The extreme heat in the southwest is a reminder of the change of season. Not only is it summer, but the Southwest US Monsoon season is now in full swing. Heat is the most deadly weather-related cause of death in the U.S. with an average of over 300 deaths per year – much greater than all other weather-related deaths. The Southwest U.S. is susceptible to extreme temperatures with highs commonly reaching above 110 degrees. In addition, during the monsoon, humidity levels reach high levels and lead to exceptionally warm evenings with low temperatures sometimes not dropping below 90 degrees.
What is the North American Monsoon?
The word monsoon is derived from the Arabic word “mausim,” which means season. Traders plying the waters off the Arabian and Indian coasts noted for centuries that dry northeast winds in the winter suddenly turn to the southwest during the summer, and bring beneficial yet torrential rains to the Asian subcontinent. In North America, the Monsoon impacts Arizona, New Mexico, Utah, Colorado, Texas, California, and Mexico.
The wind shifts we see during Monsoon season, and the dramatic change in weather they bring, are all more or less driven by a similar mechanism. For much of the year, low level winds in dry subtropical regions tend to blow from the land toward the sea. However by late spring, strong solar heating causes temperatures to soar over these land areas. The intense heat causes surface air pressure to fall, forming an area of low pressure known as a thermal low. Adjacent large bodies of water are also warmed, but not as quickly. Thus air pressures remain high relative to the land. Eventually, the pressure difference increases to the point that the cooler and much more humid air over the ocean is drawn toward the hot, dry air over land. This moist air moving onto the hot land eventually becomes unstable and develops into thunderstorms. Once this occurs and rain begins to fall, humidity levels increase over land, which only triggers more thunderstorms. This cycle will continue until land areas begin to cool in the early fall and water temperatures reach their peak in early fall. This reduces the pressure difference, which in turn causes the moist onshore flow to diminish, and the monsoon gradually ends. The Monsoon seasons brings welcome rains and relief from the summer heat. For some areas, over 50% of annual rain falls during the monsoon. However for us in Arizona, this is the most dangerous time of the year weather-wise. Monsoon storms can cause damaging winds, torrential rainfall, frequent lightning and dust storms. That is why it is a very good idea to review the safety tips.
Here’s what the National Weather Service is forecasting for this year’s season:
For more info on the hazards of monsoon season and how you can prepare, click here.