Here in Southeastern Wisconsin we have been in the grips of dense fog for a few days this month. It is not unusual to have fog here in December. On average there are 14 days during the month with some fog in this area. Both fog and clouds are formed when water vapor condenses or freezes to form tiny droplets or crystals in the air. Clouds can form at many different altitudes. They can be as high as 12 miles above sea level or as low as the ground. Fog is a kind of cloud that touches the ground. Fog forms when the air near the ground cools enough to turn its water vapor into liquid water or ice.
In this part of Wisconsin, which is near to Lake Michigan, fog often forms when warm air meets the cooler are over the Lake. As Lake Michigan was largely frozen over this past winter, the water has been cooler than usual and has been more conducive to the formation of fog.
Types of Fog
Fogs which are composed entirely or mainly of water droplets are generally classified according to the physical process which produces saturation or near-saturation of the air. The main types of fog are:
This type of fog forms at night under clear skies with calm winds when heat absorbed by the earth’s surface during the day is radiated into space. As the earth’s surface continues to cool, provided a deep enough layer of moist air is present near the ground, the humidity will reach 100% and fog will form. Radiation fog varies in depth from 3 feet to about 1,000 feet and is always found at ground level and usually remains stationary. This type of fog can reduce visibility to near zero at times and make driving very hazardous.
Valley fog is a type of radiation fog that is very common in the mountains of eastern Kentucky. When air along ridgetops and the upper slopes of mountains begins to cool after sunset, the air becomes dense and heavy and begins to drain down into the valley floors below. As the air in the valley floor continues to cool due to radiational cooling, the air becomes saturated and fog forms. Valley fog can be very dense at times and make driving very hazardous due to reduced visibility. This type of fog tends to dissipate very quickly once the sun comes up and starts to evaporate the fog layer.
Advection fog often looks like radiation fog and is also the result of condensation. However, the condensation in this case is caused not by a reduction in surface temperature, but rather by the horizontal movement of warm moist air over a cold surface. This means that advection fog can sometimes be distinguished from radiation fog by its horizontal motion along the ground.
Sea fogs are always advection fogs, because the oceans don’t radiate heat in the same way as land and so never cool sufficiently to produce radiation fog. Fog forms at sea when warm air associated with a warm current drifts over a cold current and condensation takes place. Sometimes such fogs are drawn inland by low pressure, as often occurs on the Pacific coast of North America.
Advection fog may also form when moist maritime, or ocean, air drifts over a cold inland area. This usually happens at night when the temperature of the land drops due to radiational cooling.
Upslope fog forms when light winds push moist air up a hillside or mountainside to a level where the air becomes saturated and condensation occurs. This type of fog usually forms a good distance from the peak of the hill or mountain and covers a large area. Upslope fog occurs in all mountain ranges in North America. This usually occurs during the winter months, when cold air behind a cold front drifts westward and encounters the eastward facing slopes of the Rocky Mountains. As the cold, moist air rises up the slopes of the mountains, condensation occurs and extensive areas of fog form on the lower slopes of the mountains.
This type of fog forms when the air temperature is well below freezing and is composed entirely of tiny ice crystals that are suspended in the air. Ice fog will only be witnessed in cold Arctic / Polar air. Generally the temperature will be 14 F or colder in order for ice fog to occur.
Freezing fog occurs when the water droplets that the fog is composed of are “supercooled”. Supercooled water droplets remain in the liquid state until they come into contact with a surface upon which they can freeze. As a result, any object the freezing fog comes into contact with will become coated with ice. The same thing happens with freezing rain or drizzle.
Evaporation or Mixing Fog
This type of fog forms when sufficient water vapor is added to the air by evaporation and the moist air mixes with cooler, relatively drier air. The two common types are steam fog and frontal fog. Steam fog forms when cold air moves over warm water. When the cool air mixes with the warm moist air over the water, the moist air cools until its humidity reaches 100% and fog forms. This type of fog takes on the appearance of wisps of smoke rising off the surface of the water. The other type of evaporation fog is known as frontal fog. This type of fog forms when warm raindrops evaporate into a cooler drier layer of air near the ground. Once enough rain has evaporated into the layer of cool surface, the humidity of this air reaches 100% and fog forms.
Dense fog contributed to the worst automobile accident in Wisconsin history. Ten people were killed and nearly 40 more hurt in a massive pileup on Interstate-43 near Cedar Grove on October 11, 2002. The crash was blamed on extremely dense fog and drivers who continued driving at normal speeds. A total of 50 vehicles were involved in the wreck.
There are regular news reports of motorists who are involved in collisions where fog is a contributing factor. Here are some suggestions for safe driving in foggy conditions:
DRIVE WITH LIGHTS on low beam. High beams will only be reflected back off the fog and actually impair visibility even more. Your lights help other drivers see your vehicle, so be sure they all work. Keep your windshield and headlights clean, to reduce the glare and increase visibility.
SLOW DOWN – and watch your speedometer – before you enter a patch of fog. Be sure that you can stop within the distance that you can see. Fog creates a visual illusion of slow motion when you may actually be speeding. Speed is a major factor in fog-related crashes.
WATCH OUT for slow-moving and parked vehicles. Listen for traffic you cannot see. Open your window a little, to hear better.
REDUCE THE DISTRACTIONS in your vehicle. Turn off the radio and cell phone. Your full attention is required.
USE WIPERS AND DEFROSTERS liberally for maximum visibility. Sometimes it is difficult to determine if poor visibility is due to fog or moisture on the windshield.
USE THE RIGHT EDGE of the road or painted road markings as a guide.
BE PATIENT. Avoid passing and/or changing lanes.
SIGNAL TURNS well in advance and brake early as you approach a stop.
DO NOT STOP on a freeway or heavily traveled road. You could become the first link in a chain-reaction collision. If you must pull off the road, signal (people tend to follow tail lights when driving in fog), then carefully pull off as far as possible. After pulling off the road, turn on your hazard flashers(hazard lights should only be used when you pull over to show that you are parked on the side of the road). Move away from the vehicle.
Acknowledgements: NOAA and National Weather Service content was used in preparing this article.