The enveloping tule fog that once defined a Central Valley winter is shrinking away due to years of dry weather, scientists say.
Scientists at the University of California, Berkeley, last week released a study concluding that winter fog in the Central Valley decreased 46 percent on average over the span of a 32-year period ending in 2013.
The study, which used satellite data from NASA, along with other sources, confirms what scientists at the National Weather Service already know. The weather service has been monitoring tule fog since the 1950s, and in the past 20 years the amount of fog in both the Sacramento and San Joaquin valleys has plunged. The agency has nine measuring stations in the San Joaquin Valley and eight in the Sacramento Valley.
Since measuring began, weather service instruments at Sacramento Executive Airport have recorded an average of 223 hours of fog a year, said Bill Rasch, a meteorologist with the weather service in Sacramento. While the amount of fog varies from year to year, overall it has been dropping. Last year, Executive Airport recorded just 30 hours.
While motorists may welcome their ability to see the road, the lack of fog is a concern to farmers since some crops – including cherries and almonds – rely on winter chill temperatures. Most deciduous fruit and nut trees must be exposed to chilling temperatures to break dormancy and begin flowering in the spring.
Dennis Baldocchi, lead researcher on the UC Berkeley study, said less fog means more sunlight, which increases the maximum air temperature within orchards. That, in turn, reduces the number of chilly hours that allow buds to flower, he said.
Baldocchi’s study estimated there were 400 fewer chill hours in the San Joaquin Valley in 2012 than there were in 1982.
The UC Berkeley study is considered key because it used data and images from NASA and weather satellites, along with other data. The combination of these sources offers more precise and comprehensive fog monitoring over the entirety of the Central Valley. Previous measurements for tule fog have mostly been gleaned from single measuring points in the Valley, often at airports or in cities.
Baldocchi considers that method inaccurate because cities tend to be warmer than the countryside, and so they have less fog.
His study found the greatest concentration of fog in the lower half of the San Joaquin Valley, where there has also been a drop in fog days since they peaked in the early 1990s.
“This last winter, Fresno had the least amount of fog on record going back to 1950,” said Paul Iniguez, who works in the Fresno office of the National Weather Service.
Iniguez said his records show that, last winter, Fresno tallied only 18 hours of fog. The previous lowest amount of fog was tallied in the 1999-2000 winter, when Fresno saw 55 hours. The average amount of fog is 280 hours in a winter.
The decrease in fog is directly tied to the lack of rain in the region, Iniguez said.
“When there is drought, there is less fog,” said Iniguez. “When you don’t have rain, you don’t have moisture – because fog is a cloud on the ground.”
Tule fog is considered “radiation fog,” because it forms on cold, clear nights when the ground surface cools fast, condensing moisture into visible droplets. Wet ground is required to provide that moisture.
Less tule fog could bring one significant benefit: fewer multi-car pileups on freeways. Tule fog forms overnight and reduces visibility to less than an eighth of a mile on roads and freeways. In 2007, tule fog caused a 108-car pile-up on Highway 99 that involved 18 semi-trailer trucks. That pileup resulted in two deaths and nearly 40 injuries.
Call The Bee’s Edward Ortiz, (916) 321-1071. Follow him on Twitter @edwardortiz.