Let’s look at the global wind belts at the Earth’s surface in the Northern Hemisphere. In
the Hadley cell, air moves north to south, but is deflected to the right by the Coriolis Effect.
These winds therefore blow from the northeast to the southwest. They are called thetrade
windsbecause at the time of sailing ships they were good for trade. Winds in the Ferrel cell
blow from the southwest and are called the westerly winds orwesterlies. The westerlies are
the reason a flight across the United States from San Francisco to New York City takes less
time than the reverse trip. On the outbound flight, the airplane is being pushed along by
the westerlies, but on the reverse trip the airplane must fight against the air currents. In
the Polar cell, the winds travel from the northeast and are called thepolar easterlies.These
names hold for the winds in the wind belts of the Southern Hemisphere as well.
Theusualpatternofatmosphericcirculationcellsandtheglobalwindbeltsdeterminenormal
global climate, but many other factors come into play locally. The high and low pressure
areas created by the six atmospheric circulation cells generally determine the amount of
precipitation a region receives. In low pressure regions, where air is rising, rain is common.
In high pressure cells, the sinking air causes evaporation and the region is usually dry. More
specific climate affects will be described in the chapter about climate.
The junction between the Ferrell and Polar cells is a low pressure zone. At this location,
relatively warm, moist air that has circulated from the equator meets relatively cold, dry air
that has come from the pole. The result is a place of extremely variable weather, known as
thepolar front. This weather is typical of much of North America and Europe.
The polar jet stream is found high up in the atmosphere where the two cells come together.
Ajet streamis a fast-flowing river of air at the boundary between the troposphere and the
stratosphere. A jet stream can flow faster than 185 km/hr (115 mi/hr) and be thousands
of kilometers long and a few hundred kilometers in width, but only a few kilometers thick.
Jet streams form where there is a large temperature difference between two air masses. This
explains why the polar jet stream is the world’s most powerful.
Jet streams move seasonally as the angle of the Sun in the sky moves north and south. The
polar jet stream moves south in the winter and north in the summer between about 30oN
and 50oto 75oN. The location of the jet stream determines the weather a location on the
ground will experience. Cities to the south of the polar jet stream will be under warmer,
moister air than cities to its north. Directly beneath the jet stream, the weather is often
stormy and there may be thunderstorms and tornadoes.
Lesson Summary
- Winds blow from high pressure zones to low pressure zones. The pressure zones are
created when air near the ground becomes warmer or colder than the air nearby. - Local winds may be found in a mountain valley or near a coast.
- Global wind patterns are long term, steady winds that prevail around a large portion
of the planet.