By Richard Dickert
The persistent dense fog in the South Bay over the past few months is unusual, but not unprecedented.
Southern California has been dominated by an upper-level ridge of high pressure, pushing the jet stream (storm track) to our north. This pattern has kept the region mostly dry during the late autumn and through early winter
The ridges produce sinking air, which creates strong subsidence (sinking) temperature inversions over Los Angeles. During an inversion the temperature increases with altitude, which is the opposite of the “normal” pattern. This stable, capping inversion results in very little mixing between the shallow layer of air near the surface and the layer above it.
Now comes the marine air. Just enough of an onshore push off the chilly Pacific Ocean (58° water temperature currently in Hermosa Beach) advects (moves horizontally) cool-moist air across the coastal plains of Southern California. Think of that cool ocean surface along our coast like a glass of ice cold beer. What happens to that glass? Condensation forms on the outside of it. Well, condensation forms in our atmosphere underneath that strong subsidence inversion in the way of fog.
The low sun angle this time of year and short days allows for longer cool periods (less time to burn the fog away), which helps to maintain the persistent “cooling” condensation/foggy weather. I like to call this fog a “hybrid” of advection (movement) and radiation (cooling) because both dynamics are in play to produce the low visibilities that we have experienced of late.
What do we need to break the cycle? A breakdown of that relentless ridge and a southward dip in the jet stream into Southern California. This will break up that capping subsidence inversion and hopefully bring us some much needed precipitation. A stronger offshore flow (flow of air from the land towards the sea/Santa Ana winds) helps to scour out the fog as well. This has occurred a couple of times during this foggy period and is in the forecast starting Sunday and continuing through the week.
Be careful out there in the milky air.
Richard Dickert is a meteorologist. He was a weather reporter for Fox News LA for over 20 years. ER
