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glossary
Weather
Glossary
A
| B | C | D | E
| F | G | H | I
| J | K | L | M
| N | O | P |
Q | R | S | T |
U | V | W | X | Y | Z
A
ACID RAIN - Cloud
or rain droplets containing pollutants, such as oxides of sulfur and nitrogen,
to make them acidic.
ADIABATIC - changes in temperature
caused by the expansion (cooling) or compression (warming) of a body of air
as it rises or descends in the atmosphere.
ADNLY- Additionally
ADVECTION - The
horizontal transport of air or atmospheric properties. Commonly used with temperatures,
i.e., "warm air advection", or moisture, i.e., "moisture advection".
ADVISORY- Issued
for weather situations that cause significant inconveniences but do not meet
warning criteria and, if caution is not exercised, could lead to life-threatening
situations
AGL- above ground level.
AIR MASS - A large
body of air having similar horizontal temperature and moisture characteristics.
ALERT- Automated Local Event
Reporting in Real Time. Network of automatic raingauges that transmit via VHF
radio link when precipitation occurs. Some sites are also equipped with other
sensors such as temperature, wind, pressure, river stage or tide level.
ALBEDO -The percentage of
light reflected by an object.
ALTOCUMULUS (AC) -
Mid-altitude clouds with a cumuliform shape.
ALTOSTRATUS -Mid-altitude
clouds with a flat sheet-like shape.
AMPLIFICATION: (AMPLFCTN)-
Building, or sharpening, of an upper level high pressure ridge or low pressure
trough.
ANABATIC - wind flowing up
an incline, such as up a hillside; upslope wind.
ANEMOMETER- An instrument
that measures wind speed.
ANGULAR MOMENTUM - the energy
of motion of a spinning body or mass of air or water.
ANTICYCLONE- A large
area of high pressure around which the winds blow clockwise in the Northern
Hemisphere.
ARCTIC AIR - a mass of very
cold, dry air that usually originates over the Arctic Ocean north of Canada
and Alaska.
ARCTIC HIGH - a very cold
high pressure that originates over the Arctic Ocean.
ASOS -Automated Surface Observing
System. Observes sky conditions, temperature and dewpoint, wind direction and
speed, and barometric pressure.
ATMOSPHERE - the mass of air
surrounding the earth and bound to it more or less permanently by the earth's
gravitational attraction.
ATTM- AT The Moment
ATWC -Alaska Tsunami Warning
Center, located in Palmer, AK.
AVALANCHE - a large mass of
rapidly moving snow down a steep mountain slope.
AVHRR - Advanced Very High
Resolution Radiometer. Main sensor on U.S. polar orbiting satellites.
AVN - Aviation Model generated
every 12 hours by NCEP.
AWIPS - Advanced Weather Information
Processing System. New NWS computer system integrating graphics, satellite and
radar imagery.
B
BACKING WINDS - A counterclockwise
change in wind direction. Backing winds with height are indicative of cold air
advection (CAA).
BAROCLINICITY- A cold air
advection/warm air advection couplet that increases atmospheric instability.
On analysis and forecast charts it is the isotherms crossing the height contours.
BAROTROPIC- Homogeneous atmosphere
in which there are neither fronts nor any thermal advections.
BAROMETER An instrument
for measuring atmospheric pressure.
BEAUFORT SCALE - a scale that
indicates the wind speed using the effect wind has on certain familiar objects.
BLACKBODY- A mass which absorbs
and emits all wavelengths of radiation
BLACK ICE - thin, new ice
that forms on fresh water or dew covered surfaces; it is common on roadways
during the fall and early winter and appears "black" because of its
transparency.
BLIZZARD - A storm
lasting about 3 hours or longer with sustained winds 35 mph or greater with
frequent gusts to 35 mph or greater; and considerable falling and/or blowing
snow frequently reducing visibilities to less than 1/4 mile.
BLOCKING PATTERN- A situation
in which the upper tropospheric flow becomes highly meridional and stagnant.
Two common blocking patterns are the omaga block and the rex block.
BLOWING DUST OR SAND
- Small particles of dust or sand which are caused by strong winds blowing over
dry ground, that has little or no vegetation.
BLOWING SNOW - Wind-driven
snow which reduces surface visibility.
BOW ECHO - An accelerated
portion of a squall line of thunderstorms, taking on a radar indicated bow configuration,
created by strong downburst winds
BREEZY - Sustained winds of
15 to 25 mph.
BROKEN CLOUDS - Clouds which
cover between 6/10 and 9/10 of the sky.
C
CAA - Cold Air Advection.
The movement of colder air toward a fixed point on the earth's surface.
CALM - the absence of apparent
motion in the air.
CAP - Temperature inversion
which prevents convection from occurring.
CAT - A category. Usually
refers to a category of precipitation given by the forecast models.
CEILING - The height of the
lowest layer of clouds, when the sky is broken or overcast.
CELSIUS - a temperature scale
in which zero is the freezing point of water and one hundred is the boiling
point.
CHANCE (CHC) - A 30,
40 or 50 percent chance of occurrence of measurable precipitation.
CHINOOK A warm,
dry wind that blows along the east slopes of the northern Rocky Mountains.
CIRRIFORM - High altitude
ice clouds with a very thin wispy appearance.
CIRROCUMULUS - Cirrus clouds
with vertical development.
CIRROSTRATUS - Cirrus clouds
with a flat sheetlike appearance.
CIRRUS (CI) - High
clouds, usually above 18,000 feet, composed of ice crystals.
