Fog: The Lost Member of the Cloud Family

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Clouds are undeniably cool. It may seem like the only way to get an up-close look at one is to hug the window seat on an airplane; but what if I told you there's a better way... one that doesn't even involve leaving the ground. Believe it or not, all you need to do is find a patch of fog.

Not All Clouds Live High In the Sky

Yes, fog — the same phenomenon that veils your vision in the wee morning hours — is essentially a cloud. There is, however, one stark difference between the two: clouds form several thousand feet above ground, whereas fog forms at or very near to the ground.

How does fog manage this unusual act? Well, while air that forms the clouds we see floating high up in the sky must rise several thousand feet from the surface before it reaches a level where it can cool and condense, air that develops into a fog cloud need only travel a short distance up because it is already very near to the point where it can no longer hold all the water vapor it contains (this point is called saturation or 100% humidity). That's right, the air temperature and dew point temperature (two temperatures that when equal, mean saturation) in the vicinity where fog forms are never more than a few degrees (about 4 °F (2.5 °C)) of each other.

Fog Formation

Like clouds, fog begins to form when water vapor condenses (changes to liquid form) into tiny liquid water droplets suspended in the air. There are generally two ways by which air condenses into a low-lying fog cloud: 1) through cooling, or 2) by the addition of enough water vapor to cause saturation. Whichever of these two processes fog forms by determines what kind of fog develops. (I bet you didn't know there were different kinds!)  

  • Radiation (ground) fog forms on clear, cool nights when humidity is fairly high. On such nights, the ground and the air immediately above it cool rapidly. Because the RH is high, just a small amount of cooling will lower the air temperature to the dewpoint and cause fog to form. (Radiation fog takes it's name from this process of heat radiating away from the ground back out into space.) Since the air containing the fog is relatively cold and dense, it sinks downslope, leading to valley fog.
  • Upslope fog is created when humid air is forced up a slope, such as the side of a mountain or any gradually sloping terrain. The rising motion itself cools the air, and once the dew point temperature is reached, a layer of fog forms.
  • forms when warm, humid air is advected (moved horizontally) over cold ground or water. As it does so, the air is cooled from below. Once sufficiently cooled, its moisture condenses to form fog. Advection fog is very common during winter and spring thaws and is typically a dense or thick fog.
  • Steam fog is a common sight over ponds, lakes, and rivers on crisp fall mornings when the air has turned chilly, but waters (which are slower to cool than air) are still relatively warm from yesterday afternoon's warm up. When this colder, drier air blows across the water's warm surface, some of the lake's moisture evaporates into it and slightly warms it from below. The warm water vapor rises up into the layer of cold air above, mixes with it, cools, and condenses into tiny fog droplets. The fog usually forms a few inches above the water's surface -- however far up the rising air must go to be cooled to the point of condensation. Eventually the body of water cools enough that it no longer evaporates as much moisture into the cool air above it, at which point fog formation ends.  
  • Frontal (precipitation) fog is actually caused by rain and snow falling into a layer of cold air below the precipitating cloud. As the rain's water droplets or snow's ice crystals fall into the drier air below, they evaporate into water vapor. (This phenomenon is commonly known as "virga.") The water vapor increases the moisture content of the air below and also cools it. Once enough precipitation has fallen into the air to saturate it, a wall of fog forms from the ground up through the clouds.

    In the winter, you may hear of two other types of fog, freezing fog and ice fog. Freezing fog works on a similar premise to freezing rain; the fog droplets are supercooled liquid droplets that freeze onto surfaces they come into contact with, covering them in rime ice. In contrast, ice fog refers to fog where the droplets have frozen into tiny ice crystals. As you can imagine, it takes some pretty cold temperatures to suspend ice in midair -- roughly -31 °F (-35 °C) or below to be exact! For this reason, ice fog is generally only seen near the Arctic and Antarctic regions.

    Reduced Visibility Ahead

    While fog is fascinating, it isn't without its hazards. Depending on the concentration of water droplets comprising it, fog can range anywhere from light to dense and can greatly impact visibility, reducing it to almost zero in some instances. This in turn can lead to travel delays, cancellations, and accidents, as fog makes it difficult for ships, trains, cars, and planes to see each other.

    Whenever driving in fog, it's always advised to slow your speed and use your low beam headlights. (While you may be tempted to use your high beams to cut through the fog, the light will only be reflected back into your eyes, further reducing your ability to see the road.)