Rain & Monsoons

Coming from the Northeast, it was very hard for me to adapt to the terminology used when other Green Valley residents were talking about the 'monsoons'. I come from upstate New York where a 4 day torrential rainstorm is not uncommon. My first summer in Green Valley (2000), I was looking forward to the monsoons. Then we got a few days of what I would call fair rain and my neighbors all referred to it as the 'monsoon'. It was not until the Summer of 2005 that I finally saw what I considered to be a good amount of rain, that resulted in some flooding.

So, what is a monsoon? Monsoons are caused by the fact that land heats up and cools down quicker than water. In summer, land reaches a higher temperature than the ocean. The hot air over the land tends to rise, creating an area of low pressure. This creates an extremely constant wind blowing toward the land. Associated rainfall is caused by the moist ocean air being diverted upward by mountains, which causes cooling, and in turn, condensation.

The North American monsoon occurs in Arizona from mid July into September, with the average start date being July 3rd. The begining of 'monsoon season' is determined when the average daily dewpoint is 54 degrees or greater for "3" consecutive days. As much as 70% of our rainfall occurs during this season. Keep in mind that the rainfall in Southern Arizona is very small compared to many other US areas. See the graph below and the monsoon records page at the NOAA website:
Southern Arizona Monsoon Records

Many desert plants that thrive in our dry climate have adapted to take advantage of this wet season. And so have the golfers! Because of the danger of being caught in the open when these storms suddenly appear, many golf courses in Arizona have thunderstorm warning systems.