Fog Drought in the Central Valley

At Mother Jones, there is great concern about a future "California Without Fog?"

In the Golden State, since 1981 the Central Valley's winter fog from November through February "has been appearing less and less often ... according to Dennis Baldocchi and Eric Waller at the University of California, Berkeley."

Waller and Baldocchi go on to state that "the fog's disappearance is both occurring along with the state's milder winters, and is also contributing to them, as less fog in the air means warmer temperatures. The clearer air may be good news for motorists, but it's another kick in the butt to California's multitude of fruit and nut farmers."

Milder winters? Less fog means warmer temperatures? Well, in the five major NOAA climate sub-regions for the Central Valley (Bakersfield, Fresno, Redding, Sacramento, and Stockton), I see no clear evidence for "milder winters" or "warmer temperatures" between November and February since 1981:


A few scattered significant increasing trends, and a few significant decreasing trends, and an overwhelming number (96 percent) of non-significant temperature trends (a.k.a., an unchanging climate). Precipitation also hasn't been changing since 1981.

Mother Jones also quotes the following statement from NASA regarding the importance of Central Valley fog:

"The fog is important to California's crops because fruit and nut trees, like people, need sufficient rest before they can be their most productive."

The American Geophysical Union has also profiled the Central Valley winter fog decline research:

"California's winter tule fog -- hated by drivers, but needed by fruit and nut trees -- has declined dramatically over the past three decades, raising a red flag for the state's multibillion dollar agricultural industry, according to new research.

Crops such as almonds, pistachios, cherries, apricots and peaches go through a necessary winter dormant period brought on and maintained by colder temperatures. Tule fog, a thick ground fog that descends upon the state's Central Valley between late fall and early spring, helps contribute to this winter chill.

'The trees need this dormant time to rest so that they can later develop buds, flowers and fruit during the growing season,' said University of California, Berkeley biometeorologist and study lead author Dennis Baldocchi, whose father grew almonds and walnuts in Antioch and Oakley. 'An insufficient rest period impairs the ability of farmers to achieve high quality fruit yields.'"

The USDA's statistical database doesn't contain annual yield data for almonds, pistachios, cherries, apricots, and peaches in California before 2009, but yields of all these crops have been stable or even increasing over the past four years, with no evidence of a declining trend. Production data for all but apricots and peaches goes back to the mid-1990s, and over the past nearly two decades almond production has quadrupled, pistachio production has increased by five-and-a-half fold, and cherry production is up by a factor of six. Apricot and peach production are only available back to 2009, and both appear stable.

Fog may be on the decline in the Central Valley, but it doesn't appear to have a substantial impact on either overall temperature patterns or crop production.