Wind speeds and forces
for more definitions please visit Russ Rowlett's pages at the University of North Carolina

Force Name

Speed

Conditions
at Sea
Conditions
on Land
mph

knots

kph

0

Calm

< 1

< 1

< 2

Sea like a mirror. Smoke rises vertically.
1

Light air

1-4

1-3

1-5

Ripples only. Smoke drifts and leaves rustle.
2

Light breeze

5-7

4-6

6-11

Small wavelets (0.2 m). Crests have a glassy appearance. Wind felt on face.
3

Gentle breeze

 8-11

7-10

12-19

Large wavelets (0.6 m), crests begin to break. Flags extended, leaves move. 
4

Moderate breeze

12-18

11-16

20-29

Small waves (1 m), some whitecaps. Dust and small branches move.
5

Fresh breeze

19-24

17-21

30-39

Moderate waves (1.8 m), many whitecaps. Small trees begin to sway.
6

Strong breeze

25-31

22-27

40-50

Large waves (3 m), probably some spray. Large branches move, wires whistle, umbrellas are difficult to control.
7

Near gale

32-38

28-33

51-61

Mounting sea (4 m) with foam blown in streaks downwind. Whole trees in motion, inconvenience in walking.
8

Gale

39-46

34-40

62-74

Moderately high waves (5.5 m), crests break into spindrift. Difficult to walk against wind. Twigs and small branches blown off trees.
9

Strong gale

47-54

41-47

76-87

High waves (7 m), dense foam, visibility affected. Minor structural damage may occur (shingles blown off roofs).
10

Storm

55-63

48-55

88-102

Very high waves (9 m), heavy sea roll, visibility impaired. Surface generally white. Trees uprooted, structural damage likely.
11

Violent storm

64-73

56-63

103-118

Exceptionally high waves (11 m), visibility poor. Widespread damage to structures.
12

Hurricane

74+

64+

119+

14 m waves, air filled with foam and spray, visibility bad. Severe structural damage to buildings, wide spread devastation.
             
Cat 1 Hurricane 74-95 64-82 119-153    
Cat 2 Hurricane 96-110 83-95 154-177    
Cat 3 Hurricane 111-130 96-113 178-209    
Cat 4 Hurricane 131-155 114-135 210-249    
Cat 5 Hurricane 155+ 135+ 249+