What is a Heat Wave?

There is no set definition for a heat wave.  It typically depends on the climate in which you are living.  In one area, a couple days of 85 degrees in the summer may be a heat wave but in Central California that’s mild.  The World Meteorological Organization recommends it is a heat wave when temperatures are five degrees warmer than average for five days.

In the Central Valley, the sometimes uncomfortable hot and sunny summer weather is due in part to our geography.  The Central Valley is located in an interior Mediterranean climate.  We have cool, wet winters and hot, dry summers.  As summer approaches, a dominant high pressure area sets up over the four-corners (Colorado, Utah, Arizona, and New Mexico.)  The wind flow around the high pressure is clockwise with the cooler air to the east and warmer air to the west toward California.  The air sinks and warms, inhibiting precipitation and bringing cloud-free days.  When the pattern persists, temperatures soar.

Many times in the valley we’ll experience heat waves in the summer but we have some daily relief.  At night, our temperatures cool into the 50s and 60s.  The clear skies at night help to allow all the daytime heat to escape.  It's called radiational cooling.

This pattern starts to break down as we head into October.  The jet stream begins to head south out of Canada.  The shift in the upper-level wind pushes the high pressure toward the tropics.  This allows cooler wet weather to move through the region and bring relief from the heat.

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