Marine Layer Depth and Wind Blocking – California’s Famous June Gloom Explained

You check the wind forecast. It promises 20 knots of thermal wind. You drive to the beach. It is grey, cold, and dead calm. You have been blocked by the Marine Layer.

This phenomenon, famous in California as "June Gloom," is the arch-enemy of the summer wingfoil wind. It acts as a shield. It blocks the sun from heating the land. Without heat, the sea breeze engine cannot start.

The Inversion Lid

The Marine Layer is a blanket of cool, moist air trapped under a layer of warm, dry air. This setup is called a "Temperature Inversion."

• The Cap: The warm air on top acts like a lid. It prevents the cool air at the surface from rising or mixing.
• The Fog: The cool air near the water condenses into low clouds or fog. This is the "Gloom."

For a thermal wind to form, the sun must hit the dirt. It must heat the valleys inland. If the Marine Layer is too deep (thick), the sun cannot burn through the clouds. The land stays cool. The pressure gradient never forms. The wind speed stays at zero.

Depth Matters: The 2,500-Foot Rule

Meteorologists measure the depth of this layer. You can find this on a local meteogram or atmospheric sounding.

• Shallow (< 1,000 feet): Weak gloom. The sun will burn it off by 11 AM. The thermal wind will kick in. Expect 15–25 knots.
• Deep (> 2,500 feet): Strong gloom. The clouds will stick all day. The sun never touches the ground. The wind forecast will fail. Expect light wind or calm.

Predicting the Burn-Off

The "Burn-Off" is the moment the sun evaporates the cloud edge. You can watch this happen on visible satellite loops. The white cloud sheet retreats from the valleys toward the beach.

If the wind direction aloft is pushing the clouds offshore, the burn-off happens fast. The wingfoil wind will start early.

If the wind direction aloft is pushing onshore (from the West), it reinforces the layer. The clouds pile up against the coastal hills. The thermal wind is suffocated.

The "Reverse" Clearing

Sometimes, the Marine Layer clears inland but sticks to the beach. This creates a "fog line."

• On the Beach: 10 knots, foggy, cold.
• 1 Mile Offshore: 25 knots, sunny, blue water.

The thermal wind is blowing, but it is lifting over the fog bank. You have to slog through the light wind zone in the fog to reach the high wind line in the sun. This is common in San Francisco and Santa Cruz.

Summary

Do not trust a thermal wind forecast if the sky is grey. Check the depth of the Marine Layer.

• Thin Layer: Pack your small wing.
• Thick Layer: Stay home or drive inland. The wind speed is locked behind the clouds.

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