This time of the year we have many weather elements that can and do make driving on our roadways hazardous. We have snow, blowing snow, freezing rain, hail, frost, ice, and fog. How many times have you been driving along without a care in the world when you come upon an area of fog?
You turn your headlights on so you can see better and so your vehicle can be seen by oncoming traffic. Then suddenly you are blinded by the fog because the car coming at you has their high beams on. That is the first rule of driving in foggy conditions. Turn on your headlights, but low beam only. The high beam lights only light up the fog making it more difficult for you to see and for you to be seen.
You might remember being taught in school 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 description of the creation of fog. That is the dewpoint temperature. (That means the air is holding as much moisture as it can.) 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.
First we 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 particularly during the winter months, 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.
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].