Fog - The Nuisance That Can Turn Deadly
Fog may not be the most dramatic of weather's hazards, but it can create serious problems by causing airport closures and road pile ups.
Fog is basically a cloud in contact with the ground. By definition it exists when visibility is less than 1000m (O.6 miles) and is classed as thick when visibility is reduced to less than 100m or 220 yards, depending on whose system you use.
Fog can form in many ways, and almost all relate to temperature differences between air and whatever it is in contact with - land, water, ice or snow.
Like cloud, fog will form when, things like air pressure andwater content being equal, the temperature drops below thedewpoint. Effectively, at this temperature the air is saturatedwith water vapour - and vapour condenses to water droplets.
Please don't ask me to explain the dewpoint, or dewpoint temperature. Every so often I feel I have it nailed, butif I take my eye off it understanding slips away.
Fortunately the dewpoint doesn't matter too much when airtemperature is falling reasonably rapidly, because oncecomplete saturation is reached it will stay that way untilthe temperature starts to rise (all other things beingequal)
So, to recap, fog will form when the temperature of the airfalls to a point when water vapour must begin to condenseand form the water droplets that fog is composed of.
Fog comes in many forms, and the two most important areradiation fog and advection fog. Both have the powerto disrupt transport over large areas for considerablelengths of time.
Valley Fog, a form of Radiation Fog
Source; NOAA Photo Gallery
Radiation refers to the nighttime cooling of land by
radiation of heat into the atmosphere. Calm conditions
and a clear sky are necessary, while slow movement
of cool air (air drainage) into the lowest areas, such
as valley bottoms can result in local fogs.
As the ground cools it chills the air in contact with it,
causing condensation and the formation of fog. As time
passes and the fog thickens, the zone of maximum cooling
of the air moves to the top of the fog bank, and
considerable thicknesses of fog can build up over large
Fogs also tend to be thicker on calm clear nights after
rain, where air in contact with the ground can contain
more water vapour than usual. The combination of all
these factors leads to the notorious Tule fogs of the
San Joaquin Valley in California, with their
spectacular vehicle pile ups on bad days.
Radiation fog usually dissipates quite quickly in the
morning as the atmosphere warms up and air begins
to move around. Tule fogs can last all day.
Golden Gate Bridge disappearing into
sea (advection) fog. Source; NOAA
Advection is meteorologist's talk for mostly horizontal
air movement or wind, and advection fog forms when warmer
moist air moves over a cooler surface. The air cools and
water vapour condenses near the contact with land or
water. The wind continually feeds in more moist air and
thick, extensive moving fog banks can form.
Advection fogs commonly form over the sea, and can
disrupt air and ground traffic if they move in over land.
They can also form over cold land during the passage of
a warm front, particularly if the land surface is covered
Freezing fog occurs when the air is cold and the ground
is colder. The water droplets may be supercooled - below
freezing point but lacking tiny particles to provide
nuclei for the droplets to freeze around.
When they come in contact with cold ground, structures
or vegetation they freeze instantly, forming coatings
of frost-like ice called rime which can extend well
above ground level.
Freezing fog can also form an icy glaze on paths and roads,
similar to freezing rain. If only fog is involved, this
is unlikely to cause problems. But often drizzle
accompanies fog, and the glaze can thicken to levels
approaching those of an
creating driving problems.
Freezing fog can also cause major agricultural losses
if it occurs at critical times in the growing season.
Other Forms of Fog
Fog forms in several other ways, but these rarely create
dangerous conditions or economic loss. These include
Avoiding Fog Hazards
Most fog related problems occur on roads, and major
pile ups often occur when fast moving traffic enters
thick fog. Common sense is your best defence, along with
a fair bit of luck.
Slowing down as quickly as possible, turning on your lights,
and keeping as much distance between yourself and the vehicles
in front will help. Praying that the vehicles behind you are
doing the same thing may or may not be as effective. Here's a graphic
account of one man's account of a
Fog lights will help a while driving in fog - at
the very least you will be more visible to other vehicles.
Valley fog below, strato-cumulus above
Mt Washington. Source; NOAA Photo Gallery
Data from home weather stations can be very interesting
during times of fog, particularly if your area experiences
different types. The interaction of wind speed, temperature,
outside humidity and particularly dewpoint is worth
keeping an eye on.
page has links to a number ofarticles on other forms of hazardous and dangerous weather.
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Last update 05/24/2011