Field Guide / Clouds & Weather Signs
Fog
ground cloud
A cloud at ground level that cuts visibility and can make it easy to lose your way. Fog that forms on a clear, calm night often burns off by mid-morning.
- Height
- Special & Ground-Level
- Abbreviation
- fog
- Typical altitude
- At the ground
- Usually means
- Dull and damp
How to recognize it
- A white or gray mist that thickens near low ground, valleys, and water.
- It forms on clear, calm, cool nights, often at dawn.
- Visibility may drop to a few yards.
What it usually means
- Fog on a clear, calm morning often burns off by late morning, leaving a fair day.
- Fog that stays all day usually comes with a low, stable layer of damp air.
- Sea fog can last all day along the coast.
What to do outdoors
- Do not leave the trail or road in dense fog. Use a map, compass, or GPS to keep track of where you are.
- If you are lost, stop, stay put, and use a whistle or light to signal.
- Wear bright clothing when hunting or walking along roads, and slow down on the water.
Often confused with
- Stratus: the same kind of layer, but above the ground.
Field notes
Fog is often calm and quiet, which makes it easy to forget how hard it is to see. Navigation skills matter most when you can't see more than a few yards.
Related guides
Cloud signs are rough guides, not forecasts. Always check the National Weather Service forecast at weather.gov before and during a trip, carry a way to receive alerts, and treat any watch or warning as a reason to change your plans. If you can hear thunder, you are close enough to be struck by lightning.