Desert Rain   

While we get little precipitation here in Central Arizona most of the year, Arizona has a monsoon season. When you think of a monsoon, you probably think of India. However, Mexico also has a monsoon, and her northern neighbors in the U.S. Southwest share in the effects.

So, what is a monsoon? Meterologists point out that the word actually refers to a change in an area's normal wind pattern, not to the torrential rains and other weather phenomena caused by that change:

Actually one of the reasons that India has its more famous monsoon is largely due to the huge Rajasthan Desert in western India. But more fundamentally a 'Monsoon' is linked more to a wind shift rather than precipitation. In fact, the name "monsoon" is derived from the Arabic word "mausim" which means "season" or "wind-shift". Again, for India, during the winter dry period, the airflow comes from high pressure to the north�the dry Himalayas and Siberia. For the summer, the desert of western India heats up and low pressure forms. This causes air to swirl in from the west, the south and the east�all oceans! The result? HEAVY RAIN!

The Arizona Monsoon is a well-defined meteorological event (technically called a meteorological 'singularity') that occurs during the summer throughout the southwest portion of North America. During the winter time, the primary wind flow in Arizona is from the west or northwest�from California and Nevada. As we move into the summer, the winds shift to a southerly or southeasterly direction. Moisture streams northward from the Pacific Ocean and the Gulf of Mexico. This shift produces a radical change in moisture conditions statewide.

By the way, the term "monsoons" as in "when the monsoons arrive ..." is a meteorological no-no. There is no such beast. The word should be used in the same manner that "summer" is used. Consequently, the proper terminology is "monsoon thunderstorms" not "monsoons."

Monsoon thunderstorms are convective in nature. By that, we mean that the thunderstorms are powered by intense surface heating. In addition, strong moisture influx into Arizona is also required. The operational criterion for the onset of "monsoon" conditions used in Arizona is "prolonged (3 consecutive days or more) period of dew points averaging 55�F" or higher." There is nothing magical, however, about 55�F. It originally was linked to the total amount of water in the atmosphere above the weather station (a precipitable water amount of 1", a quantity thought to be necessary for convective thunderstorm activity). In general, for Phoenix, the temperature limits for the production of monsoon thunderstorms are 100� to 108�F with the optimum temperature being about 105�F. Temperatures needed to produce Tucson's thunderstorms are somewhat lower.

Basics of the Arizona Monsoon & Desert Meteorology, ASU School of Geographical Sciences

Of course, not all of our rain is caused by the monsoon. During cooler weather, we do get some rain, and, sometimes, we even get hail! Here are photos from a recent hailstorm: