The University of Arizona
Maricopa County Cooperative Extension Home Horticulture:
Environmentally Responsible
Gardening & Landscaping in the Low Desert

Honored by Access Arizona as Site of the Day July 6, 1998

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!

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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

Ligntning Safety Rules

When the weather forecast calls for thundershowers or thunderstorms, take it seriously. It means that lightning is possible. Lightning kills 125 people on the average each year in the United States and injures over 500. This makes it one of the most dangerous weather events in terms of lives lost.

Follow these lightning safety rules and it may save your life.

When thunderstorms are forecast, keep an eye on the sky and when a thunderstorm threatens, stay indoors or in an automobile (not a convertible).

Do not use the telephone except for emergencies.

If you are caught outside, avoid tall, isolated trees and utility poles. Avoid projecting above the landscape; don't stand on a hilltop. In a forest seek shelter in a low area under a thick growth of small trees. In open areas go to a low place such as a ravine or valley.

Get off and away from open water. Avoid tractors, farm equipment, motorcycles, golf carts and other metal vehicles. Avoid lines, wire fences, metal pipes and railings. Put down golf clubs. If you are in a group in the open stay several yards apart.

If you are caught in an open area far from shelter, and you feel your hair stand on end, lightning may be about to strike you. Drop to your knees and bend forward putting your hands on your knees. DO NOT lie flat on the ground.

Persons struck by lightning receive a severe electrical shock and burns, but they carry no electrical charge. Handling the victim will cause you no harm. Prompt action can revive someone "killed". When a group has been struck, treat the "apparently dead" first.

The American Red Cross says that if a victim is not breathing, give mouth-to-mouth resuscitation until medical help arrives. If the victim is not breathing and has no pulse, the properly trained should administer cardiopulmonary resuscitation (CPR).

Victims who appear stunned or otherwise unhurt may also need attention. Check for burns especially at fingers and toes and next to buckles and jewelry.

® Copyright 1997 by Ed Phillips/KTAR Radio. Information provided courtesy of KTAR via Ed Phillips Arizona Almanac.

To Home Horticulture in Maricopa County, AZ To Weather in the Valley of the Sun

The Arizona "Monsoon"

visitors since August 1, 1997 visitors since April 1, 1998
Last Updated July 6, 1998
Author: Cathy Rymer, Instructional Specialist, University of Arizona Cooperative Extension, Maricopa County
Reviewed by: Lucy K. Bradley, Extension Agent Urban Horticulture, University of Arizona Cooperative Extension, Maricopa County
© 1997 The University of Arizona, College of Agriculture, Cooperative Extension, in Maricopa County

Comments to Cathy Rymer, 4341 E. Broadway Rd., Phoenix, AZ 85040
(602) 470-8086 ext. 308