The fatal pile-up of vehicles on a foggy Friday morning on I-43 is a reminder of a weather phenomenon we often take for granted. Fog has been described as a cloud sitting on the ground. While that description is accurate, it doesn't tell the whole story. Fog formation occurs under various circumstances, and its dissipation is equally interesting. Understanding the vagaries of fog can help us be aware of potentially dangerous situations.

The locally dense fog on the morning of October 11 was called "radiation fog." It is a result of clear skies and calm winds. Under those atmospheric conditions, the air nearest the ground cools rapidly once the sun sets. You may recall a recent weather column where I talked about frost formation when temperatures reach 35, 36 or 37 degrees. Temperatures at ground level can reach the freezing mark (32 degrees) even though standard thermometer readings at a height of 5 feet above the ground are 3-5 degrees warmer than freezing.

The word "radiation" refers to the cooling process of the air near the ground. It is radiational cooling, meaning the heat from the previous day radiates or moves upward away from the Earth's surface. If the overnight hours feature an extended period of clear sky and calm wind, radiational cooling is at its maximum. The air within the first few feet of the ground cool to such a level that the water vapor in the air begins condensing out into liquid water droplets. These tiny droplets form fog by the same process that clouds form high above us.

Cooler air causes the water molecules to move more slowly. With warmer temperatures and more rapid movement, the molecules of H2O (two hydrogen atoms attached to one oxygen atom) do not bind together. They move freely as a gas (or vapor). Cooler temperatures cause the molecules to move more slowly, and they begin to attach to each other to form liquid water droplets.

Radiation fog is very shallow, often no more than 30 feet thick. That was the situation on the morning of October 11. Forward visibility was reduced to less than 1/8th mile in many areas, yet stars were visible in the clear sky if you looked straight above you. Sunrise posed a problem, too. The accident occurred only 20 minutes after sunrise. The glare of the low Sun angle as it rose above the horizon caused a "white-out" effect. The tiny water droplets in fog can scatter sunlight and create a zero-visibility situation without warning. Also, the accident site is in a slightly lower part of the surrounding landscape.

Radiation fog is most dense in low-lying areas. Because cold air is denser (heavier) than warm air, it tends to sink down to the lowest level due to gravity. Valley fog is very dense. But even a slight dip in the road can cause forward visibility to vanish because of the cold air.

While radiation fog is the most common, there are other ways in which fog can form. Anytime the air cools to its dew point, water vapor will begin condensing to water droplets. "Advection fog" is created when warm, moist air moves over colder ground. The "advection" refers to air moving horizontally.

Advection fog is common in winter when we have an extensive snow cover over southeast Wisconsin. If warm and humid air moves in from the south, the snow cover acts as a refrigerator and cools the air enough to cause condensation. Advection fog covers a wide area, sometimes hundreds of miles, because of the large extent of winter snow cover.

"Steam fog" or "sea smoke" is formed over water when very cold air moves over warmer water. We see this often in winter over Lake Michigan, the Milwaukee River, or even the neighborhood pond. Anytime there is open water (no ice), the warmer water evaporates into the cold air above, increasing the dew point and forming fog that appears to rise from the water's surface like steam from a pot on the stove.

Fog is a common weather element and often adds a beauty all its own to the landscape. The process of fog formation is the same as the formation of clouds above us. While its formation may be a simple process, knowing where and when it may occur may help keep us out of danger.