Name: How fog works in 90 seconds
Uploaded: 2026-01-20T13:50:10.471Z
Duration: 1 min 26 s
Description: Whether you call it radiation fog, ground fog, valley fog or a temperature inversion, it all comes down to the same physics: the atmosphere flipping upside down. CBC’s Johanna Wagstaffe breaks down how a high-pressure system can turn our valleys into bowls of clouds.

Whether you call it radiation fog, ground fog, valley fog or a temperature inversion, it all comes down to the same physics: the atmosphere flipping upside down. CBC’s Johanna Wagstaffe breaks down how a high-pressure system can turn our valleys into bowls of clouds.

The article provides a concise explanation of how fog forms, focusing on the role of temperature inversions where cold, heavy air settles at the ground, trapping moisture and creating a cloud at ground level. It discusses how high pressure can reinforce this effect, leading to persistent fog, especially in the morning when the ground cools overnight and chills the air above.

The piece also notes that the fog layer is typically shallow, ranging from 50 to a few hundred meters thick, which can cause whiteout conditions at ground level while higher elevations remain clear. The article concludes by highlighting that fog, though quiet and persistent, can be among the most disruptive types of weather.

How fog works in 90 seconds

Fog. It's hard to ignore, hard to escape, and hard to forecast. Here's what's happening in 90 seconds. So, most of the time, the sun heats the ground and then heats the air right above it. That air starts to rise, air expands, cools, and sinks back down. It's this constant churning, this vertical motion that keeps the atmosphere moving. But every so often, that whole structure flips. Cold, heavy air settles right against the ground. Meteorologists call this an inversion. You can think of it like an atmospheric lid. And right now, that lid is being held down by high pressure, sinking air that traps everything underneath. With nowhere to go, that moisture condenses into tiny droplets, a.k.a. a cloud on the ground. It's also why you almost always see fog in the morning. Overnight, without the sun to keep things churning, the ground loses heat, chills the air right above it, and that cold air becomes heavy and just sits there. That's all fog is. The air's stuck in the wrong order. And that cold layer is usually very shallow. We're talking 50 to a few hundred meters. So whiteout at a bus stop, but if you go up a mountain, clear blue skies. Sometimes the most disruptive weather in Canada isn't the loud crashing kind. It's the kind that just settles in and refuses to move.

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