General information about the monsoon
Average rainfall in Tucson 12.17 inches. (For the whole year.)
Average monsoon rainfall: 6.43 inches.
Average of monsoon rainfall for Tucson last 30 years: 6.06 inches.
Earliest start: June 17, 2000. Average starting date: July 3.
Latest start: July 25, 1987.
Average of monsoon days: 62 days.
Most monsoon days: 84 in 1999.
Least monsoon Days: 30 in 1979.
Wettest monsoon season: 13.84 inches of rain in 1964.
Driest monsoon season: 0.81inches of rain in 1926.
In 2008 the National Weather Service identified the Monsoon season as being from June 15 through September 30.
“Monsoon season is here—yes, really.” Arizona Daily Star. June 15, 2008. Page B1.
The older definitions of the monsoon:
Craig Ellis, a meteorologist with the National Weather Service in Phoenix, gives two definitions of the term "monsoon" which literally means "season" in Arabic and refers to seasonal winds.
The first definition is based on a seasonal wind shift. The summer monsoon, which provides slightly more than half of Tucson's annual rainfall, is a flow of humid air from the south and east.
The second definition is based on dew-point rules. Therefore, the monsoon begins when the average dew point in Phoenix is 55 degrees or higher for three consecutive days.
"Worrying about Monsoons." Tucson Citizen. June 22, 2001, Page 1.