Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 50

point of lower air pressure. It thus wants to migrate from an isobar with the pres-
sure p 1 towards an isobar with the pressure p 2 (Fig. 2.22). Through the movement
from p 1 towards the lower pressure level p 2 , the gradient force accelerates the
particle at constantly increasing speed. At the same time, the influence of the
Coriolis force increases. This force is defined as the result of the product of parti-
cle mass, angular velocity of the rotating system, and the particle velocity relative
to the rotating reference system. As it impacts always vertically to the direction of
the movement (Fig. 2.22), it constantly causes a change of the direction of the
velocity vector. The resulting change of the directional movement lasts as long as
the magnitude of the Coriolis force is equal to the gradient force. The particle is
then not any longer subject to a resultant force; it is in equilibrium. Its speed and
the Coriolis force hence remain unchanged, it is moving in parallel to the isobars.
The consequent wind category, where air moves alongside the isobars, is called
the geostrophic wind (Fig. 2.23, left).

Isobar with air pressure p 2 < p 1

Trajectory of air particle

Gradient force vector
Coriolis force vector
gradient and Coriolis force
Speed vector

Vector resulting from

Isobar with air pressure p 1

Air particle

Fig. 2.22 Origin of geostrophic wind (Northern hemisphere; see /2-6/)

The larger the pressure gradient, the closer the isobars are together and the
higher is the gradient force. Therefore the air particles are accelerated more, and
the speed of the particle moving from isobar with the pressure p 1 to the isobar
with the pressure p 2 increases. The amount of the Coriolis force, however, in-
creases proportionally to the speed of the particle the force impacts on.
Therefore for isobars located in parallel the equilibrium of forces between
Coriolis force and gradient force, and thus the straight movement of the particle
along the isobars always occur, independently of the difference in pressure or the
gradient force. The speed of the geostrophic wind itself depends solely on the size
of the pressure differences.
In areas with a low or a high-pressure core the isobars are curved. In addition
to the two forces already mentioned, a third force, the centrifugal force, also acts
on the air particle. It points radially outwards (Fig. 2.23, centre and right). The
resulting wind is called the gradient wind. In the northern hemisphere, it blows
anti-clockwise, and clockwise in the southern hemisphere, around low pressure
areas. For a high pressure area, the situation is reversed. As the centrifugal force