What is Fog?
Posted by drdavescience on February 7, 2008
Fog can be thought of as a cloud that is on the ground. Clouds are usually very thick and almost impossible to see through, while fog can vary in thickness.
Did you know that Meteorologists (scientists that study the weather) officially define a day as foggy if the visibility is about 6/10 mile or less? If the visibility is greater than 6/10 mile but less than 1 1/4 miles, it is officially call mist. I don’t know who came up with those numbers but I know that fog poses a challenge to safe driving or flying.
Here is a really cool picture of a plane landing through the fog from www.airliners.net.
Recipe for fog
Fog typically occurs when the air is calm, cool, and full of water vapor.
Depending on where you live, and what season it is, your local TV weather person will give the temperature, dew point, and humidity. The temperature tells us how hot or cold the day will be. The dew point is the temperature the air needs to be cooled to form dew, fog, or clouds.
Both the dew point and humidity both tell us how much water vapor is in the air.
Key points
- If the temperature and dew point are within a few degrees of each other, then it means that the humidity is high and it is probably raining, snowing, or foggy.
- If the temperature and dew point are far apart, it means the humidity is low.
- Humidity is relative to the temperature. Cool air holds less water than warm air. This means that a hot, humid summer’s day holds more water vapor than a cold, humid winter’s day.
Putting it all together
Chicago recently experienced fog so thick that hundreds of flights were canceled into the area airports and there were many car accidents. The fog came on the heels of serious snowfall resulting in 10 inches of snow on the ground. A few days later the air began to warm slight above freezing and the snow to melt and evaporate. This filled the air with water vapor and, because it was not windy, the air was clam enough for fog to form.
In an earlier post on Hurricanes and Latent Heat, I discussed condensation, the process that causes water to form on the outside of a cold glass of soda on a hot and humid day. Did you know that fog and clouds form in a similar fashion? It’s true! Tiny particles of dust and pollution give water a surface to condense on to form visible moisture.
If you would like to read more about fog, this link to Wikipedia has a section that discusses it in greater detail.
-Dr. Dave
