Global pattern of temperature and rainfall; climate zones


Fig. 1  Schematic depiction of the major climate belts on Earth.
 

If we look at the climate of Planet Earth without going into very much detail, we
can see a number of very basic features that seem to depend only upon latitude,
as sketched in Fig. 1: (a) an equatorial rain belt which shows up clearly in the annual
mean rainfall (the blues and purples in Fig. 2)

Fig. 2  Annual mean rainfall based on station data over land and infrared satellite
imagery over the sea.  For monthly mean maps and animations see
http://tao.atmos.washington.edu/legates_msu/index.html)
 

(b) the tradewind belts (northeasterly trades in the Northern Hemisphere and
southeasterly trades in the Southern Hemisphere), flanking the equatorial rain
belt (Fig, 3),

Fig. 3 Annual-mean surface winds over based on the NCEP/NCAR Reanalyses
 

(c) the subtropical deserts (indicated by the yellows and browns in Fig. 2)
(d) the midlatitude westerly belts (Fig. 3) which are disturbed by an unending
sequence of disturbances called extratropical cyclones.  We viewed an extratropical
cyclone over the North Pacific on the web in class.  To see what's out there now,
check out the visible and infrared satellite imagery on the  departmental web site at
http://www.atmos.washington.edu/data/weather.html#satellite.
 
 

Seasonal distribution of rainfall


The above figure shows the climate zones on an idealized continent. The equatorial
belt experiences ample rain year round. It's wetter and wider about the equator on
the eastern side of the continent where the trade winds carry greater amounts of
moisture from the ocean onto the land. Pole-ward of the equatorial rain belt is the
monsoon belt in which the rains are largely restricted to the summer season. The
climate in this belt is still tropical but the winters may be a few degrees cooler
than the summers. The hottest month of the year is often in late spring or early
summer, just before the monsoon rains set in.

The climate becomes drier as one proceeds pole-ward into the sub tropics where we
encounter the sinking branch of the Hadley cell. The notable exception to the
dryness is the eastern part of the continents, where the flow of warm, moist air
around the western side of the oceanic sub tropical anticyclone brings summer
rains, sometimes in the form of hurricanes and tropical cyclones. The driest
regions tend to be toward the western sides of the continents: the location of the
world's great deserts.

Proceeding pole-ward into the westerly belt, the dry summers persist up to
latitudes of around 45 degrees, but extra tropical cyclones embedded in the
westerlies bring winter rains that tend to be especially heavy over and on the west
slopes of mountain ranges. Because of the pronounced summer dry season, the western
sides of continents tend to receive most of their annual rainfall during winter.
Pole-ward of 45 degrees these seasonal contrasts in rainfall are less pronounced and
they are less easy to characterize in terms of an idealized continent.