I felt like taking a break from typing out more articles about the Old Spanish mine to look at monsoons. I could just blabber on about it, but I found some great information fromĀ so enjoy:

Probably the most confusing and often debated weather event in Arizona is our so called “monsoon”. Just the word conjures up visions of torrential rains and widespread flooding followed by heat and drought. The drastic wet/dry combination is the example that most of us learned about in school. However, the monsoon actually occurs in varying degrees, and ours is marginal, just not as intense as those found in other parts of the world.The word “monsoon” comes from the Arabic “mausim” which means “a season.” It was first used to describe the winds over the Arabian sea which blow from the northeast for six months and from the southwest for six months. Over the years, monsoon has been extended to include Europe, Africa and the western coasts of Chile and the United States.

Strong annual variations of temperature over land masses is the primary cause of the monsoon. This causes an excess of high pressure in the cold months and low pressure in the warm months. This deficit of pressure coupled with the storm track well to the north in the summer, allows the tropical moisture to literally be sucked northward toward the lower pressure in the low levels of the atmosphere. The end result is a shift in the winds over an area and enough moisture to trigger seasonal rains.

In Arizona, the process 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. Most of our humid air comes from the Sea of Cortez, but a good portion also comes from the Gulf of Mexico. Once the moist air arrives, our strong summer sun heats the moist air causing the familiar thunderstorm (cumulonimbus) clouds.

Our monsoon is the most pronounced in southern Arizona and becomes more marginal over northern Arizona. The monsoon lasts longer in the south, usually beginning around the middle of June. In the Phoenix area, the moisture is usually here by the first or second week in July. The end of the hot and humid weather normally comes in the latter half of September state wide.

Statistically, we consider it a “monsoon day” when the average daily dew point is 55 degrees or higher. This can easily be measured and gives us a way of comparing one year to another.

Still not convinced? 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!

Monsoon Facts and Figures

Average date of monsoon beginning July 7
In 2 out of 3 years the monsoon begins July 1 through July 16
Earliest Monsoon beginning on record June 16, 1925
Latest monsoon beginning on record July 25, 1987
Average date of first break in monsoon August 16
Average total number of monsoon days 56
Greatest number of monsoon days on record 99 in 1984
Greatest number of consecutive monsoon days on record 72 in 1984
(June 25 through September 5)
Least number of monsoon days on record 27 in 1962
Wettest monsoon on record (July, Aug. and Sept. rainfall) 9.38 inches in 1984
Driest monsoon on record (July, Aug. and Sept. rainfall) .35 inches in 1924
Average monsoon rainfall (July, Aug. and Sept.) 2.45 inches

And some pictures:



Courtesy of Arizona Daily Star

Courtesy of

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2 Responses to Monsoons

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