Fog and Visibility Estimator

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. Aviation works to a 4°F rule, which is 2.2°C: once the spread is inside that, the cloud base is effectively on the ground and fog is on the cards. The cloud base calculator on this site uses the same threshold, so the two pages describe the same air the same way.

The calculator also reports the exact relative humidity from the temperature and dew point rather than the familiar RH ≈ 100 − 5 × depression shortcut. The shortcut is easy to do in your head and runs a few points high, which matters here because 95% is the line between mist and haze.

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 and clears after sunrise. Advection fog is sea fog blown inland, and it can sit for days. Upslope and valley fog are terrain doing the lifting for you.

Radiation fog is the predictable one, and it needs three things at once: clear skies so the ground can radiate its heat away, near-calm air so the cooled layer stays put, and a small dew point depression so it does not have far to fall. Take away any one of those and you get a cold damp night instead. That is why this calculator can only tell you whether fog is possible, and why the forecaster looks at the wind and the cloud before committing.