Oregon Chapter
American Meteorological Society

December 24, 2009

 

Fog – Not a Dreary Subject

By John Rinier
OR - AMS Treasurer

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

Advection Fog:
Warm moist air moves horizontally across colder water or air and the warm moist air cools enough to reach the dewpoint or condensation point where a fog cloud is formed.

Radiation Fog:
This fog is formed by the cooling of the land and adjacent air after sunset by thermal radiation in calm (or windless) conditions with a clear sky. Thermal radiation occurs when the sky is clear of clouds and stored thermal or ground energy is allowed to radiate back into space which produces condensation in the nearby air through the heat transfer process of heat conduction. In perfect calm conditions, the fog layer can be less than a meter deep but a slight turbulence can create a thicker layer. Radiation fogs occur in autumn and early winter, and during the night lasting not long after sunrise.

Upslope Fog:
The fog occurs when relatively humid air moves up a gradually sloping plane which causes the air to expand and adiabatically cool.  What is adiabatic cooling?  Adiabatic cooling is basically the cooling from the expansion of air (from low to high elevation).  This type of fog occurs very often in the mountains and can be seen when you drive over a mountain pass.

Adding or increasing moisture to air produces steam fog or frontal fog

Steam Fog:
When cool air moves over a warm water or moist land, it causes moisture to evaporate from the water surface and saturate the cool air.  The process produces a steaming appearance similar to a boiling pot of water.

Frontal Fog:
Frontal fog forms when frontal wedging or warm moist air is lifted over colder air during the movement of a low pressure system.  This is seen when a warm front moves over cold air.

Wow that was too much information about fog; can we talk about snow instead?

 

References:
American Meteorological Society Website 2009. Glossary of Meteorology
Lutgens, Frederick K. & Tarbuck, Edward J. 1989. The Atmosphere, An Introduction to Meteorology
Christian, Spencer & Biracree, Tom 1993.  Weather Book
Willams, Jack 1992.  The Weather Book, An Easy-to-Understand Guide to the USA’s Weather.