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Rainbow Rings: Here’s a WWII-Era Trick to Predict Ground Fog

Pilot’s unexpected encounter with weather element brings to mind a visual warning sign that’s still relevant.

One of the first skills a pilot should develop is the ability to acquire a weather briefing. [Credit: Meg Godlewski]
One of the first skills a pilot should develop is the ability to acquire a weather briefing. [Credit: Meg Godlewski]
Gemini Sparkle

Key Takeaways:

  • Fog forms rapidly when the temperature and dew point spread is 5 degrees Fahrenheit or less and converging.
  • A practical trick to detect impending ground fog is to look for rainbow rings around taxiway lights, which indicate water droplets in the air.
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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.”

Meg Godlewski

Meg Godlewski has been an aviation journalist for more than 24 years and a CFI for more than 20 years. If she is not flying or teaching aviation, she is writing about it. Meg is a founding member of the Pilot Proficiency Center at EAA AirVenture and excels at the application of simulation technology to flatten the learning curve. Follow Meg on Twitter @2Lewski.

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