Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 52

out the entire year, from North-East and South-East (North-East and South-East
trade winds). The trade winds flow from the so-called subtropical high pressure
cells, which are located in the vicinity of the 30 th latitude on each hemisphere.
This subtropical high pressure belt is characterised by weak winds and clear
weather. On the polar side follows a zone where the Western winds of the me-
dium latitudes are predominant. For example Central Europe is located within this
zone of influence. Wind direction and wind velocity change significantly depend-
ing on the wandering cyclones and anticyclones in this area. This West wind area
is limited by a low pressure trough towards each of the poles. In the polar areas
the wind conditions show strong fluctuations. On average, weak high pressure
areas prevail in the deeper strata.

North pole

High pressure
Low pressure trough
West winds
of the median latitudes

Subtropical anticyclon

Tropical zone
North-east trade winds

Equator Tropical convergence Equator

Tropical zone
South-east trade winds

Subtropical anticyclon

West winds
of the median latitudes

Low pressure trough

High pressure

South pole

Fig. 2.24 Planetary air flow at high altitudes (see /2-3/)

Due to these complex interdependencies, influenced additionally to a large ex-
tent by sea and land, as well as seasonal and other effects, a global air circulation
system is created. It is responsible for the global exchange of air. For energetic
utilisation, these air movements are only of minor importance, as it is currently
almost impossible to use the energy of the moved atmosphere at great altitudes,
where these air circulation systems are active.

Local air circulation systems. The forces responsible for wind generation are
active everywhere within the atmosphere. But with increasing closeness to the
surface of the earth, they are also increasingly influenced by local effects. There-
fore, the so-called free atmosphere at great altitudes, where the described global
air circulation systems are active, and the planetary boundary layer, located close
to the surface of the earth, are differentiated.
Geostrophic wind and gradient wind only occur if the pressure gradient and the
Coriolis force are predominant. This is only the case within the free atmosphere.