CLASSIC EVENT- Having all
the ingredients for a specific type of weather (i.e. classic severe weather
event, classic Nor'easter)
CLEAR - Sky condition of less
than 1/10 cloud coverage.
CLIMATE - The historical record
of average daily and seasonal weather events.
CLOSED LOW (CUT OFF
LOW) - A low pressure center having a closed circulation, which is
used in reference to systems in the upper levels of the atmosphere. Closed lows
that become cut off from the main flow pattern are called cut-off lows.
CLOUD CONDENSATION NUCLEI
- small particles in the air on which water vapor condenses and forms cloud
droplets.
CLOUDY- the state of the sky
when 9/10ths or more of the sky is covered by clouds.
COASTAL FLOODING - Inundation
of coastal areas from waves and storm surge.
COLD CORE HIGH- A dome of
cold surface high pressure that originates from the high latitudes.
COLD CORE LOW- A low pressure
which has the coldest temperatures located near its center. A cold core low
is deep and is often associated with the polar jet stream. Mid-latitude cyclones
are cold core lows.
COLD FRONT - The boundary
between a cold air mass that is advancing and a relatively warmer airmass. Generally
characterized by steady precipitation followed by showery precipitation.
CONDENSATION - The
process of gas changing to liquid. The process by which water vapor changes
into water droplets and clouds.
CONDENSATION PRESSURE DEFICIT
(COND PRES DEF)- On an isentropic chart (a layer of constant potential temperature),
condensation pressure deficit represents the amount of lift, expressed in millibars,
needed to saturate an air parcel.
CONDUCTION - the transfer
of heat between bodies that are in contact.
CONFLUENCE- A 2-D process
in which airstreams move toward each other at some angle.
CONQ- Converging Q vectors.
Indicates air will rises due to low level warm air advection and/or upper level
divergence
CONSIDERABLE CLOUDINESS -
Between 7/10 and 8/10 cloud cover.
CONTINENTAL AIR MASS - A dry
air mass originating over a large land area.
CONVECTION (CNVTN) - the transfer
of heat within a gas or liquid by their movement.
CONVECTIVE INSTABILITY-
Instability caused by very dry air advecting in the mid-levels of the troposphere
over a warm and moist lower troposphere. Dynamic lifting causes the mid-levels
to cool at a greater rate than the lower troposphere.
CONVERGENCE- A 1-D process
in which higher momentum air moves into lower momentum air.
CORIOLIS FORCE -
An apparent force caused by the rotation of the earth. In the Northern Hemisphere
winds are deflected to the right, and in the Southern Hemisphere to the left.
CORONA - a disk of light surrounding
the sun or moon; this is a result of the diffraction of light by small water
droplets.
COUNTY WARNING AREA (CWA)
- The area of responsibility that each National Weather Service office
has for its warnings and forecasts.
CUMULONIMBUS - A
vertically developed cumulus cloud, often capped by an anvil shaped cloud. Also
called a thunderstorm cloud, it is frequently accompanied by heavy showers,
lightning, thunder, and sometimes hail or gusty winds.
CUMULUS CLOUD (CU)
- A cloud in the shape of individual detached domes, with a flat base and a
bulging upper portion resembling cauliflower.
CWA- County Warning Area
CYCLONE - An area
of low pressure around which winds blow counterclockwise in the Northern Hemisphere.
Also the term used for a hurricane in the Indian Ocean and in the Western Pacific
Ocean.
D
DAMBREAK FLOODING - Dambreaks
present a unique flood warning problem. Dams may erode and empty relatively
slowly. In these situations, when river forecasts are available, flood warnings
and flood statements are used. Dams may also fail catastrophically during a
heavy rainfall event or earthquake, resulting in a very dangerous flash flood-type
situation. For these events, Flash Flood watches, warnings and statements will
be used. Similar action may be required for the failure of certain levees.
DEFORMATION ZONE (DFRMTN ZN)
- An area in the atmosphere where winds converge along one axis and diverge
along another. Deformation zones (or axis of deformation as they are sometimes
referred to) can produce clouds and precipitation.
DENSE FOG - A cloud,
with its base on the surface, which reduces visibility to 1/4 of a mile or less.
DEW - Moisture that has condensed
on objects near the ground, whose temperatures have fallen below the dewpoint
temperature.
DEWPOINT - The temperature
to which the air must be cooled for water vapor to condense.
DEWPOINT DEPRESSION (TDD)-
The positive numerical distance between the temperature and the dewpoint.
DIFFERENTIAL ADVECTION- The
increasing or decreasing of advection with height. Vorticity advection increasing
with height is favorable for uplift as well as warm air advection increasing
when moving from aloft to the surface.
DIFFLUENCE- A 2-D process
in which airstreams spread apart.
DIGGING- Deepening, becoming
more amplified with time. Often in reference to a trough deepening and building
to the South.
DIRTY HIGH OR RIDGE - High
pressure usually brings clear weather. A high pressure is termed "dirty"
if it has a canopy of clouds associated with it. These clouds can form due to
orography, a saturated PBL (with low level fog and/or stratus building under
a middle level inversion), or lifting mechanisms within the region of high pressure
(WAA, moisture advection).
DISTURBANCE - a disruption
of the atmosphere that usually refers to a low pressure area, cool air and inclement
weather.
DIURNAL EFFECTS- A reference
to an effect that has its origins due to daytime heating, such as afternoon
cumulus cloud development or the formation of a lake/sea breeze. These phenomena
dissipate once the sun goes down and surface heating is lost.
DIVERGENCE - refers to the
spreading out of winds.
