Approach of Arizona Monsoon
July 13, 2004
The photos above are a preview of the coming Arizona monsoon. Monsoons aren't just a weather event of south-east Asia. Any large continent can develop seasonal pressure gradients. In North America, cold, dense air (high pressure area) settles over the U.S. mountain West during the winter months. Cool winds blow out from this stationary high pressure center to create dry conditions in the desert southwest. These winds warm (adiabatic warming) as they descend into the low Sonoran desert. The driest months in Phoenix, Arizona are usually April-June.
Warming of the intermountain high deserts reverses this air flow in the summer. With increasing temperatures, the air pressure drops creating a zone of convergence (low pressure center) that begins to draw in moist air off the Pacific Ocean and the Gulf of Mexico. As this air ascends (blows up into) the mountains, it's cooled by expansion, forming clouds. Although the monsoon winds generally come from the south, clouds more often build in from the north.
The official beginning of Monsoon Season in Phoenix occurs when we have three days in a row of dew point temperatures above 55°F (13 C) -- still dry by east coast standards. On average, this occurs about the second week in July. This year? Who knows?