The six Kinds of Fog

A fogbank is a cloud at the earth’s surface. Fog forms when water vapor condenses into droplets of water or particles of ice. This can happen either because moisture is added to the air or because the temperature of the air is lowered so that it can hold less moisture. The various ways that condensation happens create the six kinds of fog.

Advection Fog

Advection fog forms when warm moist air moves over a cold surface. In meteorology, advection means the horizontal transfer of heat. Near San Francisco, seasonal currents bring cold water to the ocean surface. Warm moist air passing over the surface is cooled as its heat is transferred to the water, and the water vapor in the air condenses into fog. Meanwhile, air over inland California heats up and rises, and cool Pacific fog flows in to take its place. This is natural air conditioning. It happens on coasts around the world.

Radiation Fog

Radiation fog begins at night. The land cools by radiating its heat into the night air. Then the cold land surface draws heat away from the bottom layer of the atmosphere. The cooled air can then hold less water vapor. If the air is calm, the moisture will become dew or frost, condensed onto the ground. If it is windy, the air will clear as the moisture is mixed into large volumes of air. If the atmosphere stirs just the right amount, there will be fog.

The coldest air in a region drains to valley bottoms and can produce what appears from the hillside to be a river or lake of fog.

In California, the Central Valley is famous for its tule fog, which forms in the rainy season. It is named for the tulares, the old marshes full of tule (pronounced Too-lee) reeds, where early inhabitants believed the fogs originated. In reality, tule fog is a radiation fog. Causing huge pile-ups on the highways, it is the commonest cause of weather-related car accidents in California. Sometimes the tule fog trails through the San Francisco bay and out to sea, reversing the usual situation.

Freezing Fog

Freezing fog is made up of very cold water. This is water that is below freezing temperature, but that has not frozen. It does not freeze because it has no nucleus to condense onto. (Even ordinary raindrops need a nucleus to form.) If the vapor gets cold enough, it will freeze anyway, but between 32 and 14 degrees Fahrenheit, it is below freezing temperature but still liquid. Water in this state is called supercooled. When it hits a surface that is below freezing it will freeze onto it. If there is enough precipitation, it will coat cars, trees, houses, and roads with ice.

Ice Fog

Ice fog is composed of crystals condensed out of sub-freezing air. The temperature must usually be below -30 degrees Celsius for ice fog to occur. This most commonly happens in winter in high latitudes. Air that is already full of moisture must have water added to it, whether by the exhalations of a herd of caribou, by a body of water, or by combustion from cars and power plants. The extra water vapor is instantly crystallized.

Evaporative fog

Steam fog, also called sea smoke, is one kind of evaporative fog. It forms when cold air moves over relatively warm water. The cool air mixes with warm moisture that evaporates from the water, and the water condenses into fog. The water can look like it is covered with wisps of smoke. This happens regularly over the Great Lakes in fall. Steam fog is different from ice fog because the particles that make it up are not frozen.

Steam devils are whirling columns of fog that may seem to dance on the water. The convection currents that form them are similar to the ones that produce dust devils. They are visible because they are traced out in steam fog.

Frontal fog, another evaporative fog, forms when rain falling at a weather front evaporates as it reaches a dry layer near the ground. Once enough rain evaporates into the air, the humidity at ground level becomes high enough to form fog.

Upslope fog

As winds push air up a mountain, it usually cools as it rises. In more technical terms, orographic lift causes adiabatic cooling. Eventually, the air can get cool enough that it cannot hold the water vapor it is carrying. The vapor may condense into fog. This often happens before the fog reaches the top of the range, so viewers on the plain may see mountain peaks poking out of middle levels shrouded in fog.

Fog is mysterious and romantic, in small amounts. It is also dangerous to drivers and passengers. Too much fog is dreary, but a foggy Saturday morning is perfect for daydreaming, or for reading about fog.