It's that time of year again. For much of the United States, fall weather, and the rapid cooling that happens after sunset, leads to radiation fog. Here are the best ways to recognize the conditions that might get you stuck on the ground, or worse, in the air and diverting to another airport.

1) Terrain

Radiation fog tends to hide in valleys and other low spots of terrain. Rapid cooling, mixed with the calm nights and sheltered valleys, provides the perfect conditions for low-lying fog formations. In some parts of the US, like the central valley of California, the phenomenon is easily visible from space. This particular example of radiation fog is called the 'Tule Fog'.

If you're considering flying to an airport in a basin or valley, be aware that fog could easily and quickly form as you near sunset.

2) Dew Point

When you're getting your weather briefing, look at the current temperature-dewpoint spread. The dew point is the temperature to which air must be cooled to become completely saturated.

When the temperature-dewpoint spread is within 3 degrees Celcius, it might be worth further investigation.

3) Clear Skies

Radiation fog thrives on clear, cloudless nights. On stable fall and winter evenings, cool surface temperatures allow the air to cool to the point of condensation, where fog will form along the surface of the terrain. As more air above the fog bank cools throughout the night, the layer of fog becomes deeper.

Since radiation fog relies on the Earth's surface cooling, the fog banks that form are often patchy. That's due to different parts of the Earth's surface cooling at different rates.

4) Calm Winds

Radiation fog won't form in light to moderate winds. Unlike advection and upslope fog, radiation fog needs calm air. This provides an advantage to pilots though, because radiation fog is highly predictable: once it forms, it tends to stay in the same place until the sun rises and starts heating the Earth's surface.

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