It Comes On Little Cat Feet.

Dense Fog At PeaceHealth Sacred Heart Hospital River Bend | Photo By Tim Chuey

Carl Sandburg wrote about it. London is famous for it. Airline flights have been delayed and even cancelled because of it and if the surface temperature is at or below freezing it can cause very slippery road conditions. It should be obvious that I am talking about fog.

Carl Sandburg's Fog.jpg | image by
Carl Sandburg’s Fog.jpg | image by

Here is the short and sweet of Carl Sandburg’s famous poem. ” The fog comes on little cat feet. It sits looking over the harbor and city on silent haunches and then moves on.”

You might remember being taught way back in your general science class that fog is just a cloud that touches the ground. That is a gross understatement of the facts which are not quite that simple. It takes specific conditions of temperature and humidity to produce fog and there are actually many kinds of fog determined by exactly how it is formed. One thing you have to understand before we proceed is what it takes to make fog. To do that we need to define a meteorological term that is used in the creation of fog. That is the dewpoint temperature. The dewpoint temperature is the temperature to which air must be cooled at constant pressure for saturation to occur, followed by condensation.The relationship between the dewpoint temperature and the ambient air temperature is shown by the relative humidity expressed as a percentage. The closer the dewpoint temperature is to the ambient air temperature the higher the relative humidity and when they are the same number the relative humidity is 100%. Some of the varied types of fog are advection fog, radiation fog or ground fog, upslope fog or hill fog, steam fog or evaporation fog or sea smoke, precipitation fog, freezing fog, valley fog, and fog stratus. I’m sorry if I am taking away some of the mystique of fog but now is the time to explain the various kinds I have listed for you. Some of the difference are subtle, but I think it’s interesting to see how much something we take so much for granted has been studied and explained.

Yaquina Head Lighthouse In Fog | Photo By Tim Chuey
Yaquina Head Lighthouse In Fog | Photo By Tim Chuey

Well start with advection fog. It forms when moist air pushed by the wind passes over a cool surface and is cooled. Advection fog is most commonly found over the ocean as the moist air sweeps over the cooler water. Radiation fog, also called ground fog, is formed by the cooling of the ground after sunset by thermal radiation that rises when the wind is calm with clear skies. Upslope fog or hill fog is produced when winds blow air up a slope cooling it as it rises and causing the moisture to condense out. This can also cause freezing fog on mountain tops. Steam fog, evaporation fog, or sea smoke is created by cold air passing over warmer water or moist land. It often causes freezing fog or sometimes even hoar frost. Precipitation fog, also called frontal fog, forms as precipitation falls into drier air below the cloud and the liquid droplets evaporate into water vapor. The resultant water vapor cools forming fog. Freezing fog, something we have been familiar with lately, is fog that  has its droplets of moisture freeze to a surface the temperature of which is at or below freezing. Valley fog forms in mountain valleys often in winter. It is a radiation fog that is trapped in a relatively narrow space.

Dense Fog Stratus In South Hills | Photo By Tim Chuey
Dense Fog Stratus In South Hills | Photo By Tim Chuey

The last type of fog I have listed is fog stratus which is a layer of fog that acts like a cloud in that it can stay higher above the ground surface and then lower to the ground often causing very dense fog and even freezing fog if the surface (particularly streets, driveways and sidewalks) or objects like roofs are cooled to levels at or below freezing. There may be a few other kinds of fog, but they would be combinations of what I have already described or so close in definition that it wouldn’t be significant. I don’t know about you, but I still am in awe  of one of natures coolest creations, fog.

Let me know what you would like me to talk about or explain. You can comment below or email me at: [email protected].

Tim Chuey is a Member of the American Meteorological Society and the National Weather Association and has been Awarded Seals of Approval for television weathercasting from both organizations.

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