Tule FogCalifornia's Tule Fog season usually begins in November, after the first substantial rain and lasts until the end of March. Tule fog is a dense ground fog produced over land by the cooling of the lower atmosphere and is also known as radiation fog. Areas of dense fog can reduce visibility to less than 1/8 of a mile. Since the Central Valley of California has no air drainage other than mountain passes, day time heating or turbulent winds are required to break up the fog layer once it develops. Tule fog can be accompanied by drizzle and black ice. Accidents caused by Tule fog are the leading cause of weather related casualties in California. Examples of major accidents include:
December 1997 - 25 cars and 12 big rigs collided in fog near Elk Grove, south of Sacramento. 5 killed, 28 injured.
February 2002 - 80-plus car pileup on Hwy 99 near Kingsburg resulted in 2 killed.
November 2007 - a pileup involving 108 cars and 18 big rigs resulted in 2 deaths and 39 injuries.
During periods of dense fog the California Highway Patrol runs traffic breaks and pace cars on major highways to help slow traffic when visibility drops below 500 feet. Message signs provide information on changing conditions. (NOAA National Weather Service, FEMA Region IX)
