We all talk about the monsoon season in the summer here, but I've never understood the actual mechanics of this weather phenomenon. Here is a quote that explains the monsoon season from Understanding the Southwest Monsoon, an article by the University of Arizona's Zach Guido for the Southwest Climate Change Network:
In Arizona and New Mexico, monsoon storms typically begin in early July after several complex and dynamic weather phenomena collide. By July, the Four Corners region has baked in the sun for months. Air has risen like a helium balloon, creating a low pressure trough in the lower atmosphere. Off the coast of Baja California, the sun’s energy has boosted ocean temperatures to around 85 degrees Fahrenheit. But the ocean has a moderating effect on the air and has kept it at temperatures below those over the deserts of the Southwest. This temperature imbalance becomes large enough that a change in the high and low altitude atmospheric movement occurs. The winds aloft over the Southwest, near an altitude of 30,000 feet, take a U-turn westward, opposite their trajectory for nine months. They carry with them moisture from the Gulf of Mexico. At approximately the same time, the near-surface air over the Gulf of California rushes northward into Arizona and New Mexico, carrying with it moisture from the gulf.
The moist air flowing into Arizona and New Mexico hits the mountains and rises. As the air ascends, it expands and cools. The air temperature decreases, falling below the dew point temperature—the temperature below which the air can not hold all the moisture and condenses to form rain. Thunderstorms begin. Vegetation grows. Humidity increases over land. Then more rain falls, creating a cycle that continues until the temperature difference between the land and sea is reduced, sometime in early fall.
Although the article was written about the monsoon season in 2008, it contains plenty of useful information about monsoons in general in this part of the world. The rest of the website contains constantly updated information about weather patterns, models, and changes in the Southwest.