Some of the thickest fog we’ve seen in years enveloped the Valley on Friday, and look for it to continue this weekend. If you’re driving… slow down, allow extra time to get to where you’re going, and switch on those low beams!
Visibility was knocked down to just a few hundred feet at times both Friday morning and again Friday night in the San Joaquin Valley, and a Dense Fog Advisory is in effect until Noon Saturday. The visibility at Fresno-Yosemite International Airport was down to 1/16th of a mile between Midnight and 10 a.m. Friday… 10 consecutive hours of exceptionally low visibility. That’s the longest stretch of visibility that low at Fresno-Yosemite since January of 2011… 15 years! I’ve heard many people say it just hasn’t been very foggy in recent years, until this year.
The main reason for this year’s serious fog has been our very wet start to the rain season. You can’t make fog without water, and there’s been plenty of that. The rain came early, soaked the ground and left standing water in places as well. Then, high pressure returned and pushed the jet stream northward, clearing our skies out. Clear skies and long winter nights allow the temperature to cool down to the dew point, which put the relative humidity at 100%. The moisture in the air then has nowhere left to go, so it condenses out into a cloud on the ground… which is fog. Strong high pressure overhead forces air down to the surface, preventing any of this saturated air from mixing out with surrounding drier air. The San Joaquin Valley is surrounded by mountains on 3 sides, and the tallest mountains in the lower 48 to the east at that… so this airmass can’t move. When was the last time you felt some kind of wind? So, this whole weather pattern just keeps repeating itself, like the movie Groundhog Day, until a new storm blasts on in and mixes all of this out. When will that happen? Not until the very end of January, at the earliest. Maybe Mother Nature will have a sense of humor and wait until February 2nd, which is Groundhog Day.
Until then, we’ll continue to see fog.
Although… it may evolve somewhat, like it did in early December.
In this latest round of fog, we’ve been managing to clear the skies by the afternoon. That’s important for how thick the fog will be later that night, as clear skies in the evening allow the temperature to drop quickly after sunset to the dew point. That late afternoon sunshine also warms up the air near the surface a little, which allows it to soak up just a little more moisture before sunset. Then the sun goes down, temperatures cool quickly under clear skies until the humidity nears 100%, and fog forms.
In early December, the skies didn’t clear out. At first, it was just mildly foggy all day, and our temperatures barely moved. Remember the streak of 365 hours (over 2 weeks!) of temperatures in the 40’s (and only 40’s) in Fresno in early December? With no sunshine, there was very little temperature change, and the ground level cloud (Fog) lifted to around 1000’… and just sat. And we were cloudy and cold… but not super foggy.
The difference between these 2 events is that there was more available moisture in early December. Back then, we went straight from heavy rain to fog, and there was just too much moisture to clear out. This time, we had a big storm around Christmas, another on New Year’s, and then… a weak, but DRY storm swept through. That cooled us down and DRIED the air out behind it. There’s still so much available moisture in the soil and standing water that fog is inevitable, but going into this second event with drier air allowed our skies to clear in the afternoon, leading to mild afternoons and very foggy nights.
So, looking ahead to the next week or two, which kind of fog will we see? Thick, dense morning fog and afternoon sun, like recently? Or all day gloom and doom, cold, but not terrible visibility, like early December? The answer should be in the afternoon sunshine. If the fog clears a bit, look for more very foggy nights. However, if it stays foggy all day, that should eventually evolve into the cloudy and cold pattern from early December.
The heart of of fog season runs from Veterans Day to Valentine’s Day. We can still get fog in late February and March, but it becomes harder as the sun gets stronger. Spring is just around the corner.