Dear Tom, 

Why is it that there is severe drought everywhere (particularly in the west), but there should be plenty of water in the atmosphere?

Richard Kassner, Western Springs

Dear Richard, 

     You are correct in assuming that water vapor is always present in the atmosphere. However, that amount varies widely from place to place and season to season depending on variables such as temperature, air mass type, and topography. Even in moisture-rich environments the presence of water vapor does not guarantee clouds and precipitation.

In Chicago, a dramatic case in point occurred on July 13, 1995, during the city’s “Killer Heat Wave”. Temperatures topped 100 degrees with rainforest-level dew points near 80, yet the day went by with hardly a cloud in the sky. The reason is that in order for cloud and precipitation growth to occur there must be areas in the atmosphere where the air is rising. Upward vertical motion in the atmosphere results in lowering temps, perhaps to the point where condensation occurs. Rising air motion results from low pressure development, air being forced up mountain slopes, motion along frontal boundaries, and instability (when warm/moist air becomes buoyant). 

Along the West Coast, despite its proximity to the Pacific ocean, much of the warmer months are dry due to the absence of Pacific storms systems and relatively cold ocean water offshore. Most rain/snow occurs in winter when opposite conditions exist. Western (upwind) slopes of the Cascades and Sierras receive much more precipitation while eastern (downwind) slopes remain bone-dry.