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Whether you're flying VFR or IFR, fog can ruin your day. Here's what you should know about the 6 most common types of fog.

How Does Fog Form?

Fog may be present when a small temperature/dew-point spread exists (usually within 5 degrees Celsius). It's most common in areas with abundant moisture, and it generally forms when...

• Air is cooled to the dew-point (temperature decreases).
• Moisture is added to the air (dew-point increases).

1) Radiation Fog

Common both at night and dawn, radiation fog forms when the ground cools, subsequently cooling surrounding air to its dew-point. This fog tends to "burn-off" in the morning as the sun re-heats the ground and air.

The best conditions for the formation of radiation fog are clear skies, little to no wind, and high relative humidity.

2) Advection Fog

When moist air moves over cooler ground or water, advection fog forms. It's most common along the coast, as sea breeze blows moist air over land.

This type of fog is very common in the Pacific Northwest, and is usually much more persistent than radiation fog.

3) Steam Fog

Common during cold weather months over bodies of water, steam fog forms when cold and dry air moves over warm water.

This type of fog tends to be very low-level, affecting seaplane pilots and pilots flying on runways next to the water.

4) Upslope Fog

As moist, stable air moves up along terrain, it cools. This causes upslope fog, which can extended hundreds of feet above surrounding terrain. It's one common reason that an AIRMET Sierra may be issued.

5) Precipitation Fog

As warm rain falls through cool air, the precipitation saturates the cool air. This fog is dense and long-lasting. Mixed in with rain, it can be tough to determine exactly where precipitation fog is...and isn't.

6) Freezing Fog

Freezing fog occurs when tiny droplets of water are supercooled in the air. Once they contact a surface (or condensation nuclei) the droplets freeze on contact. It typically needs to be very cold for freezing fog to form, roughly 15 degrees Fahrenheit or colder.

When's the last time you were grounded by fog? Tell us in the comments below.

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