Yards Moving Forward - GL Group

Halley also noticed that the winds somehow had to
be connected with rising air after it had been
warmed by the sun. The air heated in the tropics
expands, becomes lighter and rises, whilst at the
same time cooler air from the north or south enters
in exchange. This cooler air, Halley reasoned, was
the cause of the trade winds. The stronger the wind
blows, the greater the pressure difference between
the various air masses must be.
However, there was a physical conundrum
which Halley could not crack: according to his
hypothesis, the trade wind to the north of the equator
should have blown straight from the north and
the trade wind south of the equator directly from
the south. In actual fact, the trade wind north of the
equator blew from the northeast, and the south
trade wind from the southeast.
It was the British physicist George Hadley (1685-
1768), who clarified the matter in 1735. The cool air
from the north moves more slowly than the air at the
equator. When the cool air moves south, it loses
speed in relation to the faster rotation of the earth
from west to east. For this reason, the trade wind
blows from the northeast. The same principle
applies south of the equator, so that the wind there
blows from the southeast. Conversely, air masses
that are displaced from the equator northwards
move comparatively faster than the earth surface
lying below them, which leads to the typical west
winds.
All these observations and meteorological models
were already expressed in mathematical terms in
Beaufort’s lifetime. In 1835, the French physicist
Gaspard- Gustave de Coriolis (1792-1843) calculated
how strongly the circulatory pattern of air currents
between the equator and the poles was influenced
by the earth’s rotation. The fact that each air
current, or even water current, on the northern
hemisphere is deflected to the right and those on
the southern hemisphere to the left is now known as
the Coriolis effect. This force may lead to varying
circular vortices, thus creating storms or hurricanes.
In Beaufort’s time, the Coriolis force had to be taken
into account for the ballistic calculation of artillery
fire, and today it is, for example, considered when
launching satellites. And even in the load calculations
for the growing wind turbines, the deflecting
force of the earth’s rotation must be included in the
calculation: thanks to the Coriolis force, it is quite
possible that, for turbines with a hub height of over
80 metres, a different wind direction will dominate
at the lower blade tip than at the uppermost blade
tip. ■ CG
THE GL WIND BEAUFORT SCALE – EDITION 2006
nonstop 3/2006 65