California Winter Fog Drastically Declines: Trouble for Fruit and Nut Crops

A changing climate doesn't just mean that ice will melt and sea levels will rise; it also means that the weather will be affected, and this can be bad news for crops. Scientists have found that California's winter tule fog, which is needed by fruit and nut trees, has declined dramatically over the past three decades due to climate change.

Crops such as almonds, pistachios, cherries, apricots and peaches go through a necessary winter dormant period maintained by colder temperatures. The tule fog in California, which is a thick ground fog that descends on the state's Central Valley between late fall and early spring, helps continue this winter chill. Yet it turns out that this fog may soon be a thing of the past.

The researchers used NASA and NOAA satellite records in combination with data from a network of weather stations, covering 32 consecutive winters, in order to examine the prevalence of tule fog. They found that while there was a great variability from year to year, overall there was a 46 percent drop in the number of fog days over time.

"The year-to-year variability we saw was likely influenced by whether the season was relatively wet or dry," said Dennis Baldocchi, one of the researchers, in a news release. "Generally, when conditions are too dry or too wet, we get less fog. If we're in a drought, there isn't enough moisture to condense in the air. During wet years, we need the rain to stop so that the fog can form."

The fact that the researchers found an overall decrease in fog over the years, despite variability, could be bad news. The fog not only helps continue the winter chill, but also shields buds from direct sunlight. This means that a decrease in fog could spell some major issues for crops.

"The trees need this dormant time to rest so that they can later develop buds, flowers and fruit during the growing season," said Baldocchi in a news release. "An insufficient rest period impairs the ability of farmers to achieve high quality fruit yields."

The findings are published in the journal Geophysical Research Letters.