Fog is a cloud that forms at ground level, and in aviation it matters because it can reduce visibility below safe limits for takeoff, landing, and taxi. Pilots must understand not only that fog is present, but also why it formed and how it is likely to change. Radiation, advection, upslope, and steam fog each develop under different temperature, moisture, wind, and terrain conditions.
Knowing the fog type helps pilots predict whether visibility may improve quickly after sunrise or persist for many hours.
Key Facts
- Fog forms when air near the ground becomes saturated, so relative humidity is about 100 percent.
- Radiation fog forms on clear, calm nights when the ground loses heat and cools the air above it to the dew point.
- Advection fog forms when warm, moist air moves over a colder surface and cools from below.
- Upslope fog forms when moist air is forced uphill, expands, and cools to the dew point.
- Steam fog forms when cold air moves over warmer water and water vapor evaporates upward into the cold air.
- Temperature-dew point spread = T - Td; a small spread means fog or low clouds are more likely.
Vocabulary
- Fog
- Fog is a cloud at or near the ground that reduces horizontal visibility, often to less than 1 kilometer in meteorology.
- Dew point
- Dew point is the temperature to which air must cool for water vapor to begin condensing into liquid droplets.
- Radiation fog
- Radiation fog is fog caused by nighttime ground cooling that chills nearby moist air to saturation.
- Advection fog
- Advection fog is fog caused by moist air moving horizontally over a colder surface and cooling to its dew point.
- Upslope fog
- Upslope fog is fog caused when moist air rises along sloping terrain and cools as pressure decreases.
Common Mistakes to Avoid
- Assuming all fog forms the same way, which is wrong because each fog type depends on a different cooling or moisture process.
- Ignoring wind speed, which is wrong because calm air favors radiation fog while light to moderate wind can support advection or upslope fog.
- Using temperature alone to predict fog, which is wrong because the temperature-dew point spread shows how close the air is to saturation.
- Expecting fog to disappear at sunrise every time, which is wrong because radiation fog may burn off with heating, but advection and upslope fog can persist if the air mass and wind pattern continue.
Practice Questions
- 1 At an airport, the air temperature is 8°C and the dew point is 7°C just before sunrise under clear skies and calm wind. What is the temperature-dew point spread, and which fog type is most likely?
- 2 A coastal runway has 18°C moist air moving over ocean-cooled land at 10°C. If the air cools to its dew point near the surface, which fog type is likely, and what horizontal air motion causes it?
- 3 A valley airport reports fog on a clear, calm morning, while a nearby hill airport reports fog during steady moist wind blowing uphill. Explain why these two locations may have different fog formation mechanisms.