Now I Know: Southern California’s June Gloom

June Gloom is the phrase many Southern Californians use to describe the generally overcast skies that hang over the region in late spring and early summer. The gloom is deepest just after dawn and heaviest near the beaches. But by midday — earlier over points inland — the clouds burn off and the basin’s familiar over-saturated sunlight washes over all. Well, the sunlight washes over all except those of us who live within a mile of the shoreline, where the overcast may linger deep into the afternoon.

The gloom and overcast are a result of the marine layer, which is a kind of temperature inversion layer. Normally, higher air temperatures are found near the ground and cooler temperatures are found aloft. An inversion layer sees this relationship turned upside down: cooler temperatures are found near the surface and warmer temperatures lie above. One effect of an inversion layer is a “capping” of normal convection currents — the rising and falling of air due to thermals — which traps dust and other particles under the inversion layer. LA’s famously bad air quality is partly a result of this meteorological quirk.

Our local inversion layer is generated by the especially chilly waters of the Pacific Ocean. The California Current carries frigid sea water south from the Gulf of Alaska to the tip of Baja. The sea water cools the air above, creating a temperature inversion. If there is enough moisture in the air, and the cooling effect is strong enough, then clouds and fog are generated within the marine layer. Depth of the marine layer is affected by the movement of much larger weather systems in the atmosphere above. High pressure systems squish the marine layer so that only coastal areas lie under the gloom. Low pressure systems allow the marine layer to expand upward and outward; fog along the shore rises and pushes inland.

It’s just after 2:00 pm PDT here in Long Beach, CA, and the sun is just beginning to burn through the clouds above. June Gloom has stretched all the way through July. But when the alternative is thunderstorms and sweltering heat, I don’t think many of the locals mind.