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It's officially fall, and if you fly after sunset, you need to be on the lookout for unexpected fog formation.

Most parts of the US don't see a lot of fog in the summer, but all of that changes this time of year. Here's why.

How It Happens

There are several types of fog, but the one that's most likely to happen quickly in the fall is radiation fog.

There are three reasons why radiation fog is more common in the fall. First, when the sun sets, the temperature usually drops quickly. Second, the nights are longer, which allows the Earth's surface to cool for a longer period of time. And third, there tends to be plenty of warm, moist air during the day, creating the perfect recipe for radiation fog.

Here's how it happens.

As the sun sets, the ground no longer receives solar radiation, and it starts to cool. And if the sky is clear, it cools very quickly, as the surface heat escapes to space.

When that happens, conduction causes the air directly above the ground to start cooling as well. As the air cools, it becomes more saturated (remember, cold air can't hold as much water vapor). Eventually, the air reaches 100% relative humidity. And at that point, fog can start to form.

The Best Recipe For Fog

There are actually a few more factors needed for fog to form, and they're things you can easily look for when you're getting your preflight briefing.

The best recipe for radiation fog is: clear skies, calm winds, high relative humidity, and a stable atmosphere.

Knowing that, you can look at a weather briefing and know within minutes if you're in an area that's ripe for fog formation.

1) Are skies clear, few or scattered? That means there will be a lot of heat escaping the Earth's surface as soon as the sun goes down.

2) Are winds calm or light and variable, and is there a high-pressure area over you? That means the atmosphere is stable, which means fog can form right over the ground. (If winds are higher, fog typically doesn't form. Instead, low stratus clouds can develop, but they're detached from the surface).

3) And finally, is the air humid? A quick check of the temperature/dewpoint spread can tell you that. If the spread is only a degree or two apart, you know that fog could easily become a factor after sunset.

The Fog Isn't Going Away, At Least Until Morning

When radiation fog happens, it doesn't usually move very much. The good news is that it's typically patchy, meaning it doesn't cover the ground uniformly. But the bad news is when (and where) it forms, it usually stays until the morning.

That's because at night, the earth's surface is continuously cooling, causing the air above it to keep cooling and stay at 100% relative humidity throughout the night. The fog starts shallow along the surface, and then thickens as the air above it continues to cool.

There are only a few things that will make the fog go away. An increase in wind or a dry air mass moving into an area can make it happen, but the most likely factor is the sun.

When the sun rises in the morning, the Earth's surface and atmosphere start heating up again, causing the top of the fog bank to start mixing with the air above it. At the same time, tiny droplets of water suspended in the fog start evaporating as the relative humidity decreases with the increase in temperature.

Eventually, the fog bank detaches from the ground and starts lifting into low stratus clouds.

How To Avoid It

Avoiding an unexpected fog layer starts with your weather briefing. If you know there's stable air above you, the skies are clear, and the temp/dewpoint spread is only a few degrees or less, you know there's a good chance of fog forming when the sun sets.

But keeping an eye on the weather when you're aloft is just as important. Most ASOS weather stations have a 50-mile range, and they update their weather every minute.

Keep an eye on the temperature/dew point spread during your flights. If they start closing in on each other, it might be time to get yourself on the ground. Otherwise, you risk getting caught on top of a fog layer, and diverting somewhere you didn't plan on going.

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