One of natures most beautiful sights from a far; yet potentially dangerous when caught in it, is fog. Fog is a cloud on the ground and is technically defined as water droplets suspended in the air at the earth’s surface. While most folks know what fog is, did you know that there are different types of fog? Thick fog reduces visibility, creating a hazard to motorists as well as air traffic, as airports may close due to thick fog, but the intensity and duration of fog varies with the location and type of fog. From early morning ground fog that burns off easily, to prolonged valley fog that can last for not just hours, but days, morning sunshine, and breezy/windy conditions tend to prevent fog from forming.


Looking at a yearly calendar and it can be said that fog can form anytime of year, but many folks think of spring and/or fall nights often being a perfect time to see fog, because during the months of March, April, September and October, one particular type of fog forms, known as “radiation” or “ground” fog. Its called radiation fog because the process begins with the “radiation” of heat from the Earth into space on clear nights, as fall nights can supply the additional ingredient of light winds which help to keep the fog in place. As the ground cools on a clear night, it cools the layer of air near the ground. If the air is humid enough, the water vapor in it begins condensing into tiny water droplets that float in the air. The end result is ground fog, as it hugs the ground, often no more than a couple of hundred feet thick. A second type of fog is called Valley fog. The factors needed to create this type of fog are slightly different than ground Fog. With ground fog, clear nights and light to calm wind is needed, but for valley fog cold surface air and a weak winter sun are needed. Due to the fact that fog often forms in low places because cooler air is heavier than warmer air, the fog flows downhill. Valley fog can build to a height of more than 1,500 feet and is the type that can last for days, until winds are strong enough to scour out the cold air.

Advection fog forms in a completely different way to Radiation and Valley fog. The factors needed to create this fog, are a bit of a horizontal wind, warm, humid air and winter temperatures. This fog forms as the wind pushes warm, humid air over the colder ground or water, and as the air cools enough that the air temperature and dew point temperature are the same, fog begins to form. As an aside, the dew point temperature is the temperature the air needs to be cooled to, in order to achieve a relative humidity of 100%. The effects of advection fog can cover wide areas of the U.S. in the winter and this type of fog may be enough to close airports. Upslope fog is opposite to valley fog, as instead of the fog forming at the base of a mountain and in a valley, this type of fog is associated with winds blowing up hills or mountains. As humid air is pushed up hills and mountains, it cools to its dew point temperatures and fog forms, which then drifts up the mountain. This type of fog is, as you would expect, common and widespread around the Rockies and Allegany Mountains.

The last two types of fog are not as common as the above mentioned ones, but still they do occur and are called Steam Fog/Sea Smoke and Precipitation fog. Once again Dew Point temperature is a key component in helping these fog form. When it comes to steam fog, cold air blows over warmer water, where the water evaporates into the cold air,, increasing the humidity to the dew point temp. This type of fog forms on fall days over ponds and streams. Precipitation fog is associated with warm rain and cooler air surrounding it. When this fog happens, rain evaporates and the added water vapor in the air increases the dew point, and then the vapor condenses into fog. Basically this type of fog forms on cool, rainy days and nights.

Justin Sanner, of San Francisco, rides through heavy fog that engulfs Twin Peaks in San Francisco, Monday, July 10, 2006. (AP Photo/Benjamin Sklar)

So who is most prone to seeing fog on a yearly basis? The distribution of fog is greatest along the Pacific Coast, the Appalachian highland region and New England, with the reasons based on topography and the general microclimate that occurs in these locations. The foggiest spot in the U.S. is Cape Disappointment, Washington, with the state of Washington being the most overcast state in the Union. On average it sees 165 foggy days a year, compared to D.C. which sees and average of 130 days a year. Some other places where fog is a way of life are San Francisco and Point Reyes in California, and Mistake Island, Maine. The Atlantic Coast’s fog capital is Maine’s Moose Peak Lighthouse on Mistake Island, where foggy conditions are reported nearly 1,600 hours annually, thanks to the chilly Atlantic Ocean. When it comes to San Francisco no place on Earth is more associated with fog. Who hasn’t conjured up the image of the famous Golden Gate Bridge peeking out from the clouds at the very mention of the city? During the winter months, San Francisco commonly gets enveloped by radiation fog which develops in humid conditions, calm winds, and cooling of air temperatures especially during the longer, winter night. When it comes to the other foggy city in California, Point Reyes is tagged with two labels of notoriety: not only is it known as being the second foggiest place on the North American continent, it is also the windiest place on the Pacific Coast, with occasional speeds clocked at hurricane force levels!

While other weather related systems get far more headlines, such as tornadoes and hurricanes, fog has also played its role in accidents and unfortunately deaths, as well. With so many different types of fog seen throughout the year, and as harmless as fog may seem, one always has to be careful of your location and time of day. According to the Federal highway Administration, each year over 38,700 vehicle crashes occur in fog, with over 600 people killed and more than 16,300 people injured in these crashes annually.