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Fog Delay Calculator — How to Predict School and Flight Delays
Fog delay calculator predicts whether overnight conditions will produce dense fog by morning — giving you a heads-up before your commute, school run, or flight.
Fog Delay Calculator — How to Predict School and Flight Delays
A fog delay calculator predicts whether overnight conditions will produce dense fog by morning — giving you a heads-up before your commute, school run, or flight.
The key metric is the dew point spread: the gap between current air temperature and dew point temperature. When that gap closes to within 2-3°F (about 1-2°C), fog becomes likely. Pair that with calm winds under 5 mph and you have the classic setup for radiation fog. Learn more in Fog Day Predictor — How to Know If Fog Will Form Tonight. Learn more in Best Weather App for Hiking in 2026.
Why fog forms overnight
After sunset, the ground loses heat rapidly. If the air near the surface cools to its dew point, water vapor condenses into tiny droplets — that's fog. Clear skies accelerate this cooling (clouds trap heat near the surface). Calm air prevents mixing that would disrupt the fog layer from forming.
What inputs matter:
- Current temperature (°C)
- Dew point temperature
- Wind speed — under 5 mph dramatically increases fog probability
The closer your overnight low temperature gets to your current dew point, the more certain fog becomes by 6-8am.
When delays typically trigger:
- Schools: Visibility below 1/4 mile usually prompts a 1-2 hour delay
- Airports (Category I ILS minimums): Roughly 1,800 ft runway visual range (1/3 mile)
- Highway advisories: Dense fog advisory issued when visibility drops below 1/4 mile
The evening forecast tells you "fog possible." The calculator tells you how likely it actually is given tonight's specific conditions.
