Commonly Used Weather Terminology Advisory Issued when hazardous weather or hydrologic conditions ex

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Commonly Used Weather TerminologyAdvisory -
Issued when hazardous weather or hydrologic
conditions exist, are imminent or are likely tc
occur. Advisories are issued for conditions that
may cause major inconvenience and could lead to
situations that may threaten life or property.
Anticyclone - An area of high pressure. Winds
blow in a clockwise direction in the Northern
Hemisphere and in a counter clockwise direction
in the Southern Hemisphere. Fair weather is
associated with an anticyclone. Backdoor Front -
A front that moves east to west in direction
rather than the normal west to east movement.
Backing Wind - A wind that changes its direction
in a counter clockwise motion. An example would
be a Northwest wind changing to a West wind.
Beaufort Wind Scale - A scale classifying wind
strength in terms of observable effects both on
the sea and over land. Copies of the Beaufort
Wind Scale for both land and water are included
with this terminology guide. Ceiling - The
height of the lowest layer of broken or overcast
cloud layer. Cold Front - The transition zone
where a colder air mass overtakes and replaces a
warmer air mass. Cut-Off Low - An upper level
low pressure system that is no longer in the
normal west to east upper air flow. Usually a
cut-off low will lie to the South of the
established upper air flow. Cyclone - An area of
low pressure. Winds blow in a counter clockwise
direction in the Northern Hemisphere and in a
clockwise direction in the Southern Hemisphere.
Inclimate weather is associated with a cyclone.
Degree Day - A measure of departure from the
mean daily temperature. One degree day occurs
when the daily mean temperature is above or below
65 degrees Fahrenheit. Dense Fog Advisory -
Issued when dense fog covers a widespread area
and frequently reduces visibility to 1/4 of a
mile or less. Dense fog usually results in
traveling problems and/or delays. Downburst - A
strong downward rush of air which produces a
blast of damaging wind on or close to the
surface. Downslope Flow - Air that descends an
elevated plain and consequently warms and dries.
Occurs when prevailing wind direction is from the
same direction as the elevated terrain and often
produces fair weather conditions. Fog - A cloud,
with its base on the surface, reducing
visibility. If visibility is frequently reduced
to 1/4 of a mile or less, the fog is termed dense
fog. Ground Fog - A shallow layer of fog (less
than 20 feet thick) on the ground that reduces
visibility more in the horizontal than in the
vertical. High Wind Warning - Issued when
sustained winds of 40 mph or greater are
occurring or expected to persist for an hour or
longer or if winds of 58 mph or greater are
expected for any duration.
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High Wind Watch - Issued when conditions are
favorable for the development of high winds over
all of or part of the forecast area but the
occurrence is still uncertain. The criteria of a
high wind watch are listed under the high wind
warning and should include the area affected, the
reason for the watch and the potential impact of
the winds. Inversion - A layer of the atmosphere
where the temperature increases with height.
Surface based inversions occur during long nights
when calm conditions and dry air exist. Isobar -
A line on a weather map that connects points of
equal pressure. On a weather map, isobars extend
around areas of high and low pressure. Jet
Stream - Relatively strong winds that are
concentrated in a narrow band in the atmosphere.
Jet Streams are usually thousands of kilometers
long, hundreds of kilometers wide but only a few
kilometers thick. They are usually found between
6 and 10 miles above the surface. Leeward Side -
The side of an object that is facing away from
the direction that the wind is blowing.
Lenticular Clouds - A cloud that generally has
the form of a smooth lens. They usually appear in
formation as the result of oragraphic origin.
Viewed from the ground, the clouds appear
stationary as the air rushes through them.
Macroburst - A large downdraft of air with an
outflow diameter of 2.5 miles or greater and
damaging winds lasting from 5 to 20 minutes. May
reach tornado intensity. Microburst - A small
downdraft of air with an outflow diameter of less
than 2.5 miles with the peak winds lasting from 2
to 5 minutes. May affect aircraft performance.
Occluded Front - A combination of two fronts
that forms when a cold front catches up to and
overtakes a warm front. Overrunning - A
condition that exists when an air mass moves up
and over a denser air mass on the surface. The
result is usually low clouds, fog and steady,
light precipitation. Relative Humidity - The
ratio of water vapor contained in the air
compared to the maximum amount of moisture that
the air can hold at that particular temperature
and pressure. Ridge - An elongated area of high
pressure. Stable Air - Air that is colder than
its surroundings and as such is resistant to
upward movement. Trough - An elongated area of
low pressure. Unstable Air - Air that is warmer
than its surroundings and as such tends to rise,
leading to the formation of clouds and possibly
precipitation. Upslope Flow - Air that gradually
rises along an elevated plain and consequently
cools to the dew point. Occurs when easterly
winds prevail and often lead to low clouds, fog
and generally steady and light precipitation.
Veering Wind - A wind that changes its direction
in a clockwise motion. An example would be a west
wind changing to a northwest wind. Virga -
Precipitation that falls from clouds but
evaporates in dry air beneath the cloud before
reaching the ground. Virga resembles streaks of
water extending from the cloud.
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Warm front - The transition zone where a warm air
mass overtakes and replaces a colder air mass.
Warning - Issued when hazardous weather or
hydrologic conditions exist, are imminent or are
highly likely to occur. Warnings are issued for
conditions that pose a threat to life or
property. Watch - Issued when hazardous weather
of hydrologic conditions are possible but its
occurrence, location and/or timing is still
uncertain. A watch is intended to provide lead
time so those who need to set plans in motion may
do so. Wind Advisory - Issued for the occurrence
of wind gusts between 40 and 57 mph for any
duration. Windward Side - The side of an object
that is facing into the direction that the wind
is coming from.