Written by Raymond Nakamura
Once upon a time, Raymond earned his doctorate studying the hydrodynamics of sand dollars. Nowadays, when he’s not employed as personal assistant to his lovely and demanding daughter, he enjoys creating fun and educational experiences in science and history using facts and fiction, words, pictures and whatever else is handy. Follow him on Twitter @raymondsbrain


fog occurs when the temperature equals the dew point

Created date

Tuesday, January 23, 2018 - 2:08pm

Ever Wonder About Fog?

On New Year’s Day, I found myself having trouble finding my way to my cousin’s house – not because of any residual effects from the night before, but because it was so foggy outside! I could hardly see the street signs on my drive over. We ended up arriving to our destination safely, but I continued to wonder about what causes fog.

Fog is made up of tiny little water droplets – kind of like a cloud, except closer to the ground. When the air temperature reaches the dew point - the temperature at which the air becomes 100% saturated with moisture - water condenses to form fog.

(A quick warning to those who dislike the word “moist”. I may use it a few times throughout this post.)

Vancouver has an average of 62 foggy days a year. This may sound like a lot but it’s nothing in comparison to Argentia, Newfoundland. Deemed the foggiest place in Canada, Argentia sees about 206 days of fog a year!

Fog tends to be more common in industrial areas because droplets tend to form around particles. Have you ever heard the saying “the fog is as thick as pea soup”? This saying comes from the brownish-yellow colour that results from water droplets condensing around microscopic particles of coal. Vancouver used to be a lot foggier because of all the air pollution!

There are many different types of fog. But here are a few of the kinds you are likely to come across:

  1. Radiation Fog

    Not to be confused with nuclear radiation, radiation fog refers to the ground radiating heat in the form of infrared radiation, also known as ground fog. This is more likely to occur after a sunny day, or on a calm, clear night. Because there isn’t any wind to mix the air or clouds to trap the heat, as the ground cools, the air above can become cooled below its dew point, resulting in fog suitable for the set of a horror movie. The temperature of bodies of water tend to remain more stable and do not usually cause this kind of fog.
     
  2. Valley Fog

    In valleys, cold dense air can drain down mountain slopes at night to collect along the valley floors and produce fog. A temperature inversion, with warm air passing above the valley can add to the stability of this kind of fog under calm conditions. In the Central Valley of California, valley fog is referred to as Tule fog – named after a marsh that grows in the local wetlands.

    During the day, the sun can usually clear out the fog by drying out the edges and proceeding inwards. During the winter, however, the sun is lower in the sky which sometimes can cause the fog to linger all day.
     
  3. Upslope Fog

    Upslope fog occurs when warm air rises up the slope of a mountain and cools off below its dew point.
     
  4. Advection Fog

    Advection fog occurs when warm, moist air – like air over the ocean – moves across a cool surface (i.e. a snow-covered ground). If the air is cooled past its dew point, you can get fog moving along or near the surface. Strong winds can cause enough turbulence to disperse the water droplets and dissipate the fog.
     
  5. Evaporation Fog

    When cold air moves over a warm body of water, then steam fog can form as the water vapour evaporates from the surface and becomes cooled. Colder air tends to sink, resulting in convection. You may see this kind of fog rising from a lake, at an outdoor hot tub, over an ocean (sometimes referred to as sea smoke), or when warmer raindrops fall into colder, drier climates.
     
  6. Freezing Fog

    When fog forms above a surface that exists in below freezing temperatures, the water droplets can freeze on contact with the surfaces. Sometimes the accumulation of hail at the ground can chill the surrounding air enough to produce fog. If it’s even colder, then instead of water droplets, the fog can consist of ice crystals. But that probably won’t happen in Vancouver, until a certain some-place-else freezes over.

I hope you’ve enjoyed learning all about fog with me! Don’t forget to share your questions, thoughts and experiences in the comment box below!

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