The Monsoon

Ed Phillips, Staff Meteorologist
Newsradio 620 KTAR

This may be a shock, but we were taught something wrong in school. It was probably around fifth or sixth grade when we had the chapter in our science book on weather. Our teacher dutifully recited from the text that the monsoon was in India or Southeast Asia or somewhere like that. The teacher went on to say how these exotic places had torrents of rain for six months and it was dry for six months. That was it for the monsoon, and we moved on to the next topic. Just to set the record straight, here's the adult version: the monsoon is a period of dry alternating with wet, and we have our own version right here in the Southwest.

A large annual change of temperature over land masses is the primary engine that drives the monsoon. This causes an excess of high pressure in the cold months and low pressure in the warm months. This deficit of pressure, along with the storm track retreating to the north in the summer, sets the stage for the summer monsoon. Tropical moisture is literally sucked northward toward the lower pressure in the low levels of the atmosphere. The result is a shift in the winds over and area and enough moisture to trigger seasonal thunderstorms.

In Arizona, the monsoon starts with the hot and dry weather of May and June. Usually, the winds are from a dry, westerly direction, so humidity is low and temperatures soar above 100 degrees in the deserts. As the atmosphere warms, the jet stream retreats northward. This allows the winds to shift to a more southerly component and bring in the moisture from the Sea of Cortez and Gulf of Mexico. Our strong summer sun heats the moist air causing thunderstorms and rain.

The monsoon is the most pronounced in southern Arizona and becomes more marginal over northern Arizona. In the Phoenix area, the moisture usually arrives by the first or second week of July. The end of the hot and humid weather normally comes in late September statewide. During the dry monsoon (April, May, and June), we get only 6% of our normal yearly rainfall. During the wet monsoon (July, August, and September), we get 32% of our normal yearly rainfall.

So that's the adult version of the monsoon. It makes me wonder about all the other stuff I learned in sixth grade! For more weather, visit our new Weather & Coffee shop with free wireless internet, 3607 E. Bell Road., Suite 1, Phoenix, or call 602-485-JAVA.