Fundamentals of wind energy - Wind Power Generation and Wind Turbine Design - page 5

towards the poles and cold air with higher density fl ows from the poles towards the
equator along the earth's surface. Without considering the earth's self-rotation and
the rotation-induced Coriolis force, the air circulation at each hemisphere forms a
single cell, defi ned as the meridional circulation.
Second, the earth's self-rotating axis has a tilt of about 23.5° with respect to its
ecliptic plane. It is the tilt of the earth's axis during the revolution around the sun
that results in cyclic uneven heating, causing the yearly cycle of seasonal weather
changes.
Third, the earth's surface is covered with different types of materials such as vegeta-
tion, rock, sand, water, ice/snow, etc. Each of these materials has different refl ecting
and absorbing rates to solar radiation, leading to high temperature on some areas (e.g.
deserts) and low temperature on others (e.g. iced lakes), even at the same latitudes.
The fourth reason for uneven heating of solar radiation is due to the earth's
topographic surface. There are a large number of mountains, valleys, hills, etc. on
the earth, resulting in different solar radiation on the sunny and shady sides.
2.2 Coriolis force
The earth's self-rotation is another important factor to affect wind direction and
speed. The Coriolis force, which is generated from the earth's self-rotation, defl ects
the direction of atmospheric movements. In the north atmosphere wind is defl ected
to the right and in the south atmosphere to the left. The Coriolis force depends on
the earth's latitude; it is zero at the equator and reaches maximum values at the
poles. In addition, the amount of defl ection on wind also depends on the wind
speed; slowly blowing wind is defl ected only a small amount, while stronger wind
defl ected more.
In large-scale atmospheric movements, the combination of the pressure gradient
due to the uneven solar radiation and the Coriolis force due to the earth's self-
rotation causes the single meridional cell to break up into three convectional cells
in each hemisphere: the Hadley cell, the Ferrel cell, and the Polar cell (Fig. 1).
Each cell has its own characteristic circulation pattern.
In the Northern Hemisphere, the Hadley cell circulation lies between the equa-
tor and north latitude 30°, dominating tropical and sub-tropical climates. The hot
air rises at the equator and fl ows toward the North Pole in the upper atmosphere.
This moving air is defl ected by Coriolis force to create the northeast trade winds.
At approximately north latitude 30°, Coriolis force becomes so strong to balance
the pressure gradient force. As a result, the winds are defected to the west. The air
accumulated at the upper atmosphere forms the subtropical high-pressure belt and
thus sinks back to the earth's surface, splitting into two components: one returns to
the equator to close the loop of the Hadley cell; another moves along the earth's
surface toward North Pole to form the Ferrel Cell circulation, which lies between
north latitude 30° and 60°. The air circulates toward the North Pole along the
earth's surface until it collides with the cold air fl owing from the North Pole at
approximately north latitude 60°. Under the infl uence of Coriolis force, the mov-
ing air in this zone is defl ected to produce westerlies. The Polar cell circulation lies
between the North Pole and north latitude 60°. The cold air sinks down at the