Geoscience Reference
In-Depth Information
6
CHAPTER SIX
in action
From the geostationary images, individual cloud pat-
terns can be identified and followed and from successive
photographs cloud movements can be calculated and pre-
dicted. Unfortunately the satellite photographs show the
circulation only in cloudy areas. In clear areas, wave pat-
terns can be identified to estimate wind speed and direc-
tion. Together with cloud images, these provide us with a
less detailed picture of the pattern of wind circulation over
most of the continental areas of the globe (
Figure 6.1
).
Earth's atmosphere is in perpetual motion: movement
which is striving to eradicate the differences in tempera-
ture and pressure between different parts of the globe.
It is this motion which produces the winds and storms
with which we are all familiar. It is this circulation
which plays a basic part in maintaining a steady state
in the atmosphere and generating the climatic zones
which characterize Earth. So far we have considered the
upward movements which transfer energy from the
surface to the atmosphere. Let us now consider the more
obvious horizontal movements that transfer air around
the globe.
Causes of air movement
Why do we have winds at all? To answer this question it
is useful to consider some of the basic principles of
motion. Our understanding of these is due in large degree
to Isaac Newton. Many people know the story of how Isaac
Newton 'discovered' gravity when sitting beneath an apple
tree, but he also formulated laws of motion. There are two
main laws. The first states that: a particle will remain at
rest or in uniform motion unless acted upon by another
force. The second law states that: the action of a single
force upon a particle causes it to accelerate in the direction
of the force. If there is more than one force the particle is
accelerated in the direction of the resultant (
Figure 6.2
).
These forces are particularly important for movement
in the atmosphere because forces are continuously acting
on particles of air, causing them to accelerate or decelerate
and change their direction. The explanation of movement
is not unique to Earth, for similar patterns of atmospheric
circulation have been identified on other planets with
atmospheres.
Sources of information
With modern satellite technology we can watch and
monitor these movements. We are no longer dependent
solely upon balloons to provide information about the
upper atmosphere. Geostationary satellites (e.g. Meteosat,
GOES) orbit the globe with the same rate of rotation as
Earth, permitting the same portion of Earth to be viewed
continuously, using visible light by day and using infra-
red imaging by day and by night. The images show the
main cloud features of the atmosphere. Polar-orbiting
satellites with their lower altitudes provide more detailed
information about clouds and about polar regions, but
they pass above a particular part of Earth's surface only
twice a day, so wind determination is more difficult.
