The Monsoon or Summer Rain Season in Northwest Mexico

What causes the Summer rains in Northwest Mexico?

We all know that air tends to leave a region under higher pressure, and go where the pressure is lower. For instance, a tire or a balloon are under greater pressure than that of the Atmosphere. And when they are puctured, the inside air will escape.

Now, when we also consider the rotation of the Earth, the air located within High Pressure systems, on escaping from them will also follow the rotation of the Earth, and rotate clockwise in the Northern Hemisphere. This is the Coriolis effect. And viceversa, air located inside a Low Pressure System will rotate counterclockwise.

Well, in Northwest Mexico we are most of the year the time under a High Pressure System, as we see in the map. This system forces the air currents in Sonora to come from the Southwest. Therefore, these winds don't carry with them as much humidity as those located near the Ecuator, and our climate is drier.

However, around the beginning of July, this High Pressure System moves North, centering now over New Mexico and Texas, in the United States.

Therefore, the prevalent winds over Northwestern Mexico will also change, and now will come from the Southeast, bringing with them tropical air currents, which have more humidity.

These humid winds will gradually advance from the South, until they cover Sonora and then Arizona.

 

How is the Summer Rainy Summer in Sonora?

At the same time than the High Pressure system brings warm, humid air to Northwest Mexico, during the Summer, the temperature of water of the Gulf of California will also rise due to the Sun heating, until it reaches aroung 80° F during July. This phenomena contributes to increase even more the humidity of the currents of air going North, along the Gulf of California.

As we see in the map (World Meteorological Association: Atlas 1975), the Western Coast of Mexico receives a lot of water in the form of rain during July. The curves represent the Average Precipitation, in milimeters, that fall over North Mexico and Southern United States.

The Eastern portion of Sonora, where the ranges are located, receives during July some 150 mm. of water through precipitation. Nogales receives aroung 92 mm in July, and 108 in August. That is over 46% of the whole year average!

This rainy season lasts until the last days of September or first days of October, ending with the so called "cordonazo de San Francisco", when the hurricanes that originate in the Mexican Pacific Coast enter Sonora attracted by the High Pressure system.