![One of the first skills a pilot should develop is the ability to acquire a weather briefing. [Credit: Meg Godlewski]](https://flyingmag1.b-cdn.net/wp-content/uploads/sites/2/2025/01/FOGGY-scaled-1.jpeg?width=2560&height=1912)
Question: I was preparing for a night flight to get night current. It was a cold but clear night with a full moon. In the middle of the taxi to the runway at the nontowered airport, the fog dropped like a blanket. It was a surprise, since the TAF didn’t indicate fog. I know fog forms when the temperature and dew point converge, but is there a way to tell if it is on the way besides monitoring the temperature?
Answer: You are correct, as indicated in the Pilot’s Handbook of Aeronautical Knowledge, Chapter 12, “when the temperature of the air is reduced to the dew point, the air is completely saturated and moisture begins to condense out of the air in the form of fog, dew, frost, clouds, rain, or snow.”
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When the temperature dew point spread is 5 degrees Fahrenheit or less and converging, you expect fog to form quickly, like snap-your-fingers quick.
A trick I learned from a World War II-era Army-trained aviator was to look for rainbows appearing around the taxiway lights at the airport. Rainbows are created when light passes through water droplets. Those water droplets collect and form fog—so consider the rainbow rings a warning that ground fog is not far behind.
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