A cloud on the ground
What Is Fog?
Fog is a cloud that formed at your height instead of a kilometre up.
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Air can only hold so much water vapour.
Warm air holds a lot; cold air holds much less. Fill air to its limit and any further cooling forces water out as visible droplets.
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Step 2 of 5
Cool the air and the water comes out.
Cooling does not remove water — it lowers the limit until the air is over-full. The extra condenses onto dust as droplets. That is fog.
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Clear, still nights make the most fog.
With no cloud blanket, the ground radiates its heat to space and chills the air just above it. Wind would mix it away, so calm is essential.
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It can also form when warm air meets cold ground.
Mild damp air drifting over cold ground or cold sea is chilled from below. This is what produces the persistent sea fogs of some coasts.
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It blocks your view by scattering light.
Each droplet bounces light in a random direction. After millions of bounces, light from distant objects is thoroughly jumbled before it reaches you.
A cubic metre of thick fog contains only a fraction of a gram of water. Almost nothing blocks almost everything.
The short version
Fog is a ground-level cloud, formed when air cools past the point where it can hold its water.
Try it yourself
Open a freezer on a humid day. The visible wisp is warm air being chilled below its limit — fog you made.
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