Wind Characteristics - Wind Energy: Renewable Energy and the Environment - page 40

TABLE 3.4
Typical Values of the Roughness Parameter, z 0

Terrain Description z 0 , m

Snow, flat ground 0.0001

Calm open sea 0.0001

Blown sea 0.001

Snow, cultivated farmland 0.002

Grass 0.02-0.05

Crops 0.05

Farmland and grassy plains 0.002-0.3

Few trees 0.06

Many trees, hedges, few buildings 0.3

Forest and woodlands 0.4-1.2

Cities and large towns 1.2

Centers of cities with tall buildings 3.0

¤
¦ ¥

³
µ ´

50
1.2

ln

ln 41.7
ln

( ) 3.7

2.1

v 8 8 8 14.1 m/s

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³
µ ´

(8.3)

10
1.2

ln

This compares to 10 m/s using the power law with a shear exponent 0.14

3.5 WIND DIRECTION
Changes in wind direction are due to the general circulation of atmosphere, again on an annual basis
(seasonal) to the mesoscale (4-5 days). The seasonal changes of prevailing wind direction could be
as little as 30° in trade wind regions to as high as 180° in temperate regions. In the plains of the
United States, the predominant directions of the winds are from the south to southwest in the spring
and summer, and from the north in the winter. Traditionally, wind direction changes are illustrated
by a graph, which indicates percent of winds from that direction, or a rose diagram ( Figure 3.11 ) .
There can also be change in wind direction on a diurnal basis. However, a wind shear of change
in wind direction with height is generally nonexistent or small, except for very short time periods as
weather fronts move through. Wind direction data (hour average wind speeds) from sixteen stations
in Texas and one in New Mexico [5] did not find any significant wind shear of change in direction.
Even on Padre Island, Texas, the land-sea breeze was not significant. Pivot tables were used to
check on the relation between wind speed, wind direction, and time of day for the above seventeen
met stations, plus two tall-tower met stations.

3.6 WIND POWER POTENTIAL
The most comprehensive, long-term source of information on wind speeds, pressure, and temper-
ature is data collected at National Weather Stations. Other sources in the United States on record at
the National Climatic Center, Asheville, North Carolina, are from Federal Aviation Administration
stations, U.S. air bases, Coast Guard, etc. In the early 1960s anemometers at National Weather
Stations were changed from their previous locations (20-30 m heights) on airport control towers,
hangers, etc., to towers (around 6 m height) close to the runways and at least 1 km from buildings.