SAN DIEGO (KGTV) — The foggy season has arrived in San Diego, often called May Gray and June Gloom. But how does it form, and why does it clear some days and not others?
Let’s start with why we see the marine layer on the West Coast and not the East Coast. The eastern Gulf Stream brings warmer tropical water from the south to the north. The water along the west coast moves from the north to the south, bringing cold water from the Gulf of Alaska south to the California coast. The water off the California coast can be as much as 30°F colder than water at the same latitude on the East Coast.
That cold ocean water cools the layer of air above it, this is called the marine layer. While the layer of air above that is warmer, creating an inversion, basically a cap trapping the cold air down below.
The air below the inversion can cool to the point at which condensation occurs, and clouds or fog form.
Marine layer clouds often reach their maximum extent around sunrise, when the air near the surface usually reaches a minimum temperature.
Once the sun comes up, the sunlight starts to penetrate through the clouds, warming the surface and air above it, mixing it upwards, and the clouds start to clear.
A decrease in air pressure above the marine layer allows it to rise and deepen, like when we have a storm approaching the California coast. Near the beach, we’ll experience overcast skies while the marine layer extends farther inland, bringing fog with it. When we have high pressure aloft, we have sinking air leading to a shallower marine layer, and fog confined to the coast.
A thicker marine layer clears more slowly as it takes more time for enough warming to occur to evaporate the clouds. Sometimes the clouds get trapped and are too thick for there to be enough warming, mixing doesn’t happen, and overcast skies can last for days.
This phenomenon is most pronounced in May and June because the ocean water temperature lags behind the warming air overhead, leading to a greater temperature difference and thus a stronger inversion layer.
