Palmén-Newton schematic model of general air circulation between the pole and the equator.
emission and begins to descend back to the Earth's
surface around 20-30° north and south of the solar
equator, forming high pressure at the ground. Upon
reaching the Earth's surface, this air either moves
poleward or returns to the equator to form a closed cir-
culation cell, termed a Hadley cell . Because of Coriolis
force, equatorial moving air forms two belts of easterly
trade winds astride the equator. Air tends to converge
towards the solar equator, and the uplift zone here is
termed the intertropical convergence. Over the western
sides of oceans, the tropical easterlies pile up warm
water, resulting in convection of air that forms low
pressure and intense instability. In these regions, intense
vortices known as tropical cyclones develop preferen-
tially. These will be described in Chapter 3.
At the poles, air cools and spreads along the Earth's
surface towards the equator. Where cold polar air meets
relatively warmer, subtropical air at mid-latitudes, a cold
polar front develops with strong uplift and instability.
Tornadoes and strong westerlies can be generated near
the polar front over land, while intense extra-tropical
storms develop near the polar front, especially over
water bodies. Tornadoes will be described in Chapter 4,
while extra-tropical storms are described in the next
chapter. A belt of strong wind and storms - dominated
by westerlies poleward of the polar front - forms around
40° latitude in each hemisphere. These winds, known as
the roaring forties, are especially prominent in the
southern hemisphere where winds blow unobstructed
by land or, more importantly, by significant mountain
ranges. Maps of global winds support these aspects of
the Palmén-Newton model (Figure 2.3).
The Palmén-Newton model has two limitations.
First, the location of pressure cells within the model
is based upon averages over time. Second, because
the model averages conditions, it tends to be static,
whereas the Earth's atmosphere is very dynamic.
Additionally, the concept of Hadley circulation is
overly simplistic. In fact, rising air in the tropics is not
uniform, but confined almost exclusively to narrow
updrafts within thunderstorms. At higher latitudes,
large-scale circulation is distorted by relatively small
eddies. Other factors also control winds. For example,
over the Greenland or Antarctic icecaps, enhanced
radiative cooling forms large pools of cold air, which
can accelerate downslope under gravity because of the
low frictional coefficient of ice. Alternative models that
overcome some of these limitations will be presented.