resistance felt walking against wind

8

34-40

Gale

Moderately high (13-20 ft) waves of greater length, edges of crests begin to break into spindrift, foam blown in streaks

Whole trees in motion, resistance felt walking against wind

9

41-47

Strong Gale

High waves (20 ft), sea begins to roll, dense streaks of foam, spray may reduce visibility

Slight structural damage occurs, slate blows off roofs

10

48-55

Storm

Very high waves (20-30 ft) with overhanging crests, sea white with densely blown foam, heavy rolling, lowered visibility

Seldom experienced on land, trees broken or uprooted, "considerable structural damage"

11

56-63

Violent Storm

Exceptionally high (30-45 ft) waves, foam patches cover sea, visibility more reduced

12

64+

Hurricane

Air filled with foam, waves over 45 ft, sea completely white with driving spray, visibility greatly reduced