DIVQ- Diverging Q vectors.
Indicates air will sink due to low level cold air advection and/or upper level
convergence
DLAD- Delayed
DPNG - Deepening. Usually
in reference to a low pressure intensifying.
DOPPLER RADAR -
A type of weather radar that determines whether atmospheric motion is toward
or away from the radar. It uses the Doppler effect to measure the velocity of
particles suspended in the atmosphere.
DOWNBURST - A strong
downdraft of air which induces an outburst of damaging winds on or near the
ground.
DRIFTING SNOW -
An uneven distribution of snowfall/snow depth caused by strong surface winds,
but does not reduce surface visibility.
DRIZZLE - Fairly
uniform precipitation composed exclusively of fine drops very close together.
Drizzle appears to float while following air currents; however unlike fog droplets,
it falls to the ground. Drizzle drops are too small to disturb appreciably still
water puddles.
DRY SLOT- Refers to a influx
of dry air (especially toward a mid-latitude cyclone)
DYNAMICS- Refers to upward
forcing caused by jet streak divergence or PDVA
DUST DEVIL- A small,
rapidly rotating wind that is made visible by the dust, dirt, or debris it picks
up. Also called a whirlwind, it develops best on clear, dry, hot afternoons.
DUST STORM - an area where
high surface winds have picked up loose dust, reducing visibility to less than
one-half mile.
E
ECMWF - European Centre for
Meteorology Forecast model.
EL NINO- A major
warming of the equatorial waters in the Pacific Ocean. El Nino events usually
occur every 3 to 7 years, and are characterized by shifts in "normal" weather
patterns.
ENSO - El Nino-Southern Oscillation.
ENTRAINED / ENTRAINMENT- Refers
to the drawing in of moisture (or lack of moisture) into a system. Dry air entrainment
into the mid levels of a thunderstorm can enhance the potential for damaging
wind gusts. Moisture being entrained into a storm system can enhance precipitation
amounts.
EQUIVALENT POTENTIAL TEMPERATURE-
The temperature of a parcel of air after all moisture and latent heat is condensed
out of an air parcel then descended to the 1000 mb level. Also known as Theta-E
ETA- "Eta" (from
Greek) model generated every 12 hours by NCEP
EVAPORATION - the process
of a liquid changing into a vapor or gas.
EXTENDED OUTLOOK - a basic
forecast of general weather conditions three to five days in the future.
EXTRAPOLATE- Determining the
value of a meteorological value beyond the bounds of a data sample.
F
FA - Forecast Area.
FAA- the Federal Aviation
Administration.
FAHRENHEIT - the standard
scale used to measure temperature in the United States; in which the freezing
point of water is thirty-two degrees and the boiling point is two hundred and
twelve degrees.
FAIR - Less than 4/10 opaque
cloud cover, no precipitation, and no extremes in temperature, visibility or
winds.
FCST- Forecast
FIRE UP- A favorite phrase
of forecasters. It means storms are developing or will develop.
FLASH FLOOD - A flood that
occurs within a few hours (usually less than six) of heavy or excessive rainfall,
dam or levee failure.
FLOOD - High flow, overflow
or inundation of a normally dry area which causes or threatens damage.
FLOOD STAGE- The
level of a river or stream at which considerable inundation of surrounding areas
will occur.
FLY IN THE OINTMENT- A favorite
phrase of some forecasters. Refers to a forecast problem or a potential forecast
problem. Could also be in reference to a forecasting problem that caused a "busted"
forecast.
FOEHN - A warm dry wind on
the lee side of a mountain range. The heating and drying are due to adiabatic
compression as the wind descend downslope.
FOG - Water droplets
which are suspended in the air near the Earth's surface and causing reduced
visibilities.
FORECAST PERIODS
- Routine forecasts issued before noon contain three forecast periods: Today
(or this Afternoon), Tonight, and the Next Day. Routine forecasts issued after
noon contain four forecast periods: Tonight, the Next Day, the Next Night, and
the following Day.
FREEZE - When the
temperature at or near the surface is expected to be 32 or below, during the
growing season. Adjectives, such as "killing,' "severe," or "hard," are used
when appropriate. A freeze may or may not be accompanied by the formation of
frost.
FREEZING LEVEL The
altitude in the atmosphere where the temperature equals 32F.
FREEZING RAIN (DRIZZLE) -
Rain (drizzle) which freezes on contact to produce a coating of ice on the ground
and other exposed surfaces.
FRONT - The transition zone
between two distinct airmasses. The basic frontal types are cold fronts, warm
fronts and occluded fronts.
FROPA- Frontal Passage
FROST - The formation
of thin ice crystals on the ground or other surfaces in the form of scales,
needles, feathers, or fans. Frost develops under conditions similar to dew,
except the temperatures are colder.
FUJITA SCALE - System developed
by Dr. Theodore Fujita to classify tornadoes based on wind damage. Scale is
from F0 for weakest to F5 for strongest tornadoes.
FUNNEL CLOUD - A
rotating column of air forming a pendant from a cumulus/cumulonimbus cloud with
circulation not reaching the ground.
FWC- Forecasted Weather Conditions
G
GALE - Wind speeds from 39
to 54 mph (34 to 47 knots).
GEOSTATIONARY SATELLITE
A satellite that rotates at the same rate as the earth, thus remaining over
the same spot above the equator.
GOES Geostationary
Operational Environmental Satellite.
GLAZE - a layer or coating
of ice that is generally smooth and clear, and forms on exposed objects by the
freezing of liquid raindrops.
GRADIENT - the time rate or
spatial rate of change of an atmospheric property.
