Definition
fog n. a cloudlike mass or layer of minute water droplets or ice crystals near the surface of the earth, appreciably reducing visibility.
-Webster's New Universal Unabridged Dictionary, 1994
Introduction
How does fog affect Fort Collins?
Fog is nothing more than a cloud whose base is right on the ground. Fog droplets are so small that they remain suspended in the air - that is, they don't fall out as rain. There are several ways in which fog can form.
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Types of Fog
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Radiation
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Radiation fog develops as a result of cooling at night. ("Radiation" is what meteorologists call the process in which heat energy is given off by the earth.) For radiation fog to develop, winds must be light so that a cold layer of air can form in a relative undisturbed layer near the ground. The humidity must also be high, so that the air can cool down to the dewpoint (the temperature at which water droplets precipitate out of moist air). You can see this effect in your home if it is a humid day. Take a glass of ice water and set it on the counter. Droplets will form on the side of the glass because it's cold...below the dewpoint.
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Advection
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Advection fog develops when warm, moist air streams into a colder region. The cooler surface chills the air to the dewpoint. Advection fogs occur frequently over cool bodies of water, such as along sea coasts, or in river valleys.
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Upslope
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Upslope fogs develop when a very humid air mass moves gradually up to higher terrain. One of the laws of chemistry states that when air pressure decreases, the temperature of the air cools. If the air is humid enough, the pressure decrease at higher elevations can lower the air temperature to the dewpoint.
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Evaporation
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Evaporation fogs develop when moisture from water or snow on the ground moistens the air to the dewpoint. This moistening of the air most commonly brings about fog in combination with nighttime cooling.
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In Fort Collins, our densest fogs occur in winter. They form through a combination of evaporation and nighttime cooling. Our most dangerous fog situation takes place on the day or two following a snowstorm, whenever the following conditions are met:
- a warm-up is taking place,
- winds are light, and
- nighttime skies are clear.
Here's how it works. Warm temperatures during the day begin evaporating the snow, and the air nearest the ground (50 to 100 feet, or so) becomes very humid. As night falls, the clear skies and light winds allow very rapid cooling of the air. A fog forms. This type of fog can be very patchy, depending on how much snow was evaporated in a particular area during the day. Sometimes the fog will be thicker near canals an creeks due to the extra moisture.
Our research on the subject of fog in the Fort Collins area has yet to reveal a major accident which was caused or contributed to by fog. However, anywhere you have the possibility of fog mixed with moderate to heavy traffic traveling at speeds above 40 miles per hour, you have the possibility of having one of those major traffic accidents involving many vehicles in a chain reaction collision. So maybe a few quick foggy weather driving tips are in order:
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Foggy Driving Tips
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Slow down!
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When you can't see past your hood ornament, it is extremely dangerous to be driving at normal speeds. With such poor visibility you have virtually no time to react if you are going too fast. Drive like you are passing through a school yard during recess and you will have the best chance of avoiding an accident.
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Keep right
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Slower traffic should always keep to the right. Unfortunately there is always someone who is driving too fast for the foggy conditions. Watch your mirrors for them and let them go by.
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If you turn on your high beams to try to see better, your headlamps will reflect off of the fog and back into your face, actually making it harder to see.
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Do not stop in the roadway
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If you feel that you must stop because the fog is too thick, pull well off of the right side of the roadway so that your parking lights don't confuse other drivers approaching from the rear. Do not stop or leave your vehicle on the roadway for any reason except in an emergency.
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If you have an accident
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If you have an accident and your vehicle is still driveable, do not stop in the roadway. Very cautiously, pull well off of the roadway to the right as described above. If your car is disabled, turn on the lights and emergency flashers and get away from the car. Very carefully move well off of the roadway and go for help. Do not stand around in the roadway next to your vehicle while waiting for help. Remember that other approaching traffic cannot see you in the fog. In major chain reaction accidents, most of the damage and injuries stem from secondary crashes caused when people and vehicles stopped in the traffic lane are struck by other traffic.
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