Natural Hazards: Causes and Effects - Disaster Management Center ...

In addition to the droughts in the African Sahel, there have recently been major droughts in
northeast Brazil, Chile, Ethiopia, the Philippines, the Bolivian altiplano, and India. Near-drought
conditions in the grain-producing regions of the United States and the Soviet Union have also
occurred, affecting greatly the international food supply and demand.
Trends in the occurrence of droughts indicate they are becoming more frequent on the edges of
desert lands and where agricultural, lumbering, and livestock grazing practices are changing.
Natural Preconditions for Drought
Drought differs from other natural disasters in its slowness of onset and its commonly lengthy
duration. Before the rise of modern water-consuming cities, drought was an agricultural
disaster. Now, with cities having expanded faster than water supplies can be made available,
the specter of drought faces both the farmer and the urban dweller. Shifts in atmospheric
circulation, which cause drought, may extend for time scales of a month, a season, several
years or even a century. The latter might be termed a climatic shift, but the effect on humans
and their environment is equally great. Because of the economic and environmental importance
of drought, determined efforts are being made to solve the problem of prediction of the
atmospheric circulation patterns that produce droughts.
The following are main causes of drought:
• Widespread and persistent atmospheric calm areas called subsidence, which do not
cause precipitation. These areas result from the present-day atmospheric circulation,
which tends to create subsidence in the subtropical latitudes of both hemispheres.
• Localized subsidence induced by mountain barriers or other physiographic features. Most
such areas lie in the lee of mountains across the westerly belts. They are hence in
midlatitudes. The dryness is caused by the warming of westerly currents as they descend
east of the summits. This allows them to hold moisture and carry it away.
• Absence of rainmaking disturbances causes dry weather even in areas of moist air. In
general, rain is caused by the travel of organized disturbances across a region—i.e.,
systems that involve actual uplift of humid air. Thus the aridity of the Mediterranean
summer, though in part due to subsidence, arises mainly from the absence of cyclonic
disturbances that bring the rains of winter. There is plenty of water in the air, but nothing
to bring it down as rain.
• Absence of humid airstreams. The relationship between the water available for
precipitation (precipitable water) and the precipitation that actually falls is by no means
simple. As we have just seen, dry weather may be prolonged in areas of high humidity. In
addition to having rainmaking atmospheric disturbances, regions of abundant rainfall must
have access to humid airstreams. Some innercontinental regions are quite remote from
such sources.
These four causes are interdependent, but their relative effect depends on season and locality.
One can broadly distinguish between:
1. Almost continuously dry climates, leading to desert surface conditions, in which there is no
season of appreciable rainfall;
2. Semiarid or subhumid climates with a short wet season in which humid airstreams or
rainmaking disturbances penetrate;
3. The rare subhumid areas in which rainfall is infrequent but not confined to a special season. 4