May Gray and June Gloom, explained
In late spring the ocean off California is still cold while the land is heating up. That sets up a thick, stubborn marine layer: cool, damp ocean air trapped under warm air, with a flat gray cloud deck on top that can hang over the coast all day.
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Why late spring
The coastal ocean stays cool well into summer, while inland valleys and deserts heat up fast in May and June. Hot air inland rises, pressure drops, and cool ocean air is pulled onshore underneath a warm, sinking layer from the large high-pressure system over the Pacific. The warm air on top acts like a lid. The marine air under it fills with cloud, and nothing pushes it out.
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Why it sometimes never burns off
The sun heats the land under the cloud and thins it from the edges, so inland neighborhoods often clear by late morning. Right at the beach the cool ocean keeps feeding it, and a deep layer can last all day. A swirl of air off the Southern California coast called the Catalina Eddy can deepen the marine layer suddenly, pushing the gray farther inland and up against the mountains.
Why radar shows nothing
The clouds are made of droplets far too small for weather radar, so the radar is blank under a solid gray sky. Sometimes it drizzles lightly. Air Story's sky view shows the low cloud deck and how high it sits.
Questions people ask
- When does June Gloom end?
- Usually by July, as the ocean warms and the pattern shifts, though gray mornings can show up at the coast all summer.
- Is June Gloom fog?
- It's the same cloud. Where its base touches the ground, on the coast and hills, it's fog; a few miles inland it's a low cloud deck.