Why is it so foggy?

Fog is a cloud sitting on the ground. It forms when air cools down to its dew point and the water in it condenses into tiny droplets. Along the West Coast it's usually the marine layer: cool, damp ocean air trapped under a lid of warm air.

The two common kinds

• Radiation fog forms on clear, calm nights inland. The ground cools, the air touching it cools to its dew point, and fog fills valleys by dawn. It usually burns off by mid-morning.
• Advection fog forms when moist air moves over a cold surface, like warm air drifting over the cold California Current. It can sit on the coast all day and roll inland at night.

The marine layer, May Gray and June Gloom

Off the West Coast, cool ocean water chills the air just above it. Higher up, sinking air from a large high-pressure system is warm and dry. The warm air acts like a lid (an inversion) that traps the cool, damp layer underneath, often only 1,000 to 2,000 feet thick. When that layer is moist enough, its top turns into a flat gray deck of cloud. Sun heats the land and thins it through the day; at night it rebuilds and pushes back inland.

In late spring and early summer the contrast between cold ocean and warming land is strongest, which is why Southern California gets “May Gray” and “June Gloom”.

Why radar can't see it

Fog and marine-layer droplets are a few hundredths of a millimeter across. Weather radar is built to see raindrops and snowflakes, which are hundreds of times bigger, so a completely gray sky can look empty on radar. Air Story shows both views side by side.