Name: Nathan C.
How long does it actually take for a drought to set in?
There is no absolute definition of drought, as to percentage of normal precipitation during a
particular period of time, although a precipitation deficit of 20% might be considered a mild
drought and a deficit of 40% or more might be considered to be severe.
So, a drought may take many months or several seasons to form, if precipitation is not much
below normal, to only a month or so if precipitation is well below normal.
What really defines the severity of a drought is the effect that it has on water supplies and
agricultural activities. Good examples of the first of these are the severe decreases in water
supplies in the Denver, CO and Roanoke, VA areas over the past few years as a result of drought.
Good examples of the second of these are the periodic very dry spring and summer seasons in the
southern plains and the southeastern parts of the USA.
Southern California, Arizona, Nevada, and Utah have experienced very severe drought the last few
years in all seasons, leading to the many fires that we are now seeing in that area of the country.
David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory
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Update: June 2012