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Author Topic: Fog or mist?  (Read 8441 times)

Offline DoctorBeaver

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Fog or mist?
« on: 13/02/2008 22:17:04 »
What's the difference?  ???


 

paul.fr

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Fog or mist?
« Reply #1 on: 13/02/2008 22:29:41 »
The short, simple answer is that fog (suspended Water droplets in the atmosphere that affect visibility) reduces visibility to below 1km. And mist (an aggregate of microscopic water particles suspended in the air) has a visability greater than 1km.

Then you can get in to all sorts of stuff like diameter of the particle, relative humidity and the different types.

I did write something about fog a while back, in this topic:
What causes the formation of fog, and what makes it go away again?
« Last Edit: 13/02/2008 22:51:12 by paul.fr »
 

Offline DoctorBeaver

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Fog or mist?
« Reply #2 on: 14/02/2008 15:20:41 »
Paul - what is the difference between "suspended water droplets in the atmosphere" and "an aggregate of microscopic water particles suspended in the air"? Is it just that the droplets in mist are smaller?
 

paul.fr

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Fog or mist?
« Reply #3 on: 15/02/2008 20:44:51 »
I was trying to write a quick definition that we all could understand, but seeing as you asked for more information. Yes droplet size is a factor, and they are different for both, you also have mist to consider. In terms of particle size, they are in this order.
Fog
Mist
then Haze

This is really hard to write as there are so many factors to consider, and so many different types of fog, there are two types of mist which refract light differently...and so on... So with that in mind, the easiest way is for me to just put down the standard definitions...hope that is OK?
you may also want to check out this page:
kohler theory.

mist

A suspension in the air consisting of an aggregate of microscopic water droplets or wet hygroscopic particles (of diameter not less than 0.5 mm or 0.02 in.), reducing the visibility at the earth's surface to not less than 1km.
The term mist is used in weather reports when there is such obscurity and the associated visibility is 1000 m or more, and the corresponding relative humidity is 95% or more, but is generally lower than 100%. These hydrometeors form a thin greyish veil that covers the landscape. It also reduces visibility, but to a lesser extent than fog.


fog

Water droplets suspended in the atmosphere in the vicinity the earth's surface that affect visibility.
According to international definition, fog reduces visibility below 1km. Fog differs from cloud only in that the base of fog is at the earth's surface while clouds are above the surface. When composed of ice crystals, it is termed ice fog. Visibility reduction in fog depends on concentration of cloud condensation nuclei and the resulting distribution of droplet sizes. Patchy fog may also occur, particularly where air of different temperature and moisture content is interacting, which sometimes make these definitions difficult to apply in practice. Fogs of all types originate when the temperature and dewpoint of the air become identical (or nearly so). This may occur through cooling of the air to a little beyond its dewpoint (producing advection fog, radiation fog or upslope fog), or by adding moisture and thereby elevating the dewpoint (producing steam fog or frontal fog). Fog seldom forms when the dewpoint spread is greater than 4°F. According to U.S. weather observing practice, fog that hides less than 0.6 of the sky is called ground fog. If fog is so shallow that it is not an obstruction to vision at a height of 6 ft above the surface, it is called simply shallow fog. In aviation weather observations fog is encoded F, and ground fog GF. Fog is easily distinguished from haze by its higher relative humidity (near 100%, having physiologically appreciable dampness) and gray color. Haze does not contain activated droplets larger than the critical size according to Köhler theory. Mist may be considered an intermediate between fog and haze; its particles are smaller (a few μm maximum) in size, it has lower relative humidity than fog, and does not obstruct visibility to the same extent. There is no distinct line, however, between any of these categories. Near industrial areas, fog is often mixed with smoke, and this combination has been known as smog. However, fog droplets are usually absent in photochemical smog, which only contains unactivated haze droplets. 
 


haze

Particles suspended in air, reducing visibility by scattering light; often a mixture of aerosols and photochemical smog.
Many aerosols increase in size with increasing relative humidity due to deliquescence, drastically decreasing visibility. On Köhler curve plots of saturation relative humidity versus aerosol particle radius, equilibrium haze particles are to the left of the peak, while growing cloud droplets are to the right. Many haze formations are caused by the presence of an abundance of condensation nuclei which may grow in size, due to a variety of causes, and become mist, fog, or cloud. Distinction is sometimes drawn between dry haze and damp haze, largely on the basis of differences in optical effects produced by the smaller particles (dry haze) and larger particles (damp haze), which develop from slow condensation upon the hygroscopic haze particles. Dry haze particles, with diameters of the order of 0.1 μm, are small enough to scatter shorter wavelengths of light preferentially though not according to the inverse fourth-power law of Rayleigh scattering. Such haze particles produce a bluish color when the haze is viewed against a dark background, for dispersion allows only the slightly bluish scattered light to reach the eye. The same type of haze, when viewed against a light background, appears as a yellowish veil, for here the principal effect is the removal of the bluer components from the light originating in the distant light-colored background. Haze may be distinguished by this same effect from mist, which yields only a gray obscuration, since the particle sizes are too large to yield appreciable differential scattering of various wavelengths.
« Last Edit: 15/02/2008 22:23:27 by paul.fr »
 

Offline DoctorBeaver

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Fog or mist?
« Reply #4 on: 15/02/2008 22:14:14 »
Paul - thank you for that very comprehensive reply.

The reason I asked is that my friend calls fog what I call mist & vice versa. According to your reply, I use the correct terms.

 

Offline fishwhiz

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Fog or mist?
« Reply #5 on: 01/04/2008 03:43:35 »
could it been fog is suspended and mist is actually falling, albeit only slightly?
 

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Fog or mist?
« Reply #5 on: 01/04/2008 03:43:35 »

 

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