We had our first good rain of the season today here in Prescott.

The smell and the sounds are awesome.

You actually see people looking up in anticipation of the Monsoon season.

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.

This pic was taken this morning at 5:33 AM.

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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