December 24, 2009
Fog – Not a Dreary Subject
By John Rinier
What is Fog?
Simply defined, it is a cloud near the ground or Earth’s surface. But you say, yeah I know what fog is, it is what I see almost every morning (in the winter) when I drive to work.
But do you?
Fog or a cloud is made of water vapor or more specifically tiny water droplets (sometimes ice particles) floating in the atmosphere. The fog/cloud is created when vapor condenses on microscopic nuclei (particles) to form droplets. When the vapor condenses, it changes from a vapor state into a liquid state. Fogs of all type originate when the temperature and dewpoint (wet-bulb temperature) of the air become nearly identical and seldom forms when the temperature versus dewpoint spread is greater than 4°F.
The National Weather Service defines fog as “water droplets suspended in the air at the Earth's surface” and is “hazardous when the visibility is reduced to ¼ mile or less”. More scientifically, fog is defined as “a hydrometeor consisting of a visible aggregate of minute water droplets (or ice crystals), suspended in the atmosphere near the Earth's surface”.
What is a hydrometeor? …and what happens when it hits Earth from outer space? Do not worry; “a hydrometeor is any particle of condensed water (liquid, snow, ice, graupel, hail) in the atmosphere”. These tiny droplets can be suspended in the air indefinitely, theoretically when conditions remain constant.
How is that different from a cloud? Basically, it differs only in that the base of the fog is at the Earth’s surface while clouds are above in the surface.
Yeah, duh!! Ok, tell me something I do not know!
Do you know that fog is formed by either cooling the air to just beyond its dewpoint or by adding moisture to the air, thus elevating the dewpoint?
Cooling or evaporation produces; advection fog, radiation fog or upslope fog
Adding or increasing moisture to air produces steam fog or frontal fog
Wow that was too much information about fog; can we talk about snow instead?