This is the counterclockwise circulation of air that develops when the prevailing northwesterly winds swirl around Point Conception. This upper-level ridging compressed the marine inversion, confining the marine layer clouds and fog to the immediate coast.Īnother mechanism that can spin low clouds inland in Southern California is the Catalina, or coastal, eddy. Sometimes in these conditions, the sun breaks through just blocks from the beach.Īs Tardy points out, the weather pattern that brought heat to the Southland and monsoon-like thunderstorms to mountains and deserts May 10-22 featured high pressure at upper levels of the atmosphere. At that point, the clouds are squeezed down and are in contact with the land’s surface, forming the kind of beach fog that can seem as thick as pea soup, but leaving sunny skies just a few miles inland. When the inversion is shallow enough - down to 500 feet or so - the marine layer is confined to the coast. This fortifies the inversion as it squeezes downward on the marine layer. That’s because high-pressure air is sinking, pressing down on the air and the inversion beneath it. Under these conditions, the clouds are not able to penetrate as far inland. The warmer layer is called an “inversion” - in this case, a marine inversion - and the cool air trapped beneath it is the marine layer.Īs high pressure, or ridging, builds later this week, the inversion is expected to be squashed down, making the marine layer shallower. Above that cooler layer, the air abruptly warms before cooling higher in the atmosphere. But because of the chilling effect of the ocean, the layer of air near the water’s surface is cooler than the layer immediately above it. Normally, air temperature gets cooler as it gets higher in the atmosphere. That’s cool enough for the water to chill the layer of air directly above it. Ocean surface temperatures off Southern California were below average earlier in May but now are near normal, in the low- to mid-60s, said Alex Tardy, a meteorologist with the National Weather Service in San Diego. It heats the land and air, but the ocean remains cool, because water absorbs heat. At the same time, the sun’s angle is higher and more direct. Troughs of low pressure, the kind that bring rain in winter, still pass over the region but are much weaker. In May and June, the upper atmosphere is more stagnant. To start with, the cloud cover is a function of the season. What causes the stubborn low clouds that muscle out the sunshine day after day? Sunny Southern California has not been so sunny lately, with gray skies casting a dreary shroud over the region.
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