The Fog That Arrived Early
An aerodrome in a river valley has a runway at 300 ft elevation. At 1800 local time the temperature is 14 °C and the dewpoint is 11 °C, with a light westerly breeze of 6 kt and scattered stratocumulus at 2,500 ft. The forecast for the evening is for the wind to become calm and the sky to clear. The pilot plans a night departure at 2200. At 2100 the reported visibility is 1,500 m in mist, temperature 8 °C and dewpoint 8 °C, with the wind calm. The pilot holds a night rating but the aerodrome has no instrument approach and requires 5 km visibility for a night departure.
1. Using the 1800 observation, estimate whether radiation fog is likely and explain the physical processes involved.
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The spread at 1800 is 3 °C. A useful rule of thumb is that the temperature falls about 1 °C per hour after sunset under clear skies and light wind, and the dewpoint stays roughly constant (or rises slightly as the surface cools and moisture condenses). With sunset around 1900, by 2200 the temperature could fall to about 11 °C, matching the dewpoint, so saturation and fog are plausible. The ingredients are all present: clear skies allow strong radiative cooling of the ground, a light wind (6 kt) provides just enough mixing to spread the cooling through a shallow layer without dispersing it, and the valley setting traps cold air and moisture. The main uncertainty is whether the wind becomes truly calm, which would limit the fog to a shallow ground layer, or stays light, which would deepen it. The 2100 observation confirms the process is well advanced.
2. At 2100 the temperature and dewpoint are both 8 °C. What does this tell you about the fog's stage and likely trend?
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When temperature equals dewpoint, the air is saturated and fog is present. The visibility of 1,500 m in mist indicates the droplet concentration is still moderate, but with calm wind and continued radiative cooling the fog will thicken: visibility commonly falls to 200–500 m within an hour or two of saturation in a valley at night. The fog is now in its mature phase. The trend is for further cooling until either the ground reaches equilibrium with the air or a wind picks up. The pilot should expect the 2200 visibility to be worse than 1,500 m, not better, and should not plan on the 5 km requirement being met.
3. What should the pilot do about the 2200 departure, and what alternatives exist?
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The departure cannot legally or safely proceed: the aerodrome requires 5 km visibility for a night departure and the trend is downward from 1,500 m. The pilot should cancel or delay the flight. Alternatives include waiting until after sunrise, when insolation lifts the temperature above the dewpoint and the fog disperses (typically 1–2 hours after sunrise in a valley), or repositioning to a nearby aerodrome outside the valley with an instrument approach and better visibility. The pilot should not attempt a departure on the basis that the fog 'looks thin' - a night departure into radiation fog with no instrument approach is a high-risk decision with no escape route if the engine fails. The correct decision is to remain on the ground and re-evaluate with a new observation and a forecast for the morning.