Fog happens when the air near the ground becomes saturated with moisture and enough cooling or light wind slows mixing, letting tiny droplets hang in the air. In short, fog is basically a low-lying cloud that forms under the right humidity and temperature conditions. Key causes and kinds of fog
- Radiation fog: This is the most common for calm, clear nights. The ground loses heat, the air near the surface cools to its dew point, and water vapor condenses into tiny droplets. It often forms after a cool, calm night and burns off after sunrise as the ground warms.
- Advection fog: Occurs when warm, moist air moves over a cooler surface (land or sea), cooling to the dew point and forming fog. This type can persist if winds stay light and moisture remains high.
- Freezing fog or frost fog: When the air is near or below freezing and water droplets don’t freeze, creating a fog that feels icy to the skin.
- Plant or ground influences: Transpiration and evaporative cooling from the surface, plus moist air near water bodies, can contribute to fog formation, especially in valleys or near coasts.
Common conditions that produce widespread fog
- High humidity near or at saturation (relative humidity near 100%).
- Light or calm winds that fail to mix and disperse the moisture. Even a slight breeze can keep fog from forming or help it dissipate, but very light winds often allow fog to persist.
- Clear skies at night followed by rapid radiational cooling of the surface.
- Cool air moving over warmer water or moist ground, which can generate advection or steam fog depending on the specifics.
Practical indicators you can check locally
- You’ll notice reduced visibility, often to a few hundred meters or less, and a damp, cool feel in the air.
- Dew point and ambient temperature closeness: when the air temperature is within a few degrees of the dew point, fog is likely or already present.
- If winds are very light and the sky is clear overnight, radiation fog is a strong possibility in the following morning.
Tips for staying safe
- Drive slowly and use low-beam headlights; fog can hide hazards like vehicles and road edges.
- If you’re walking near roads, give yourself extra space and assume drivers can’t see you well.
- Check local weather updates for fog advisories or warnings, which often include expected dissipation times.
If you’d like, share your location and the current weather conditions (temperature, dew point, wind speed, and cloud cover), and can give a more precise explanation of what’s likely causing the fog where you are.
