Wind Energy Reference Manual Part 1: Wind Energy Concepts
Unit Abbreviations
m = metre = 3.28 ft.
s = second
h = hour
N = Newton
W = Watt
HP = horsepower
J = Joule
cal = calorie
toe = tonnes of oil equivalent
Hz= Hertz (cycles per second)

10 -12 = p pico = 1/1000,000,000,000
10 -9 = n nano = 1/1000,000,000
10 -6 = micro = 1/1000,000
10 -3 = m milli = 1/1000
10 3 = k kilo = 1,000 = thousands
10 6 = M mega = 1,000,000 = millions
10 9 = G giga = 1,000,000,000
10 12 = T tera = 1,000,000,000,000
10 15 = P peta = 1,000,000,000,000,000
Wind Speeds
1 m/s = 3.6 km/h = 2.237 mph = 1.944 knots
1 knot = 1 nautical mile per hour = 0.5144 m/s = 1.852 km/h = 1.125 mph
Wind Speed Scale
Wind Speed at 10 m height
Beaufort Scale (outdated)
Wind
m/s
knots
0.0-0.4
0.0-0.9
0
Calm
0.4-1.8
0.9-3.5
1
Light
1.8-3.6
3.5-7.0
2
3.6-5.8
7-11
3
5.8-8.5
11-17
4
Moderate
8.5-11
17-22
5
Fresh
11-14
22-28
6
Strong
14-17
28-34
7
17-21
34-41
8
Gale
21-25
41-48
9
25-29
48-56
10
Strong Gale
29-34
56-65
11
>34
>65
12
Hurricane
Roughness Classes and Roughness Lengths
The roughness class is defined in the European Wind Atlas on the basis of the roughness length in metres z 0 , i.e. the height above ground level where the wind speed is theoretically zero. ln is the natural logarithm function.
if (length <= 0.03)
class = 1.699823015 + ln(length)/ln(150)
if (length > 0.03)
class = 3.912489289 + ln(length)/ln(3.3333333)

You may use the calculator below and enter either the roughness length or the roughness class.
Do not use the calculator until this page and its programme have loaded completely.
CALCULATOR
Roughness Class Calculator
Roughness length in m =
Roughness class
 
Roughness Classes and Roughness Length Table
Rough- ness Class
Roughness Length m
Energy Index (per cent)
Landscape Type
0
0.0002
100
Water surface
0.5
0.0024
73
Completely open terrain with a smooth surface, e.g.concrete runways in airports, mowed grass, etc.
1
0.03
52
Open agricultural area without fences and hedgerows and very scattered buildings. Only softly rounded hills
1.5
0.055
45
Agricultural land with some houses and 8 metre tall sheltering hedgerows with a distance of approx. 1250 metres
2
0.1
39
Agricultural land with some houses and 8 metre tall sheltering hedgerows with a distance of approx. 500 metres
2.5
0.2
31
Agricultural land with many houses, shrubs and plants, or 8 metre tall sheltering hedgerows with a distance of approx. 250 metres
3
0.4
24
Villages, small towns, agricultural land with many or tall sheltering hedgerows, forests and very rough and uneven terrain
3.5
0.8
18
Larger cities with tall buildings
4
1.6
13
Very large cities with tall buildings and skycrapers
Definitions according to the European Wind Atlas, WAsP. For practical examples, see the Guided Tour section on Wind Speed Calculation.

Density of Air at Standard Atmospheric Pressure
Temperature
°Celsius
Temperature ° Farenheit
Density, i.e. mass of dry air kg/m 3
Max. water content kg/m 3
-25
-13
1,423
-20
-4
1,395
-15
5
1,368
-10
14
1,342
-5
23
1,317
0
32
1,292
0,005
5
41
1,269
0,007
10
50
1,247
0,009
15
59
1,225 *)
0,013
20
68
1,204
0,017
25
77
1,184
0,023
30
86
1,165
0,030
35
95
1,146
0,039
40
104
1,127
0,051
*) The density of dry air at standard atmospheric pressure at sea level at 15° C is used as a standard in the wind industry.

Viscosity of Atmospheric Air
Temperature
° Celsius
μ
20
1.80 E -5
1.50 E -5
50
1.95 E -5
1.79 E -5
 
Note: E -5 means exponential notation, i.e. the number should be multiplied by 0.00001
Power of the Wind **)
 m/s
W/m 2
m/s
W/m 2
m/s
W/m 2
0
0
8
313.6
16
2508.8
1
0.6
9
446.5
17
3009.2
2
4.9
10
612.5
18
3572.1
3
16.5
11
815.2
19
4201.1
4
39.2
12
1058.4
20
4900.0
5
76.2
13
1345.7
21
5672.4
6
132.3
14
1680.7
22
6521.9
7
210.1
15
2067.2
23
7452.3
**) For air density of 1.225 kg/m 3 , corresponding to dry air at standard atmospheric pressure at sea level at 15° C.
The formula for the power per m 2 in Watts = 0.5 * 1.225 * v 3 , where v is the wind speed in m/s.
Warning:
Although the power of the wind at a wind speed of e.g. 7 m/s is 210 W/m 2 , you should note, that the average power of the wind at a site with an average wind speed of 7 m/s typically is about twice as large. To understand this, you should read the pages in the Guided Tour beginning with the Weibull Dustribution and ending with the Power Density Function.
Standard Wind Class Definitions (Used in the U.S.)
Class
30 m height
50 m height
Wind speed m/s
Wind power W/m 2
Wind speed m/s
Wind power W/m 2
1
0-5.1
0-160
0-5.6
0-200
2
5.1-5.9
160-240
5.6-6.4
200-300
3
5.9-6.5
240-320
6.4-7.0
300-400
4
6.5-7.0
320-400
7.0-7.5
400-500
5
7.0-7.4
400-480
7.5-8.0
500-600
6
7.4-8.2
480-640
8.0-8.8
600-800
7
8.2-11.0
640-1600
8.8-11.9
800-2000
© Copyright 1997-2003 Danish Wind Industry Association
Updated 12 May 2003
http://www.windpower.org/en/stat/unitsw.htm
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