What is the difference between aridity and drought?

As noted by the World Meteorological Organization (1975), a distinction should be made between drought and aridity. Aridity is usually defined in terms of low-average precipitation, available water, or humidity and, setting aside the possibility of climatic change, is a permanent climatic feature of a region. Drought, on the other hand, is a temporary feature in the sense that, considered in the context of variability, it is experienced only when precipitation falls appreciably below normal. Aridity is, by definition, restricted to regions of low precipitation, and usually of high temperature, whereas drought is possible in virtually any precipitation or temperature regime.

Aridity is a natural permanent imbalance in the water availability consisting in low average annual precipitation, with high spatial and temporal variability, resulting in overall low moisture and low carrying capacity of the ecosystems. Aridity may be defined through climatological indices such as the Thornthwaite moisture index, the Budyko radiation index of dryness, or the UNESCO precipitation/evapotranspiration index (Sanderson, 1992). Under aridity, extreme variations of temperatures occur, and the hydrologic regimes are characterized by large variations in discharges, flash floods and large periods with very low or zero flows (Paulo and Pereira, 2009).

Drought is a recurrent feature of the climate. It occurs in virtually all climatic zones, and its characteristics vary significantly among regions. Drought differs from aridity in that drought is temporary; aridity is a permanent characteristic of regions with low rainfall. Drought is a normal, recurring feature of climate; it occurs in virtually all climatic regimes. It is a temporary aberration, in contrast to aridity, which is a permanent feature of climate and is restricted to low rainfall areas.

Drought is a temporary aberration, unlike aridity, which is a permanent feature of the climate. Seasonal aridity (i.e., a well- defined dry season) also needs to be distinguished from drought. There is considerable confusion among scientists and policy makers on the differentiation of these terms. Drought must be considered a relative, rather than absolute, condition. It occurs in both high and low rainfall areas and virtually all climatic regimes. The impacts of drought are, at times, enormous and result in economic and environmental impacts as well as personal hardship (Wilhite et al., 2014).

Temperatures; high winds; low relative humidity; timing and characteristics of rains, including distribution of rainy days during crop growing seasons, intensity and duration of rain, and onset and termination, play a significant role in the occurrence of droughts. In contrast to aridity, which is a permanent feature of climate and is restricted to low rainfall areas (Wilhite, 1992), a drought is a temporary aberration.