fog may exist as super-cooled liquid droplets. This
situation may produce delicate needle or platelike ice
crystals on exposed surfaces, known as hoarfrost. Rime
ice, a smooth, milky, white ice coating, or glaze ice, a
smooth coating of clear ice, may also be produced by
fog when the temperature is below freezing.
Fog is sometimes identified by the physical process
by which it forms. Examples are radiation fog, formed
by radiational cooling; advection fog, formed by moist
air moving over a cooler surface; steam fog, formed
when cold air moves over a warm body of water;
upslope fog, caused by air cooling as it rises up a hill or
mountain; and frontal fog, formed by the evaporation of
rain in a colder air mass. You will study these later in
preparation for AG2. To observe the presence of fog,
you need not know how fog forms-only if it is present.
The terms used to record fog that you, as the observer,
must be familiar with are as follows:
FogThe vertical depth of fog is greater than 20
feet and the prevailing visibility is reduced to less than
5/8 mile (1,000 meters OCONUS).
MistA fog condition that reduces prevailing
visibility to between 5/8 mile (1,000 meters) and 6 miles
(9,000 meters). The vertical depth of mist is greater than
Ground fogThis term applies to fog that has
little vertical extent, i.e., normally greater than 6 feet but
less than 20 feet. This is a local phenomenon, usually
formed by radiational cooling of the air. Ground fog
can further be described as shallow, partial, or patchy.
Shallow fogThis descriptor of fog applies to
ground fog that covers the station and visibility at eye
level is 7 miles or more, but the apparent visibility in the
fog layer is still less than 5/8 mile. Shallow fog does not
extend above 6 feet.
Partial fogThis descriptor of fog applies to
ground fog that covers a substantial part of the station
and visibility in the fog is less than 5/8 mile, and
visibility over the uncovered parts of the station is 5/8
mile or more. The vertical extent ofpartial fog is greater
than 6 feet but less than 20 feet. This type of ground fog
may be coded even when the prevailing visibility is 7
miles or more.
Patchy fogThis descriptor of fog applies to
ground fog that covers portions of the station, the
apparent visibilty in the fog patch or bank is less than
5/8 mile, and visibility over the uncovered portions of
the station is 5/8 mile or greater. The vertical extent of
patchy fog is greater than 6 feet but less than 20 feet.
This type of ground fog may be coded even when the
prevailing visibility is 7 miles or greater.
DEW.Dew is moisture that condenses directly on
surfaces. Dew will form during the evening or late at
night, usually when the winds are light. After the sun
sets, the ground and objects near the ground cool by
radiational cooling; they radiate heat energy as
infrared radiation. When the ground or objects cool
to the dew-point temperature, water vapor condenses
out of the air onto the objects surface. White dew is
dew that has frozen after the water condenses. It is
recognizable as small beads or a beaded layer of clear
ice, or sometimes milky-colored ice on surfaces.
FROST.Frost is a layer of milky white ice
crystals that sublime directly on the ground or on other
surfaces. The crystals are commonly in the shape of
needles, scales, feathers, or fans. Frost forms when
radiational cooling lowers the temperature of objects
below the freezing level. Since many objects cool faster
than the air surrounding the objects, frost may form with
ambient air temperatures above freezing, as high as
37°F. For frost to form, the object must be cooled to the
frost-point temperature, which is also referred to the
"dew-point temperature with respect to ice." This is the
dew-point temperature calculated on the "low
temperature" side of the Psychrometric Computer.
Thicker deposits of needle or platelike frost, up to
several inches thick, form in fog with ambient air
temperatures below freezing. This form of frost is
known as hoarfrost.
What type of lithometeor produces a yellow or
orange tinge when viewed against a brighter
Where are dust/sand whirls most likely to
What does the term heavy sandstorm mean?
What three factors are necessary for cloud
Fog may form when the temperature-dewpoint
spread is how many Celsius degrees?
Fog formed by moist air moving over a cooler
surface is known by what term?
Define the term "mist."
Explain the formation of frost.