It’s been five days since we’ve seen the sun at Terra Firma.  Two hundred feet outside my office window, there’s a crew harvesting cauliflower.  I can hear them chatting but I can’t see them through the fog.
Totally unlike the ocean fog that cools the Bay Area in the summer, our winter fog is result of damp soil, high humidity and the land-locked nature of the valley.  In decades past the “Tule Fog” sometimes lasted for weeks at a time.  But in the last ten years it was beginning to seem like a casualty of climate change.  I can’t remember the last time we had fog for this long.
As it is in the Bay Area, the fog here is micro-climatic.  To escape it you just need to drive uphill a few miles to the south, west or north.  We have neighbors less than a mile away who may be enjoying the sunshine right now.  And most of our citrus orchards — in the hills above the valley floor — stay sunny all day.  But in parts of the valley that are farther from the Coast Range or the Sierra Foothills, there’s no easy escape.
For our crops, fog is a mixed bag.  Winter vegetables need sunshine to photosynthesize, so long periods without it can cause stunting of the plants and reduction in harvest.  The persistent humidity also takes a toll, especially if a week of fog is followed by a week of rain, and then back again to fog.
On the other hand, our permanent crops clearly benefit from foggy, cool winters.  Tree crops, vines and even asparagus require a minimum of “chilling hours” below a certain temperature to rest each year.  Warm, sunny winters prevent the plants from going completely dormant, which disorients them and generally leads to poor production the following year.  It’s the equivalent of a human not getting a full night’s sleep.  Tule Fog is a one of lesser known reasons why the Central Valley is such an ideal place to grow fruit and nut trees.
In the past, scientists thought that as long as the chilling hours reached a certain threshold over the winter, it was sufficient.  But recent experiments have determined that plants are pickier than imagined.  Warm, sunny days can actually counteract cold nights by warming up the branches of the tree or vine as well as the soil around it.  Lately, winters have been characterized by large daily temperature swings — freezing cold nights but almost 70 degrees days.  By contrast, fog creates chilly temperatures that don’t fluctuate much.  The trees “go to sleep” more deeply.
Long stretches of foggy winter weather have a similar affect on humans — it’s hard to get out of bed, and it’s even harder to know what time it is.  When you work outside for a living, you don’t need a watch to tell you what time is, but this past week 8 a.m. has looked a lot like noon and 4:30 p.m.  And without the sun’s warmth, you need to bundle up good against the damp cold.
Season Affective Disorder and Vitamin D definiency aside, it’s good to have some “real winter” after the bizarrely warm and dry December of 2017.  Six months from now, we won’t remember how cold and depressed we were in January, but we’ll hopefully have happy trees and vines full of fruit and nuts.