Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 51

reinforces the gradient force in a high pressure zone and weakens it in a low pres-
sure zone, the gradient wind velocity in the high pressure area is greater than in
the low pressure area /2-3/, /2-5/.

Geostrophic wind Gradient wind

Isobar 1,000 mb

Isobar 1,000 mb

Isobar
1,000
mb

F
+ F Centrifugal

Gradient

F

Gradient

v

v Wi, North

Wi, South

F

Coriolis

F v

v Gradient Wi, South

Wi, North

v v

Wi, South Wi, North

F
+ F Centrifugal

Coriolis

Isobar
1,010
mb

F

Coriolis

Isobar 1,010 mb

Isobar 1,010 mb

F Gradient force vector

Gradient

v Speed vector, northern hemisphere

Speed vector, southern hemisphere

Wi, North

F Coriolis force vector
F Centrifugal Centrifugal force vector

Coriolis

v

Wi, South

Fig. 2.23 Geostrophic and gradient wind (according to /2-2/, /2-3/, /2-5/, /2-6/)

Global air circulation systems. The described mechanisms of an air movement
within the atmospheric layers are the precondition for the global air circulation
system existing on earth (Fig. 2.24).
On a global scale, the surface of the earth heats up most where the sun is in the
zenith (i.e. in the area around the equator). This causes a low pressure zone close
to the equator, where air streams in from the North and the South. Without the
influences caused by the continents this equatorial convergence zone would
stretch around the equator like a belt, shifting parallel to the seasonally changing
of the position of the sun between the tropic of Cancer and Capricorn with a cer-
tain time delay.
Because of the actual given influence of the oceans and the continents, this
convergence zone is almost always North of the equator; however, it moves
slightly with changing seasons. If the earth did not rotate, the air would flow close
to the ground from the polar areas towards the equator. Here, it would be lifted up
into the convergence zone and flow towards the poles again in higher atmospheric
layers. By sinking down in high-pressure zones above the poles, the circulation
process would finish.
Such simple flow conditions cannot occur on a rotating planet. Therefore, on a
first glance only an "ideally" rotating planet is examined, without looking at the
influence of sea and land. Thus a plant is taken into consideration where the tem-
perature is only determined by the latitude. The air then flows closely to the equa-
tor towards the tropical convergence zone. It is however diverted by the Coriolis
force. This leads to an air current, blowing practically at the same force through-