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Fog Day Predictor — How to Know If Fog Will Form Tonight
A fog day predictor uses tonight's temperature, dew point, and wind speed to estimate how likely fog is to form by early morning — before your weather app has reported anything.
Fog Day Predictor — How to Know If Fog Will Form Tonight
A fog day predictor uses tonight's temperature, dew point, and wind speed to estimate how likely fog is to form by early morning — before your weather app has reported anything. Learn more in Fog Delay Calculator — How to Predict School and Flight Delays. Learn more in Best Weather App for Runners — Heat Index, Wind Chill, and Storm Timing.
Unlike a forecast that notes fog after it's been observed, a predictor works from the atmospheric setup the night before, giving you 8-12 hours of advance warning.
The three conditions fog needs:
- High relative humidity — ideally above 90%, which happens when air temperature and dew point are within 2-4°F of each other
- Light or calm winds — wind mixes the air and breaks up the surface temperature inversion that fog depends on. Under 5 mph is ideal for fog formation.
- Clear skies overnight — clouds act like a blanket, trapping heat near the surface. Clear skies allow rapid radiative cooling, bringing surface air temperature down to the dew point.
How to read tonight's setup:
Check the overnight low. If it's within 5°F of the current dew point and winds are under 5 mph, fog probability is moderate to high. If the overnight low equals the dew point, fog is nearly certain in low-lying areas.
Valley fog is the most predictable — cold air drains downhill and pools in hollows and valleys overnight, reaching saturation faster than surrounding elevated terrain. If you're in a river valley or basin, your fog risk is higher than hilltop neighbors even with identical weather data.
