INTRODUCTION - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 4

in the US states of California, Oregon, and Washington, and in comparable passes in
other countries.
While temperature and pressure differences create the wind, it can be strongly
influenced by topography and land surface conditions as well, as the example of
coastal mountain passes attests. Where the wind is driven over a rise in the terrain,
and especially over a ridge that lies transverse to the flow, there can be a significant
acceleration, as the air mass is “squeezed” through a more restricted vertical space.
Thanks to this effect, many of the best wind sites in the world are on elevated hilltops,
ridges, mesas, and other terrain features. However, where the air near the surface tends
to be cooler and heavier than the air it is displacing, as in the sea breeze example,
it has a tendency to find paths around the high ground rather than over it. In such
situations, it is often the mountain passes rather than the mountain tops that have the
best wind resource.
Surface vegetation and other elements of land cover, such as houses and other
structures, also play an important role. This role is often represented in meteorology
by a parameter called the surface roughness length , or simply the roughness. Because
of the friction, or drag, exerted on the lower atmosphere, wind speeds near the ground
tend to be lower in areas of higher roughness. This is one of the main reasons why
the eastern United States has fewer good wind sites than, for example, the Great
Plains. Conversely, the relatively low roughness of open water helps explain why
wind resources generally improve with distance offshore.
1.2 KEY CHARACTERISTICS OF THE WIND
The annual average wind speed is often mentioned as a way to rate or rank wind
project sites, and indeed, it can be a convenient metric. These days, most wind project
development takes place at sites with a mean wind speed at the hub height of the tur-
bine of 6.5 m/s or greater, although in regions with relatively high prices of competing
power or other favorable market conditions, sites with a lower wind resource may be
viable. However, the mean speed is only a rough measure of the wind resource. To
provide the basis for an accurate estimate of energy production, the wind resource
must also be characterized by the variations in speed and direction, as well as air
density, in time and space.
1.2.1 The Temporal Dimension
The very short timescales of seconds and less is the domain of turbulence, the general
term for rapid fluctuations in wind speed and direction caused by passing pressure
disturbances, or eddies, which we typically experience as brief wind gusts and lulls.
Turbulence is a critical mechanism by which the atmosphere gradually sheds the
energy built up by solar radiation. Unfortunately, it has little positive role in power
production because wind turbines cannot respond fast enough to the speed variations.
In fact, high turbulence can cause a decrease in power output as the turbine finds