Geoscience Reference
In-Depth Information
The meteorological definition of drought has
become clouded by the subject of
desertification
,
particularly since the UN conference on the
subject in 1977 in Nairobi. This concern was
sparked by the protracted drought, resulting in
desiccation, in much of the Sahel zone. In turn, the
removal of vegetation, increasing the surface
albedo and lowering evapotranspiration, is
thought to result in decreased rainfall. The
problem for climatologists is that desertification
involves land degradation as a result of human
activities, primarily in areas of savanna and steppe
surrounding the major desert regions. These areas
have always been subject to climatic
fluctuations
(as distinct from climatic
change
) and to human
impacts (e.g., deforestation, mismanagement of
irrigation, overgrazing.) initiating changes in
surface cover, which modify the moisture budget.
Measures of atmospheric humidity are: the absolute mass of moisture in unit mass (or
volume) of air, as a proportion of the saturation value; and the water vapor pressure. When
cooled at constant pressure, air becomes saturated at the dew-point temperature.
The components of the surface moisture budget are total precipitation (including
condensation on the surface), evaporation, storage change of water in the soil or in a snow
cover, and runoff (on the surface or in the ground). Evaporation rate is determined by the
available energy, the surface-air difference in vapor pressure, and the wind speed,
assuming the moisture supply is unlimited. If the moisture supply is limited, soil water
tension and plant factors affect the evaporation rate. Evapotranspiration is best determined
with a lysimeter. Otherwise, it may be calculated by formulae based on the energy budget,
or on the aerodynamic profile method using the measured gradients of wind speed,
temperature and moisture content near the ground.
Condensation in the atmosphere may occur by continued evaporation into the air; by
mixing of air of different temperatures and vapor pressures such that the saturation point
is reached; or by adiabatic cooling of the air through lifting until the condensation level is
reached.
Rainfall is described statistically by the intensity, areal extent and frequency (or
recurrence interval) of rainstorms. Orography intensifies the precipitation on windward
slopes, but there are geographical differences in this altitudinal effect. Global patterns of
precipitation amount and annual regime are determined by the regional atmospheric
circulation, the proximity to ocean areas, orography, sea-surface temperatures, and the
atmospheric moisture budget. Droughts may occur in many different climatic regions due
to various causal factors. In mid-latitudes, blocking anticyclones are a major factor. The
primary cause of protracted drought in the African Sahel seems to be climatic fluctuations.