GRAVITY- The attraction of
two masses to one another. Large masses have higher values of gravitational
accelerations than lighter masses.
GREENHOUSE EFFECT
The warming of the atmosphere by the trapping of earth's longwave radiation
being radiated to space. The gases most responsible for this effect are water
vapor and carbon dioxide.
GROUND FOG - Fog produced
over the land by the cooling of the lower atmosphere as it comes in contact
with the ground. Also known as radiation fog, and in parts of California as
tule fog.
GROWING SEASON - the period
of time between the last killing frost of spring and the first killing frost
of autumn
GUST - A brief sudden increase
in wind speed. Generally the duration is less than 20 seconds and the fluctuation
greater than 10 mph.
GUST FRONT - The leading edge
of the downdraft from a thunderstorm.
H
HAIL - Precipitation
of small balls or other pieces of ice (hailstones) falling separately or frozen
together in irregular lumps. (Typically associated with thunderstorms and surface
temperatures above freezing).
HALOS - Rings or
arcs that seem to encircle the sun or moon. They are caused by the refraction
of light through the ice crystals in cirrus clouds.
HARD FREEZE - freeze where
vegetation is killed and the ground surface is frozen solid.
HAZE- Fine dry or
wet dust or salt particles in the air that reduce visibility.
HEAVY SNOW - In
the Inland Northwest, mountains above 3000 feet - 8 to 12 inches in 12 hours
or 12-18 inches or more in 24 hours. For the valleys and Basin below 3000 feet
- 4 inches in 12 hours and 6 inches in 24 hours.
HEAT INDEX - The
HI is the temperature the body feels when the heat and humidity are combined.
HEAT LOW - The thermal
induced surface low pressure trough that develops during the warm season in
the lee of the Cascades.
HELICITY- Streamwise vorticity
available for ingestion into a thunderstorm. Higher values are favorable for
a rotating updraft (greater than 400). A measure of low level wind shear, normally
within the lowest 3 km of the atmosphere, relative to the movement of a thunderstorm
(thus referred to as 0-3 km Storm Relative Helicity). This gives forecasters
an indication of an environment that is favorable for supporting the development
of thunderstorms with rotating updrafts, a precursor to supercell thunderstorms
(the most violent of severe storms) and tornado development. Values of helicity
greater than +150 are considered significant; however, like CAPE values, there
is no magic value of (positive) helicity under which rotating thunderstorms
will not develop. Helicity is only an index to determining thunderstorm rotation
potential.
HIC - Hydrologist In Charge.
HIGH - The center of an area
of high pressure, usually accompanied by anticyclonic and outward wind flow.
Also known as an anticyclone.
HIGH WINDS - Winds
of 40 mph or greater lasting for an hour or more , and/or gusts to 58 mph or
greater.
HORIZONTAL VORTICITY- A rotation
of air caused by vertical speed or directional wind shear.
HORSE LATITUDES - subtropical
regions where anticyclones produce settled weather.
HUMIDITY -The amount of water
vapor in the atmosphere. (See relative humidity).
HURRICANE - A severe tropical
cyclone with wind speeds in excess of 74 mph (64 knots).
HYDROLOGIC CYCLE - the composite
picture of the interchange of water substance between the earth, the atmosphere
and the seas which includes the change of state and vertical and horizontal
transport.
HYGROMETER - an instrument
used to measure humidity.
I
ICE JAM - an accumulation
of broken river ice caught in a narrow channel that frequently produces local
floods during a spring break-up.
ICE STORM - liquid rain falling
and freezing on contact with cold objects creating ice build-ups of 1/4th inch
or more that can cause severe damage.
INFLOW-
Wind speed, in knots, of the average PBL windspeed.
INDIAN SUMMER -
An unseasonably warm period near the middle of autumn, usually following a substantial
period of cool weather.
INVERSION - An increase
in temperature with height. The reverse of the normal cooling with height in
the atmosphere.
INTERPOLATION- Drawing an
isopleth between known numerical values.
INVERTED TROF- This is a trough
which bulges to the north. Mid-latitude troughs have a north to south amplitude
but in the tropics the opposite is the case. Inverted trough look like ridges
but there is lower pressure at their centers of curvature. Inverted troughs
can occur across areas of the tropics and sub-tropics.
INVOF- In the Vicinity OF
IRIDESCENCE - brilliant patches
of green or pink sometimes seen near the edges of high- or medium-level clouds.
ISALLOBAR- A line of equal
surface pressure change.
ISENTROPIC LIFT/DECENT- Lifting
or sinking of air along constant potential temperature (theta) surfaces. WAA
(especially over shallow frontal boundaries) leads to lift while CAA leads to
decent.
ISOBAR - A line of equal barometric
pressure on a weather map.
ISODROSOTHERM- A line of constant
dewpoint temperature.
ISOHYET- A line of equal rainfall.
ISOLATED - Showers covering
less than 15 percent of an area.
ISOPLETH- A line of a constant
meteorological value.
ISOTACH- A line of constant
wind speed.
ISOTHERM - the line of equal
temperature denoted on surface weather maps.
ITCZ - Inter-tropical Convergence
Zone. The region where the northeasterly and southeasterly tradewinds converge,
forming an often continuous band of clouds or thunderstorms near the equator.
J
JET STREAK- A relative maximum
of windflow within the jet stream.
JET STREAM - Strong
winds concentrated within a narrow band in the atmosphere. The jet stream often
"steers" surface features such as fronts and low pressure systems.
