Short answer
A cloud is suspended visible moisture in the air. Fog is a cloud at or near the ground, where people and vehicles move through it. The droplets can be similar; what changes is their position relative to the surface and the resulting visibility. 1 2
Fog is essentially a ground-level cloud; altitude and its effect on surface visibility distinguish the names.
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At a glance
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| Question | Cloud | Fog |
|---|---|---|
| Position | Can be high or low above ground | Touches or hugs the ground |
| Main practical effect | May shade or produce precipitation | Directly reduces visibility at surface |
| Formation | Air cools or gains moisture to saturation | Near-surface air cools or gains moisture to saturation |
The table summarizes the cited definitions and the article’s stated scope. 1 2
What each thing is
Cloud. A visible collection of tiny water droplets, ice crystals or both suspended in the atmosphere. 1
Fog. A cloud at or very near the ground that reduces horizontal visibility. 2
Key differences
Both result when air becomes sufficiently moist for tiny droplets or ice particles to be visible. Meteorologists name the near-ground form fog because it affects travel and observation differently from a cloud overhead. A cloud deck can lower until it becomes fog for a location on a hill. 1 2
How to tell them apart
Ask whether the visible moisture is at the observer’s level and obscures the view along the ground. If you are looking up at it, cloud is usually right; if you are inside it at the surface, fog is right. Visibility readings and local terrain help resolve borderline cases. 1 2
Where they overlap
Fog and clouds have similar condensed particles and can form by cooling or moisture addition. A low stratus cloud and fog can connect physically without a sharp wall between them. 1 2
Edge cases
A mountain observer can be in a cloud that is fog-like locally while someone in the valley sees a cloud overhead. Mist and haze are additional visibility terms with different particle or density considerations. Ice fog occurs under very cold conditions. 1 2
Why the distinction exists
The separate word directs attention to surface visibility, a key issue for roads, aviation and marine travel. It also describes the same atmospheric phenomenon from a useful location-specific perspective. 1 2
Common misconceptions
Fog is not a different substance from a cloud, and it does not have to form from smoke or pollution. 1 2
Examples
A gray layer passing above a city is a cloud. If the same low layer envelops a hillside road, drivers on that road experience fog. 1 2
Sources
Sources checked October 3, 2026.
- National Weather Service — Cloud Development. Fog and cloud distinction.
- National Weather Service — How Fog Forms. Formation and visibility.