Force

Mph

Knots

Description on Land

Description at Sea

0

Calm

0 - 1

0-1

Smoke rises vertically.

Sea like a mirror.

1

Light Air

1 - 3

1-3

Wind felt on face.

A glassy appearance maintained. 

2

Light Breeze

4 - 7

4-6

Leaves rustle.

Ripples formed but do not break.

3

Gentle Breeze

8 - 12

7-10

Ordinary vanes moved by wind.

Small wavelets.

4

Moderate Breeze

13 - 18

11-16

Raises dust and loose paper, small branches are moved.

Small waves - becoming longer; fairly frequent white horses.

5

Fresh Breeze

19 - 24

17-21

Small trees in leaf begin to sway; crested wavelets form on inland waters.

Moderate waves, taking a more pronounced long form; many white horses are formed - a chance of some spray

6

Strong Breeze

25 - 31

22-27

Large branches in motion; whistling heard in telephone wires; umbrellas used with difficulty.

Large waves begin to form; the white foam crests are more extensive with probably some spray

7

Near Gale

32 - 38

28-33

Whole trees in motion; inconvenience felt when walking against wind.

Sea heaps up and white foam from breaking waves begins to be blown in streaks along direction of wind.

8

Gale

39 - 46

34-40

Twigs break off trees; progress generally impeded.

Moderately high waves of greater length; edges of crests begin to break into spindrift; foam is blown in well-marked streaks along the direction of the wind.

9

Severe Gale

47 - 54

41-47

Slight structural damage occurs -roofing dislodged; larger branches break off.

High waves; dense streaks of foam; crests of waves begin to topple, tumble and roll over; spray may affect visibility.

10

Storm

55 - 63

48-55

Seldom experienced inland; trees uprooted; considerable structural damage.

Very high waves with long overhanging crests; the resulting foam in great patches is blown in dense white streaks; the surface of the sea takes on a white appearance; the tumbling of the sea becomes heavy with visibility affected. 

11

Violent Storm

64 - 72

56-63

Very rarely experienced - widespread damage

Exceptionally high waves; small and medium sized ships occasionally lost from view behind waves; the sea is completely covered with long white patches of foam; the edges of wave crests are blown into froth.

12+

Hurricane

73 - 83

64-71

The air is filled with foam and spray. Sea completely white with driving spray; visibility very seriously affected.