Geography Reference
In-Depth Information
the proliferation of an insect or critter that is instrumental to the spread of a particular dis-
ease. The connection between humid tropical climates and malaria is a case in point.
Savanna (tropical wet and dry)
Savanna climate is distinguished from its tropical climatic kin by pronounced wet and dry seasons.
The rather even duration and importance of these seasons have given rise to a self-explanatory cli-
matic alias, tropical wet and dry. Most of the savanna realm is located between latitudes 5° and 20°
North and South. These regions are alternately affected by passing low- and high-atmospheric pres-
sure belts, which bring with them the wet and dry seasons respectively.
“Savanna” refers to the natural vegetation that occurs under these conditions: a mix of trees and
grasses (see Figure 10-1). The relative abundance of these elements generally varies, however, with
annual precipitation. Accordingly, trees dominate where rainy seasons are relatively long. Grasses
dominate under opposing conditions. In some parts of the world — Africa in particular — the grasses
attract large herds of grazing animals (herbivores) and the meat-eating animals (carnivores) that prey
upon them. But the grasses and relatively fertile soil that underlie them also attract herdsmen and
farmers. The result, as described in Chapter 2, has been a steady decline in wild-animal habitat.
Going to Extremes: Dry Climates
“Dryclimate”wouldseemtobeaprettystraightforwardconcept.Wrong.Technically,itoccurswhere
warm temperatures cause potential evaporation to exceed rainfall. Don't worry if that leaves you
scratching your head. It's kind of complex, and gets worse. Herr Koeppen stipulated that the bound-
ary between “dry” and “humid” climatic zones occurs where R < 2T + 28 when 70 percent of the
rainfall occurs during . . . Like I said, it's kind of complex.
So for the sake of convenience, I choose to lose the formula and settle for the notion that a
dry climate is characterized by no more than 20 inches of precipitation during the course of a
year. Climatologists, true and exacting scientists that they are, may scream and rip their cloth-
ing upon reading this, but I think they will agree that the 20-inch threshold is pretty close to
accurate, and entirely appropriate for the purposes of this topic. The geography of dry climates
(see Figure 10-3) is made up of two areas: desert and semi-dessert (or steppe).
