Environmental Engineering Reference
In-Depth Information
Chapter 6
The atmosphere in motion
ATMOSPHERIC CIRCULATION AND WINDS
Earth's atmosphere is in perpetual motion: movement which is striving to eradicate the
differences in temperature 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.
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.
From the geostationary images, individual cloud patterns can be identified and
followed and from successive photographs cloud movements can be calculated and
predicted. Unfortunately the satellite photographs show the circulation only in cloudy
areas. What happens elsewhere is less clear. For wind speed and direction in these
cloudless regions we are dependent upon information supplied by surface or balloon
observations. 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).
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
 
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