K
KATABATIC WIND - Local winds
a result of cold, dense air flowing downhill.
KELVIN- A ratio temperature
scale using 0 as the theoretical coldest temperature.
KNOT - One nautical mile per
hour (1.15 mph).
L
LAND BREEZE - A wind that
blows from the land towards a body of water. Also known as an offshore breeze.
LA NINA - The opposite
of El Nino, when a major cooling occurs in the equatorial waters in the Pacific
Ocean which is characterized by shifts in "normal" weather patterns.
LAPSE RATE The change
in temperature with altitude in the atmosphere.
LATENT HEAT - the heat energy
that must be absorbed when a substance changes from solid to liquid and liquid
to gas, and which is released when a gas condenses and a liquid solidifies.
LEE TROUGH- Low pressure which
develops just east of the Rocky Mountains or Cascades. Often lee troughs become
organized into mature cyclones as they develop eastward.
LIFTED INDEX- The environmental
temperature at 500 millibars minus the 500 millibar parcel temperature on a
Skew-T diagram. Negative LI values are unstable.
LIGHTNING An electrical
discharge from a thunderstorm.
LIKELY - In probability of
precipitation statements, the equivalent of a 60 or 70 percent chance.
LLJ- Low Level Jet. Strong
Planetary Boundary Layer winds which advect warmer temperatures and moisture
into the forecast area.
LOW- The center
of an area of low pressure, accompanied by cyclonic and inward wind flow in
the northern hemisphere. Also known as a cyclone.
M
MACROBURST - Large
downbursts with 2.5 miles or larger outflow diameter and damaging winds lasting
5 to 20 minutes. Intense macrobursts could cause tornado-force damage.
MARINE PUSH - A
regional phenomena where the heat low shifts east across the Pacific Northwest,
along an onshore flow of cool, marine air to spill over the Cascades. It is
characterized by a gusty winds and the potential of convection across the Inland
Northwest.
MARITIME AIR MASS - Moist
air mass originating over the ocean.
MCS-
Mesoscale Convective System. A large cluster of thunderstorms and rain. Can
be a squall line, multi-cells or a mesoscale convective complex.
MEASURABLE- Precipitation
of 0.01" or more.
MESOCYCLONE -The
rotating updraft in a supercell thunderstorm
METEOROLOGY - The study of
the atmosphere and atmospheric phenomena.
MIC - Meteorologist In Charge.
MICROBURST - Small
downbursts, less than 2.5 miles in outflow diameter, with peak winds lasting
2 to 5 minutes. They may induce dangerous wind and downflow wind shears which
can affect aircraft performance.
MICROWAVE RADIATION - electromagnetic
radiation which comprises the highest frequency radio energy.
MID/UPPER LEVEL SUPPORT- This
is in reference to either positive differential vorticity advection or a jet
streak creating upper level divergence. These processes result in a dynamic
lifting of air.
MILLIBAR - A unit of atmospheric
pressure. 1 mb = 100 Pa (pascal). Normal surface pressure is approximately 1013
millibars.
MIST - very fine water droplets
at ground level
MIXING DEPTH- The vertical
distance the process of convection mixes the air from the surface to aloft.
The mixing depth is often the same depth as the PBL. The mixing depth will increase
with solar warming of the surface and increased low level wind speed. Could
also be in reference to the depth of the "transition zone" between
two air masses that are horizontally differentially advecting one over the other.
MOISTURE AXIS / RIDGE- : An
area of higher moisture values, usually in the form of a ridge of higher dewpoints
at the surface or 850 mb. Low level moisture axes enhance atmospheric instability,
which in turn promotes thunderstorm development. Existing storms can intensify
by moving into moisture axes. The concept is similar to dewpoint pooling.
MONSOON - A persistent seasonal
wind, often responsible for seasonal precipitation regime.
MOS - Model Output Statistics
(usually in reference to NGM model). These are numerical representations of
expected weather such as forecasted temperatures and precipitation chances.
MOSTLY CLOUDY - Between 7/10
and 9/10 cloud cover.
MOSTLY SUNNY (MOSTLY CLEAR)
- Between 1/10 and 3/10 cloud cover.
MM5
- the Mesoscale Model Version 5. One of these models is run at the Department
of Atmospheric Sciences at the University of Washington which covers the weather
over Washington, Oregon and most of Idaho.
MRF - Medium Range Forecast
model generated every 12 hours by NCEP.
MSLP - Mean sea level pressure.
N
NCEP - National Centers for
Environmental Prediction. Central computer and communications facility of the
National Weather Service; located in Washington, DC.
NEGATIVE TILTED TROUGH- A
trough which tilts from the northwest toward the southeast. This situation creates
instability since cold temperatures in the mid and upper levels advect over
warmer air at the surface. A negative tilt is a sign a trough is mature.
NEXRAD - NEXt Generation RADar.
A NWS network of about 140 Doppler radars operating nationwide.
NGM -Nested Grid Model generated
every 12 hours by NCEP.
NHC -National Hurricane Center.
The office of the National Weather Service in Miami that is responsible for
tracking and forecasting tropical cyclones.
NOAA - National Oceanic and
Atmospheric Administration. A branch of the US Department of Commerce, NOAA
is the parent organization of the National Weather Service.
NOAA WEATHER RADIO (NWR)-
Continuous, 24 hour-a-day VHF broadcasts of weather observations and forecasts
directly from National Weather Service offices. A special tone allows certain
receivers to alarm when watches or warnings are issued.
NOAA WEATHER WIRE (NWWS) -
Mass dissemination via satellite of National Weather Service products to the
media and public.
