A mid-afternoon thunderstorm just passed, dropping some rain but almost as exciting, lowering the temperature more than 20 degrees in a few minutes. It’s 67ยบ F right now (just after 3PM). It’s monsoon season!
Monsoon is an Arabic term for a seasonal shift in the prevailing wind.
Both the Southwest USA, including Arizona and New Mexico, and Southeast Asia, including India, Pakistan, and Bangladesh, experience the monsoon each summer. The Asian monsoon often brings heavy, flooding rains to the area, while the Southwest monsoon brings scattered strong thunderstorms to dry desert regions. The Southwest monsoon is caused by two meteorological changes during the summer:
–The northerly movement of the Bermuda High (a strong area of high pressure) into the central USA
–Intense heating of the Mohave Desert to the west, which creates low pressure over the areaSince air rotates counterclockwise around low pressure and clockwise around high pressure, the positioning of these systems allows for a strong southerly flow over the Southwest. (Prevailing winds in the winter are from the west and northwest …) These south winds bring in moist air from the Gulf of Mexico and Pacific Ocean, increasing the chance of rain and thunderstorms.
I think the Weather Guys simplify things a bit.
ASU.EDU: Researchers have proposed the Gulf of Mexico and/or the Gulf of California as the source regions but conclusive evidence has so far been elusive.
NOAA: The Gulf of California, a narrow body of water surrounded by mountains, is particularly important for low-level moisture transport into Arizona and Sonora. Upper level moisture is also transported into the region, mainly from the Gulf of Mexico by easterly winds aloft. Once the forests of the Sierra Madre Occidental green up from the initial monsoon rains, evaporation and plant transpiration can add additional moisture to the atmosphere which will then flow into Arizona. Finally, if the southern Plains of the U.S. are unusually wet and green during the early summer months, that area can also serve as a moisture source.
Monsoons are great, but leave it to USA Today to refer to The Subcontinent as Southeast Asia.