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Farmers Almanac
The 2014 Farmers Almanac
Farmers' Almanac

Fog Facts

Have you ever looked out the window and found it hard to see the trees? It could be because the windows need a good cleaning. Or it could be a sudden appearance of fog.

What is fog?

  • Fog can be simply defined as a cloud touching the ground. Ever heard the expression, he’s walking on cloud nine? Well, if you’ve walked through fog, you’ve walked through a cloud.
  • Fog or ground clouds are made of millions of tiny droplets of water floating in the air.

How, when and where does fog most likely occur?

  • Fog generally forms when the relative humidity reaches 100% at ground level.
  • The ability of fog to form suddenly and disappear as quickly is determined by what side of the dewpoint the temperature registers.
  • Long, cool autumn nights cause the air near the ground to chill, causing the formation of fog to be prevalent in fall.
  • Fog is most likely to occur at night or near dawn when the temperature of the day is normally at it’s lowest.
  • The cool ground air forms fog and dew as the air-cools and water vapors condense into tiny droplets of water.
  • Fog is typically thicker in low places as the heavy air flows downward.
  • Fog can also form over cold, snow-covered ground as warmer air moves in.
  • Fog forms often near creeks, waterways and river valleys as the water increases the humidity in the air.
  • Fog is most likely to occur when the dewpoint level is very near the current temperature reading, being no more than 5 degrees F. in difference.
  • Fog evaporates after sunrise as the sun warms the fog from the top down. The thicker the fog, the longer it takes to dissipate.

Fog Trivia

  • The foggiest area in the United States is Point Reyes, California. It is in the top two foggiest land areas in the world with over 200 days of fog a year.
  • There are as many types of fog as there are cloud formations in the sky.
  • If you thought all fogs were created equal think again. There’s Radiation fog, Sea fog, Ground fog, Advection fog, Steam fog (also called evaporation fog), Precipitation fog, Upslope fog, Valley fog, Ice fog, Freezing fog and Artificial fog.

Driving in Fog

  • Fog reduces visibility outdoors greatly producing dangerous driving conditions.
  • To avoid driving in foggy conditions, check the local dewpoint level from your local weather center before driving or boating in the evenings or early morning hours. If it is within 5 degrees of the current temperature, fog is likely to form.
  • Be particularly cautious when driving in low land areas near water when fog is present or likely to occur.

If you notice a hole in the upper left-hand corner of your Farmers' Almanac, don't return it to the store! That hole isn't a defect; it's a part of history. Starting with the first edition of the Farmers' Almanac in 1818, readers used to nail holes into the corners to hang it up in their homes, barns, and outhouses (to provide both reading material and toilet paper). In 1910, the Almanac's publishers began pre-drilling holes in the corners to make it even easier for readers to keep all of that invaluable information (and paper) handy.