WeatherWhys Blog

Fog Season Begins

Fog Season Begins
Every year in September, hunting season is inaugurated by the congregation of outdoorsmen who excitedly don their special jacket, head for the hills with a gun and the hope of bagging a trophy deer or elk. They must have a license. But no license is required for the start of fog season, and today is that day.

November 15th marks the entry into a 3-month period in which the central valley becomes prone to low visibility due to thick fog. We will be in this mode through Valentines Day. Whereas extreme heat may be a common weather complaint in Bakersfield, Tule fog is surely the most dangerous for travelers. Tule fog, named for the tule grass wetlands of the central valley and pronounced TOO-Lee, is a typical western valley radiation fog. The process is actually fairly simple. Rain falls at some point due to a transient weather system which is followed by a large ridge of high pressure and clear skies. This promotes nighttime cooling that brings on fog formation. The upper level ridge introduces a layer of warm air aloft that effectively puts a lid on the fog. Surrounded by mountains on three sides, the central valley is just a big pot of fog (with a lid on it). Since sun angles are low during the winter, solar heating is minimized and the fog does not burn off. With the temperature inversion in place there is little mixing of the low level atmosphere. So, not only is this the perfect situation for low visibility but also for poor air quality. No ventilation or turning over of the atmosphere leads to No Burn Day declarations (as is the case this weekend).

On average we get 22 thick fog days per year in Bakersfield. November has 3 days, December 7, January 9 and February 3. As you can see, the bulk of the fog season is December and January. At the winter solstice (December 21st), the highest sun angle at solar noon is only 32 degrees above the horizon. Compare that with the summer solstice in June when the sun reaches 78 degrees above the horizon. Solar insolation is 45% less in Bakersfield during the winter, hence the certainty of fog. That is, if we get rain first. And we have. Following months of dry conditions, November is running above average for rainfall as of the middle of the month. We have enjoyed 6 rain days so far (average for the entire month is 4). To date, Meadows Field has received .43” of rain. High pressure is forecast to persist over the west coast so areas of fog are likely in the central valley.

Generally the fog forms on a clear night and may persist for a couple of days with extremely low visibility. However, the fog will usually lift into a cloud deck bringing long days of slate gray skies and cool temperatures. I call this process “curing”. The dank weather may seem depressing to those who enjoy the perpetual blue skies and hot temperatures of summer. My term is “sinus headache weather”. One invigorating activity for many is to drive to the mountains where it is sunny and warmer. You can look down on the tops of the low clouds and fog. It looks like a white lake, with distant mountains visible all around. Alas, valley residents can get their sunshine fix but will only have to jump into the lake again. So if you are not thrilled with the dangerous threat to transportation Tule fog brings or the physiological consequences, just concentrate on those sunny hot days of summer with highs 100 to 110- for they will be back as surely as the sun rises.