ON THE PHYSICS OF DROUGHTS .1. A CONCEPTUAL-FRAMEWORK

A conceptual framework for the physics of droughts is provided by means of the omega equation of atmospheric dynamics. The duration and severity of droughts are dependent not only on the atmosphere but also on the hydrologic processes which feed moisture to the atmosphere. Once dry hydrologic conditions are established. the positive feedback mechanism of droughts sets in, where the moisture depletion from upper soil layers decreases the evapotranspiration rates, which, in turn, lessen the atmospheric relative humidity. The lesser the relative humidity the less probable the rainfall becomes, as it will be harder to reach saturation conditions for a regular low pressure system over the region. Only disturbances which carry enough moisture from outside the dry region will be able to produce sufficient rainfall to end the drought conditions.