NUMERICAL WEATHER PREDICTION (NWP)
- Forecasting weather by the use of numerical models, run on high speed computers.
Most of the NWP for the National Weather Service is done at the National Centers
for Environmental Prediction (NCEP).
NUMEROUS - Showers covering
more than 54 percent of an area.
NWS - National Weather Service.
O
OCCLUDED FRONT - A complex
frontal system that occurs when a cold front overtakes a warm front. Also known
as an occlusion.
OFFSHORE WATERS - Ocean waters
from 60 nm to 250 nm.
OMEGA - A term used to describe
vertical motion in the atmosphere. The "omega equation" used in numerical
weather models is composed of two terms, the "differential vorticity advection"
term and the "thickness advection" term. Put more simply, omega is
determined by the amount of spin (or large scale rotation) and warm (or cold)
advection present in the atmosphere. On a weather forecast chart, high values
of omega (or a strong omega field) relate to upward vertical motion in the atmosphere.
If this upward vertical motion is strong enough and in a sufficiently moist
airmass, precipitation results.
OPEN WAVE- A wave of low pressure
that does not have a complete circulation around it; also called a short wave
trough.
OROGRAPHIC UPLIFT
- The vertical forcing of air by terrain features such as hills or mountains.
This can create orographic clouds and/or precipitation.
OUTFLOW - Air that
flows outward from a thunderstorm.
OVERCAST - Sky condition when
greater than 9/10 of the sky is covered
OVERSHOOTING TOP
- A 'bubble' of cloud sticking up above the anvil of a thunderstorm, due to
a vigorous updraft within the storm.
OZONE - A form of oxygen containing
3 molecules, usually found in the stratosphere, and responsible for filtering
out much of the sun's ultraviolet radiation.
P
PACIFIC HIGH - A semipermanent
anticyclone located in the Eastern North Pacific.
PACKAGE- A compilation of
analysis and forecast charts and their interpretation.
PARTLY CLOUDY (PARTLY SUNNY)
- Sky condition when between 3/10 and 7/10 of the sky is covered.
PBL- Planetary Boundary Layer.
The lowest level of the atmosphere where friction is an important force and
vertical mixing is common.
PERMAFROST - a soil layer
below the surface of tundra regions that remains frozen permanently.
PD - Period
PG- Pressure gradient. Tightening
pressure gradient indicates stronger winds.
PHASING-
When two separate short waves come together to form one wave. Also, when upper
and lower level features are positioned so that each provides energy to the
other, it is said that the features are in phase with one another.
PINEAPPLE CONNECTION - A
warm and moist flow of air with origins from the tropics which precedes major
winter storms. It can bring significant rain and snow to the Pacific Northwest.
POP - Probability of Precipitation.
This is the chance the NWS gives to precipitation across there forecast region.
It may also refer to the POP a forecast model is predicting.
POSITIVE ISOTHERMAL VORTICITY ADVECTION (PIVA)- Advection of higher
values of vorticity by the thermal wind on a map of vorticity and thickness
contours. Implies an area of rising motion.
POSITIVE TILTED TROUGH- A
trough which tilts from the northeast toward the southwest. Often a trough is
positive tilted in it's development stage.
POTENTIAL INSTABILITY- Also
known as convective instability. Instability caused by dry air advecting over
warm and humid PBL air. Lapse rate of temperature increases if lifting occurs.
PRECIPITABLE WATER (PW, PCPTBL
WTR or H2O)- Total amount of water vapor in a layer of air, expressed in
inches. Normally taken between 1000 and 500 mb. Higher values of precipitable
water indicate a deep moisture layer, increasing the potential for heavy precipitation
amounts.
PRECIPITATION (PCPN)
-Liquid or solid water molecules that fall from the atmosphere and reach the
ground.
PRESSURE -The force
exerted by the interaction of the atmosphere and gravity. Also known as atmospheric
pressure.
PROFILER- A remote ground-based
sensing instrument that measures wind speed and direction at different levels
of the atmosphere. The new National Weather Service WSR-88D radar has this capability,
and its vertical wind profile is called a VAD Wind Profile (or VWP). VAD stands
for Velocity Azimuth Display.
PROG- Model forecasted output
PROGRESSIVE FLOW or PROGRESSIVE
PATTERN- An upper level flow pattern in which storm systems move along at
a fairly regular pace.
PTWC - Pacific Tsunami Warning
Center, located in Honolulu, HI.
PUNCH (or PUSH)- A fast moving
mass of air (e.g. dry punch, cold punch)
PWAT(S)- Precipitable Water
value(s)
Q
QPF- Quantitative Precipitation
Forecast
QG Forcing- Upward vertical
velocity generated by low level convergence or upper level divergence. Example
of phenomena that produce upward QG forcing include low level warm air and moisture
advection, positive differential vorticity advection, and the right rear and
left front quadrant of a jet streak. If QG forcing is in reference to sinking
air, example that cause this are low level cold air and dry air advection, negative
differential vorticity advection, and the left rear and right front quadrants
of a jet streak.
Q-VECTORS (QVEC, DIVQ)- A
mathematical entity (Q-vectors do not exist in the atmosphere) that allows forecasters
to better identify areas of vertical motion. Q-vectors essentially show vertical
motions arising from the combination of differential vorticity advection (changes
of advection with height) and temperature (thickness) advection. Areas where
Q-vectors converge implies upward motion and Q-vector divergence (DIVQ) implies
an area of sinking motion. Note: Negative Q-vector divergence is the same mathematically
as Q-vector convergence, so the statement "...negative divq..." means
Q-vector convergence.
