If you've ever been in a coastal area, you may have experienced a marine layer.

A marine layer is a dense, cool layer of air trapped near the ocean’s surface beneath a warmer layer of air above. It often forms over coastal waters when the ocean is cooler than the air above it, creating a natural “lid” that keeps the cool air from rising.

This phenomenon is especially common along western coasts, like California’s, where the contrast between chilly Pacific waters and warming land can be stark.

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Marine layers are responsible for the classic overcast or foggy mornings in coastal regions. The layer can drift inland, bringing low clouds, haze or fog that may linger until the sun warms the land enough to break through.

This cooling effect often keeps coastal cities pleasantly mild during the day, even when inland areas are sweltering.

The thickness and strength of a marine layer can vary. In strong cases, it can extend hundreds of feet deep and stretch many miles offshore.

Pilots, sailors and even local residents keep an eye on marine layers because they influence visibility, temperatures and weather conditions along the coast.

So next time you wake up to a gray, foggy morning by the ocean, you’re seeing the marine layer at work, a natural mix of air and water shaping the weather in subtle, powerful ways.

Jennifer Gray is a weather and climate writer for weather.com. She has been covering some of the world's biggest weather and climate stories for the last two decades.