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fog and tree

It’s fog season

Published on
24 November 2012

fog and tree“The fog comes on little cat feet” wrote Illinois poet Carl Sandburg in 1916 as he watched the fog roll in at the Lake Michigan shore in Chicago. The next line, “It sits looking over harbor and city on silent haunches and then moves on” describes the fog as it settles in for a while before retreating again.
When I first read those words in grade school, I admit that they didn’t make much of an impression. The cats I knew were like my pet Tuppence who thumped when he walked. Did fog thump? As a kid, I really didn’t know fog.
Reading the poem today, I actually experience a slight chill, for as an adult I have experienced the beauty and mystery of fog in its many forms.
For example, there is the winter fog that coats the tree branches with a thin film of ice, creating what is known as a hoar frost. This fog is a type of evaporation fog which results when cold air passes over water or warm, moist land. Evaporation increases the amount of moisture in the cool air, possibly to the point that the air can no longer hold all of the water it contains. And that is when the fog arrives.
Several years ago on a camping trip, I saw the fog roll in off the Pacific Ocean at Point Reyes California (one of the foggiest places in the world), and then watched it return to sea. This is an example of advection fog where wind blows warm moist air over a cool surface – in this case, the ocean.
Fog is essentially a cloud at or near the ground. Fog occurs when the air temperature reaches its dew point – meaning the point at which the air is nearly 100 percent saturated. Fog generally does not occur under windy conditions – the air will be calm, and any wind will be light.
In this area, the most common type of fog is known as radiation fog. On clear, calm nights, fog will form as the ground surface cools rapidly, dropping the air temperature to its dew point. This type of fog is most common in the fall and winter, and if the air is still, can last all night long, disappearing as the sun rises and the air warms.
For all its beauty, fog can be hazardous. In 1945, a B-52 airplane actually crashed into the Empire State Building in New York because of heavy fog and in 1977 two airplanes crashed on a runway in Tenerife (Canary Islands) because of limited visibility due to fog.
Fog has been blamed for fatal car accidents across the planet, in places as diverse as California, Dubai, and Tennessee. The fog not only reduces visibility, it also reduces contrast, muting everything to shades of grey, which in turn reduces one’s ability to judge distance and speed.
Fall and winter are the peak fog seasons in the Chicago metro area, including McHenry County. So, when driving in the fog, slow down, turn on the low beams, don’t pass other cars, and do not talk on the phone. It may come in on little cat feet, but don’t be fooled, that fog can be a killer!