R
RA - Rain.
RADAR - An instrument used
to detect precipitation by measuring the strength of the electromagnetic signal
reflected back. (RADAR= Radio Detection and Ranging)
RADIATION FOG - See ground
fog.
RADIOSONDE- An instrument
attached to a weather balloon that measures and transmits pressure, humidity,
temperature, and winds as it ascends.
RAIN - Precipitation
of liquid water particles which, in contrast to drizzle, are widely separated
and larger in size.
RAINBOW- Optical
phenomenon when light is refracted and reflected by moisture in the air into
concentric arcs of color.
RELATIVE HUMIDITY
(RH) - The amount of water vapor in the air, compared to the amount the
air could hold if it was totally saturated. (Expressed as a percentage).
RETROGRADE- To move backwards.
It relates to a ridge or trough moving from east to west. Typically in the mid-latitude,
troughs and ridges move west to east. Retrograde motion is the opposite movement
of normal.
RFC - River Forecast Center.
The California/Nevada River Forecast Center is located in Sacramento.
RIDGE - An elongated area
of high pressure in the atmosphere.
RING OF FIRE- This refers
to thunderstorms and rain across the edges of a high pressure ridge. If a ridge
occurs across the central US in summer, the ring of fire will extend across
the west coast, up into Canada and across the east coast. Underneath the main
core of the ridge weather will be stable.
ROSSBY WAVES - long waves
that form in air or water that flows almost parallel to the equator, which results
form the effect of the earth's rotation.
RUC - Rapid Update Cycle model
generated every 3 hours. The model is run each hour, with its initial conditions
updated by the inclusion of the latest surface observations available. Its primary
usage is for aviation and severe weather forecasting.
S
SCATTERED CLOUDS - Sky condition
when between 1/10 and 5/10 are covered.
SCATTERED SHOWERS - Showers
covering 25 to 54 percent of an area.
SEA BREEZE - A wind that blows
from a sea or ocean towards a land mass. Also known as an onshore breeze.
SEA SURFACE TEMPERATURE (SST)
- Surface temperature data collected using IR satellite imagery, buoy and ship
data.
SEVERE THUNDERSTORM
- A thunderstorm with winds of 58 mph or more or hail 3/4-inch diameter or larger.
Structural wind damage may imply the occurrence of a severe thunderstorm.
SHELF CLOUD - Long,
wedge-shaped clouds associated with the gust front. Shelf clouds indicate the
downdraft or outflow of a thunderstorm.
SHORTWAVE - a progressive
wave in a horizontal air flow that leads to lower air pressure and possibly
unsettled weather.
SHOWER - Precipitation
that is intermittent, in space, time, or intensity and is falling from a cumuliform
cloud.
SLEET - Solid grains
of ice which form from the freezing of raindrops or the refreezing of melted
snowflakes. These small, transparent ice pellets usually bounce when they hit
a hard surface.
SLIGHT CHANCE - In probability
of precipitation statements, usually equivalent to a 20 percent chance.
SMOG - Pollution formed by
the interaction of pollutants and sunlight (photochemical smog), usually restricting
visibility, and occasionally hazardous to health.
SNOW - Unless qualified
by such words as " occasional" or " intermittent" , a prediction of snow indicates
a steady fall of a few hours or more.
SNOW ADVISORY - An advisory
issued when snow is expected to create hazardous or restricted travel conditions,
but not as severe as expected with a winter storm.
SNOW FLURRIES -
Short duration of intermittent light snowfall with little if any accumulations.
SOO - Science and Operations
Officer.
SOUTHERN OSCILLATION - A periodic
reversal of the pressure pattern across the tropical Pacific Ocean during El
Nino events.
SPC - Storm Prediction Center.
Located in Norman, OK. This office is responsible for monitoring and forecasting
severe convective weather in the continental U.S. This includes the issuance
of Tornado and Severe Thunderstorm Watches.
SPS- Special Statement
SPRING TIDE - Semi-monthly
tide of increased height due to the new or full moon.
SQUALL LINE - A
line of thunderstorms or squalls extending over several hundred miles.
STABLE AIR - air with little
or no tendency to rise, that is usually accompanied by clear dry weather.
STACKED- A pressure system
which shows up in the same general location on every height analysis or forecast
chart. This is an indication low pressure is weakening (filling) or a pressure
system is stalling.
STATIONARY FRONT - A transition
zone between airmasses, with neither advancing upon the other.
STRAIGHT LINE WINDS-
Damaging thunderstorm winds produced as air rushes downward from a thunderstorm.
Often called gust fronts, downburst, or microburst winds these winds originate
from thunderstorm downdrafts and may produce damage similar to that of a tornado.
STRATOSPHERE- The
layer of the atmosphere above the troposphere, where temperature increases with
height.
STRATUS- Flat low
level clouds.
SUBSIDENCE- Sinking
air that is associated with warming air and little cloud formation.
SUBTROPICAL JET - The branch
of the jet stream that is found in the lower latitudes.
SUPERCELL- A highly
organized thunderstorm with a rotating updraft, known as a mesocyclone. It poses
an inordinately high threat to life and property. Often produces large hail,
strong winds, and tornadoes.
SUPERCOOLED WATER - water
that stays in liquid form if undisturbed even though it has been cooled to a
temperature below its normal freezing point.
SUSTAINED WINDS-
The wind speed obtained by averaging the observed values over a one minute period.
SWELLS - Ocean waves of regular
and longer duration than wind waves.
