|Fog is a cloud in contact with the ground. It occurs when moisture from the
surface of the Earth evaporates; as this evaporated moisture moves upward,
it cools and condenses into the familiar phenomenon of fog. Fog differs from
cloud only in that fog touches the surface of the Earth, while clouds do
not. All types of fog form when the relative humidity reaches 100% and the
air temperature drops below the dewpoint, pushing it lower by forcing the
water vapor to condense.
||Fog reduces visibility. Road vehicles have to travel slowly and use more
lights. Localised fog is especially dangerous, as drivers can be caught
Fog can occur at any time of year and in any location. Its presence can
be influenced by nearby water, the local topography, wind conditions at
the time, and even the activities of humans. The official visibility
distance in fog is less than 1km. If visibility is further than 1km, it
is not fog, but mist. Fog can appear just as suddenly as it can
disappear. When it appears suddenly like this, it is called ‘flash fog’.
There are a number of types of fog, named according to how the fog
forms. Radiation fog results from land cooling after sunset. Advection
fog or ground fog results from wind passing moist air over cool ground.
Precipitation fog or frontal fog is the result of precipitation falling
into the drier air below a cloud. Hail fog is sometimes seen after a
hail storm due to the increased moisture from the hail.
|Freezing fog is found when liquid fog freezes onto cold
surfaces, and frozen fog or ice fog is when very small droplets freeze
into midair crystal. Sea fog appears thanks to the spray and salt
crystals in the air. Sea smoke or steam fog is a localized fog created
as cold air passes over warmer water.
Fascinatingly, some plants and animals depend largely or entirely on fog
as their moisture source. Californian Redwoods receive 30–40% from the
fog coming in off the coast, while some animals and insect gather 100%
of their water from fogginess. There are even communities of people who
construct fog nets to gather the moisture in the air where other sources
of water are scarce.
Probably the most famous fog is that which descends on San Francisco
during its summer months. Amusingly, there is a Twitter account for the
San Francisco fog, which has been named Karl. His most active period is
in August (or ‘Fogust’).