The Monsoon Begins

the-monsoon-begins-1-613x380When most people think of monsoons, they think about torrential tropical rains advancing northward through the Indian subcontinent. While that’s the prototypical example of a monsoon, North America has its own version of a monsoon. In meteorology a monsoon is defined as a prevailing wind that changes direction seasonally. During the warm season the land gets very hot and is at a lower atmospheric pressure than the air mass over nearby ocean areas. This pressure difference drives moist oceanic air over the land and triggers thunderstorms and heavy rain; during the winter the situation reverses and high pressure over the land mass causes the prevailing wind to switch and blow from the land to the ocean, shutting off the precipitation.
While the monsoon in North America is not as spectacular as its Indian cousin, it does cause a significant amount of storminess in Arizona, New Mexico and the other southwestern states. The area around Tucson, Arizona is known for its particularly photogenic storms and spectacular lightning displays. The infrared satellite image shown above shows large thunderstorm complexes in southern Arizona and New and old Mexico. They’re easily recognized in the infrared image because thunderstorms have the highest, coldest clouds and in this image the coldest clouds are shaded red and green.
The monsoon typically begins in late June or early July and lasts until mid-September. More than half of the annual rainfall in Tucson and other cities in the region falls during the monsoon period.
Exactly where the monsoon moisture comes from has long been a matter of debate. The original thought was that the moisture originated in the Gulf of Mexico. One problem with this idea was that the humid air is often present at low levels, and it was hard to see how it could have made it across the high elevations of central Mexico. Now it is believed that the source of most of the monsoon moisture is the Gulf of California and the tropical Eastern Pacific, and the monsoon thunderstorms “turn on” when these bodies of water get warmer than about 80°F. The Gulf of California is only about 100 miles from the U.S. border, and the water there can approach 90°F by mid-summer. When strong winds blow from the gulf to California’s Imperial Valley the desert region can become the most humid area in all of the United States, with dew points exceeding those of places more traditionally thought of as muggy, like Key West, Florida and Corpus, Christie, Texas.