Fog, Visibility and Low-Level Obscurations
What the exam tests
This domain tests fog formation, visibility reporting and low-level obscurations that affect take-off, approach and landing.
Core rules
- Radiation fog forms on clear, calm nights when the ground cools the air to its dew point; it is favoured by light wind, high humidity and long nights, and usually dissipates after sunrise.
- Advection fog forms when warm moist air moves over a cooler surface; it can persist in wind and is common over cold water or coasts.
- Fog is visibility below 1,000 m; mist is 1,000 m to 5,000 m. RVR is runway visual range, not the same as reported meteorological visibility.
- Low cloud, haze, blowing dust, snow and precipitation can obscure the surface without fog being present.
Worked example
At 0600 local, temperature is 8°C, dew point 6°C, wind calm and the sky is clear. With only a 2°C spread, nocturnal cooling can bring the air to saturation, so radiation fog is likely. If the wind were 12 kt, mixing would probably prevent fog but could produce low stratus instead. The reasoning turns on cooling to dew point plus enough mixing, but not too much.
Traps
Equating mist with fog; using RVR as visibility; assuming fog requires calm wind; forgetting that advection fog can persist in daylight.