Fog and Visibility Estimator
Estimate visibility in fog from dew point depression and relative humidity.
Understand fog formation conditions and aviation minimums.
Fog forms when air temperature falls to the dew point (or very close to it), causing water vapor to condense into tiny suspended droplets. The closer the temperature is to the dew point, the more likely fog will form.
Dew point depression: T - T_dew. When this falls below 2°C (3.6°F), fog is likely.
Fog density and visibility: The actual visibility in fog depends on the concentration of water droplets, which relates to:
- How much the temperature has fallen below the dew point (supersaturation level)
- The size distribution of water droplets
- The presence of condensation nuclei (dust, pollutants)
Visibility categories (aviation):
- Thick fog: visibility < 200 m (656 ft)
- Dense fog: 200–500 m
- Moderate fog: 500 m – 1 km
- Shallow/thin fog: 1–2 km
- Mist: 2–5 km with relative humidity > 95%
- Haze: 2–5 km with relative humidity < 95% (due to particles, not water)
Aviation minimums (US, IFR): Ceiling below 1,000 ft or visibility below 3 miles requires an IFR flight plan. Special VFR minimums: ceiling 1,000 ft, visibility 1 mile. Cat I ILS approach: decision height 200 ft, visibility 1,800 ft (RVR).
Fog types: Radiation fog (forms overnight, clears after sunrise), advection fog (sea fog, persistent), upslope fog, valley fog. Radiation fog is most predictable — it forms when skies are clear, winds calm, and dew point depression is small.