T
TCU - Towering Cumulus
TEMPERATURE - a measure of
the warmth or coldness of an object or substance with reference to a standard
value.
THERMAL- Small rising
column of air due to surface heating.
THERMAL RIDGE or THERMAL
TROUGH- A ridge of warmer temperatures or a trough of colder temperature.
THERMODYNAMICS- In reference
to the (in)stability of the atmosphere. Important thermodynamic information
includes lapse rates, CAPE, changes in temperature / moisture with height, and
cap strength.
THUNDER- The sound
wave produced as a lightning stroke heats the air causing it to rapidly expand.
THUNDERSTORM (TS,
TSRA, TSTM) - A shower accompanied by thunder. It is always accompanied
by lightning and thunder, and occasionally by strong gusty winds, hail, and
or heavy rain.
TORNADO - A violent
rotating column of air, usually forming a pendant from a cumulonimbus cloud
with the circulation reaching the ground. It nearly always starts as a funnel
cloud and may be accompanied by a loud roaring noise. On a local scale, it is
the most destructive of all atmospheric phenomena.
TRACE -Precipitation
amounts less than 0.01".
TRADE WINDS - Persistent tropical
winds that blow from the subtropical high pressure centers towards the equatorial
low.
TROPICAL DEPRESSION - Tropical
mass of thunderstorms with a cyclonic wind circulation and winds between 20
and 34 knots.
TROPICAL DISTURBANCE - An
organized mass of tropical thunderstorms, with a slight cyclonic circulation,
and winds less than 20 knots.
TROPICAL STORM - An organized
cyclone in the tropics with wind speed between 35 and 64 knots.
TROPOSPHERE- The
lowest layer of the atmosphere where the temperature decreases with height.
Most of earth's weather occurs in this layer.
TROUGH -An elongated
area of low pressure at the surface or aloft.
TSUNAMI - An ocean wave generated
by a submarine earthquake, volcano or landslide. (Also known as a seismic seawave,
and incorrectly as a tidal wave).
TURBULENCE- Disrupted
flow in the atmosphere that produces gusts and eddies.
TURBULENCE - Disrupted flow
in the atmosphere that produces gusts and eddies.
TWEAK- To change slightly.
Often in reference to slightly changing MOS output or any other form of model
guidance.
TYPHOON - A hurricane that
forms in the Western Pacific Ocean.
U
UKMET - United Kingdom forecast
model.
UNSTABLE AIR - air that rises
easily and can form clouds and rain.
UPWELLING - The rising of
cold water from the deeper areas of the ocean to the surface. This phenomena
often occurs along the California coast during the summer.
V
VEERING WINDS - A clockwise
change in wind direction. Veering winds with height are indicative of warm air
advection (WAA).
VERY WINDY - Sustained winds
of 30 to 40 mph.
VIRGA- Precipitation
falling from the base of a cloud and evaporating before it reaches the ground.
VISIBILITY- The
horizontal distance an observer can see and identify a prominent object.
VORTICITY- A measure
of the amount of "spin" (rotation) and "shear" in the atmosphere.
W
WAA - Warm Air Advection
WALL CLOUD -An isolated
lowering of a cloud that is attached to the rain-free base of a thunderstorm,
generally to the rear of the visible precipitation area. Wall clouds indicate
the updraft of or the inflow to a thunderstorm.
WARM FRONT - A boundary between
a warm airmass that is replacing a cooler airmass.
WARNING - is issued
when severe or hazardous weather has already developed and has been reported
by spotters or indicated by radar. Warnings are statements of imminent danger
and are issued for relatively small areas near and downstream from the severe
storm or flood. For example, Tornado Warning, Severe Thunderstorm Warning, Flash
Flood Warning, Winter Storm Warning.
WATCH - identifies
a relatively large area in which hazardous or severe weather might occur. The
watch is only an indication of where and when the severe weather probabilities
are the highest, and should not be confused with a warning. Examples include
Tornado Watch, Severe Thunderstorm Watch, Flash Flood Watch, Winter Storm Watch.
WATERSPOUT - A violently
rotating column of air usually forming a pendant from a cumulus/cumulonimbus
cloud, occurring over a body of water, and having circulation reaching the water.
WATER VAPOR - water substance
in a gaseous state that comprises one of the most important of all the constituents
of the atmosphere.
WCM - Warning Coordination
Meteorologist.
WFO - Weather Forecast Office.
Designation of NWS operational offices after modernization.
WIND - air in motion relative
to the surface of the earth.
WIND ADVISORY - Sustained
winds 25 to 39 mph and/or gusts to 57 mph. Issuance is normally site specific.
However, winds of this magnitude occurring over an area that frequently experiences
such winds (e.g., the normal strong summertime winds near the San Francisco
Bay, would not require the issuance of this product).
WIND CHILL- The
apparent temperature that describes the cooling effect on exposed skin by the
combination of temperature and wind, expressed as a loss of body heat. An increase
in wind speed or decrease in temperature will accelerate the effect. A wind
chill factor of 30 degrees or lower on exposed skin will result in frostbite
in a short period of time.
WIND SHEAR -The
change of wind speed or direction with distance or height.
WIND VANE- An instrument
that determines the direction from which a wind is blowing.
WINDY - Sustained winds of
20 to 30 mph.
WRCC - Western Regional Climate
Center, Reno, NV.
X
X-BAND - a frequency band
of microwave radiation in which radars operate.
Z
ZFP - Zone Forecast Product.
The collective of public forecasts for a NWS office area of responsibility.
ZONAL WIND - the wind or wind
componant along the local parallel of latitude.
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