INTRODUCTION - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 2

The details of this process are the subject of this topic. Before diving into them,
however, we should back up a little and set wind resource assessment in context.
Where does the wind come from? What are its key characteristics? And how is it
converted to electricity in a wind power plant?
1.1 WHERE DO WINDS COME FROM?
The simple answer to this question is that air moves in response to pressure differences,
or gradients, between different parts of the earth's surface. An air mass tends to move
toward a zone of low pressure and away from a zone of high pressure. Left alone, the
resulting wind would eventually equalize the pressure difference and die away.
The reason air pressure gradients never completely disappear is that they are con-
tinually being powered by the uneven solar heating of the earth's surface. When the
surface heats up, the air above it expands and rises, and the pressure drops. When
there is surface cooling, the opposite process occurs, and the pressure rises. Owing to
differences in the amount of solar radiation received and retained at different points
on the earth's surface, variations in surface temperature and pressure, large and small,
are continually being created. Thus, there is always wind somewhere on the planet.
While uneven solar heating is ultimately the wind's driving force, the earth's rota-
tion also plays a key role. The Coriolis effect 1 causes air moving toward the poles
to veer to the east, while air heading for the equator veers to the west. Its influence
means that the wind never moves directly toward a zone of low pressure but rather, at
heights above the influence of the earth's surface, it circles around it along the lines
of constant pressure. This is the origin of the cyclonic winds in hurricanes.
By far, the most important temperature gradient driving global wind patterns is that
between the equator and the poles. Combined with the Coriolis effect, it is responsible
for the well-known equatorial trade winds and midlatitude westerlies (Fig. 1-1). At the
equator, relatively warm, moist air has a tendency to rise through convection to a high
altitude. This draws air in from middle latitudes toward the equator and thereby sets
up a circulation known as a Hadley cell (after the nineteenth century meteorologist
who first explained the phenomenon). Because of the Coriolis effect, the inflowing air
turns toward the west, creating the easterly trade winds. 2
A similar circulation pattern known as a polar cell is set up between high latitudes
and the poles. Lying between the polar and Hadley cells are the midlatitude (Ferrel)
cells, which circulate in the opposite direction. Unlike the others, they are not driven
by convection but rather by the action of sinking and rising air from the adjacent
cells. Once again the Coriolis effect asserts itself as the air flowing poleward along
1 The Coriolis effect is a property of observing motions from a rotating reference frame — in this case, the
earth. The earth's surface moves faster around the axis at the equator than it does closer to the poles. If
an object moves freely toward the equator, the surface beneath it speeds up toward the east. From the
perspective of an observer on the surface, the object appears to turn toward the west.
2 By convention, wind direction is denoted by the direction the wind comes from . If the air is moving
toward the north, it is said to be a southerly wind.