The third type of dune is the transverse dune. The transverse dune occurs on
surfaces covered thickly by sand. They develop on a locally huge supply of sand
with axes at right angles to the prevailing wind. They migrate slowly in the down-
wind direction. The transverse dune might be thought of as “what the desert is
like,” yet most of the planet’s arid zones do not have a thick sand covering.
Winds and Pressure Systems
Wind is the horizontal motion of the air. It is important in theEarthsystem as a
force capable of shaping and damage but also moves mass and energy over great
distances thus significantly affectingclimate.Windissuchanintegralpartof
our geographic environment that humans have feared its extremes like tornadoes
andhurricanesand have used its force to sail ships and grind grain and, now, to
generate electricity.
Wind is caused by differential heating. There are many reasons why places are
heated differentially (seeTemperature) and these differences occur at various
scales. Over small distances from a lawn to a sidewalk air will be forced to rear-
range itself on a summer day because much more of thesolar energyfalling on
the sidewalk is expressed as a rise of temperature than over the lawn where some
of the solar energy is used to accomplish photosynthesis and evaporate soil mois-
ture. The result of this differential heating is a modest flow of air from the lawn to
the sidewalk. For an entire hemisphere, lower latitudes are heated more than
higher latitudes because the lower latitudes have higher angles of sun in the sky.
The vast difference in heating between equator and poles causes huge transloca-
tions of air known as the global wind and pressure belts that are explained below.
One might think that the heating of air causes higher pressure as it does in a
confined pressure cooker. In the unconfinedatmosphere, however, heated air mol-
ecules become excited, increase their speeds of movement, and average greater
distances apart (lower density). Air density and air pressure are intimately related
in that one tracks the other. As air heats, its pressure lowers and wind starts
because of the differences in pressure.
Wind does not blow directly from high to low pressure. Instead, it is the product
of three forces. Pressure gradient is the rate of pressure change over distance and is
represented by lines of equal pressure called isobars; they are commonly denoted
in units called millibars. Pressure gradientforceis represented as a vector directed
from high to low pressure at a right angle across the isobars. For wind to start mov-
ing there must be pressure gradient force and when isobars are relatively close
together the wind will move quickly.
Winds and Pressure Systems 367