Below average rainfall in a region for several months.
Less precipitation for a short period of time usually do not result in drought. However, when there is no rain for a long time, there will be nothing to prove cooling, vapor and moisture that recharges the ground water.
Not enough rain to be absorbed into the ground before evaporating.
Soaking rain enable evaporation from moist soil. It can also initiate evapotranspiration by vegetation.
Cutting down trees and overuse of land.
Water vapor evaporating from trees trigger rain with the clouds that form over the forests. Less trees means less rain.
Hot dry air masses blown into a region by westerly winds.
The westerlies carry dry, warm, equatorial winds that further inhibit moisture formation.
Human activities that deplete the water table faster than it can be replaced.
Water demand is outbalanced with the supply. Humans consume more than what nature can provide. Also, excessive irrigation contributes to drought.