I don't think there is any exact definition, but anywhere a drought is considered to be happening when there is a substantial excess of evaporation over precipitation that continues for a lot longer than normal, resulting in the soil and subsoil moisture levels falling well below their long term averages for a prolonged period. Note that the terms "substantial", "well" and "prolonged" are not defined!Originally Posted by VladTepes
I suppose it is a drought when it is enough drier than normal to affect things like crops and water supplies, but for much of Australia the rainfall is so variable that we don't think there is a drought until there has been a shortage for at least a year, whereas in the UK the rainfall is so regular that even a couple of weeks without much rain can cause a crisis situation with some town water supplies, as well as farm activities. And monsoonal areas may be very dry in the dry season, but there is no drought until the monsoon fails to arrive.
John


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