7 Types of Fog You Didn't Know Had Names

Whether it "comes on little cat feet," as poet Carl Sandburg whimsically suggested, or rolls in like an ominous tsunami cloud, fog is one of Mother Nature's most magnificent displays. It can veil the dawn in delicate beauty or shroud an entire city in gloom. It has even been known to kill.

This atmospheric marvel is actually a cloud that touches the ground, formed when water vapor in the air condenses around microscopic dust, salt or other particles and morphs into suspended water droplets or ice crystals. And like overhead clouds, fog isn't a single entity that varies by degree of murkiness. It comes in several distinct varieties that are influenced by nearby bodies of water, landscape features and other local factors. Here are some of the most impressive forms of fog on the planet.

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Radiation Fog

We've all awakened to a misty low-lying ground cover that usually "burns off" in the morning sun. That's radiation fog, and it typically forms on clear, calm nights as the land cools via thermal radiation. When the air directly above the ground starts cooling too, it can't hold as much moisture. This condenses into water droplets that hang in the air.

Radiation fog — which can vary from a fine vapor to nearly white-out haze — is most common in the fall and early winter.

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Valley Fog

The name says it all. Valley fog, which settles into the hollows and basins between hills and mountains, is a type of radiation fog. When cooler, heavier air laden with condensed water droplets is trapped beneath a layer of lighter, warmer air and hemmed in by ridges and peaks, it can't escape and often lingers for days.

One of the most spectacular examples is tule fog, a pea-soup-style regular in California's Great Central Valley from late fall to early spring.

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Advection Fog

Blow moist, warm air over a cool surface (usually water) and you get advection fog. This is the legendary white stuff that blankets San Francisco (pictured).

In fact, the entire Pacific Coast of North America gets its share of advection fog in summer due to the upwelling of cooler, deeper waters near the coast. When air warmed by waters farther out in the Pacific blows in over this cooler water, fog forms and rolls inland. Advection fog can also materialize when warm air moves over cooler land surfaces or regions with a hefty snowpack.

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Upslope Fog

Just like it sounds, upslope fog (sometimes called hill fog) forms when winds blow moist, warm air up a slope. As the rising air expands due to decreasing air pressure (called adiabatic expansion) it cools and reaches the condensation point to form a cloud.

This is the fog you see artfully draped over hills and mountains. In the U.S., upslope fog usually makes an appearance in spring and winter on the eastern side of the Rocky Mountains, and in the Appalachian and Adirondack mountains.

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Freezing Fog

When water droplets in fog are cooled below the freezing point, they remain in a liquid state (unless they fall to extreme frigid lows). When these droplets hit a freezing surface, the result is white rime. These feathery ice crystals coat everything and magically transform the world into a winter wonderland.

In the West, freezing fog is often referred to as "pogonip," the Shoshone word for "cloud."

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Frozen Fog

Not to be confused with freezing fog, frozen or ice fog forms when water droplets in fog are super-cooled below the liquid-state point to extreme subzero temperatures. There they turn into ice crystals that remain suspended in the air.

For this to happen, temperatures must dip to minus 22 degrees Fahrenheit or lower. Your best bet for this bone-chilling experience? Head north to Alaska or the Arctic.

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Evaporation Fog

This type of fog goes by many names, including steam fog and sea smoke. It most often appears in fall when the air begins to chill before bodies of water do. As the cool-air layer closest to warm water begins to heat up, moisture from the water below evaporates into it. This air then rises into the colder air above, cools, and water droplets condense into fog.

You've probably seen this picturesque haze rising like morning steam off bodies of water, everything from oceans and lakes to rivers and even swimming pools!