A suspension
of numerous minute ice crystals in the air, or water droplets at temperatures
below 0 Celsius, based at the Earth's surface, which reduces horizontal
visibility; also called ice fog.
| FREEZING
LEVEL |
The altitude
in the atmosphere where the temperature equals 32F. |
| FREEZING
RAIN |
Rain
which falls as liquid then freezes upon impact, resulting in a coating
of ice on exposed objects. |
| FRONT |
The transition
zone between two distinct air masses. The basic frontal types are
cold fronts, warm fronts, occluded fronts, and stationary fronts. |
| FROST |
The covering
of ice, due to condensed water vapor, that is formed on exposed surfaces
whose temperature falls below freezing. |
| 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. |
| FUJIWHARA
EFFECT |
The Fujiwhara
effect describes the rotation of two storms around each other. |
| FUNNEL
CLOUD |
A rotating,
cone-shaped column of air extending downward from the base of a thunderstorm,
but not in contact with the ground. When it reaches the ground it
is then called a tornado. |
| G |
|
| 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. |
| GRADIENT |
The time
rate or spatial rate of change of an atmospheric property. |
| GRAUPEL |
Small
pellets of ice created when supercooled water droplets coat, or rime,
a snowflake. The pellets are cloudy or white, not clear like sleet,
and often are mistaken for hail. |
| GRAVITY
WAVE |
A wave
disturbance in which buoyancy acts as the restoring force on parcels
displaced from hydrostatic equilibrium. Waves on the ocean are examples
of gravity waves. |
| 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. |
| 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. |
| GUSTNADO |
Gust
front tornado. A small tornado, usually weak and short-lived, that
occurs along the gust front of a thunderstorm. Often it is visible
only as a debris cloud or dust whirl near the ground. |
| H |
|
| HAIL |
Precipitation
in the form of circular or irregular-shaped lumps of ice. |
| 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. |
| HAZE |
Fine
dry or wet dust or salt particles in the air that reduce visibility. |
| HEAT
INDEX |
An index
that combines air temperature and humidity to give an apparent temperature
(how hot it feels). |
| HEAT
ISLAND |
A dome
of elevated temperatures over an urban area caused by the heat absorbed
by structures and pavement. |
| HEATING
DEGREE DAY |
A form
of degree day used to estimate the required energy for heating. One
heating degree day occurs for each degree the daily mean temperature
is below 65 degrees Fahrenheit. |
| HEAVY
SNOW |
Depending
on the region of the USA, this generally means that four or more inches
of snow has accumulated in 12 hours, or six or more inches of snow
in 24 hours. |
| HELICITY |
A property
of a moving fluid which represents the potential for helical flow
(i.e. flow which follows the pattern of a corkscrew) to evolve. Helicity
is proportional to the strength of the flow, the amount of vertical
wind shear, and the amount of turning in the flow (i.e. vorticity). |
| HIGH |
The center
of an area of high pressure, accompanied by anticyclonic and outward
wind flow in the northern hemisphere. Also known as an anticyclone. |
| HIGH
WIND WARNING |
Issued
whe non-convective synoptic-scale gradient or mesoscale post-convective
wind speeds are sustained at greater than or equal to 40 mph sustained
and/or wind gusts reach or exceed 58 mph. |
| HODOGRAPH |
A plot
representing the vertical distribution of horizontal winds, using
polar coordinates. A hodograph is obtained by plotting the end points
of the wind vectors at various altitudes, and connecting these points
in order of increasing height. |
| HOOK
ECHO |
A radar
pattern sometimes observed in the southwest quadrant of a tornadic
thunderstorm. Appearing like a fishhook turned in toward the east,
the hook echo is precipitation aloft around the periphery of a rotating
column of air 2-10 miles in diameter. |
| HUMIDITY |
The amount
of water vapor in the atmosphere. (See relative humidity). |
| HURRICANE |
A severe
tropical cyclone with sustained wind speeds in excess of 74 mph (64
knots). |
| I |
|
| ICE
PELLETS |
Precipitation
of transparent or translucent pellets of ice, which are round or irregular,
rarely conical, and which have a diameter of 0.2 inch (5 mm), or less.
There are two main types. Hard grains of ice consisting of frozen
raindrops and pellets of snow encased in a thin layer of ice. |
| INDIAN
SUMMER |
An unseasonably
warm period near the middle of autumn, usually following a substantial
period of cool weather. |
| INFLOW
NOTCH |
A radar
signature characterized by an indentation in the reflectivity pattern
on the inflow side of the storm. The indentation often is V-shaped,
but this term should not be confused with V-notch. Supercell thunderstorms
often exhibit inflow notches, usually in the right quadrant of a classic
supercell, but sometimes in the eastern part of an HP storm or in
the rear part of a storm (rear inflow notch). |
| INSTABILITY |
A state
of the atmosphere in which convection takes place spontaneously, leading
to cloud formation and precipitation. |
| INVERSION |
An increase
in temperature with height. The reverse of the normal cooling with
height in the atmosphere. |
| ISENTROPIC
LIFT |
Lifting
of air that is traveling along an upward-sloping isentropic surface.
Situations involving isentropic lift often are characterized by widespread
stratiform clouds and precipitation. |
| ISENTROPIC
SURFACE |
A two-dimensional
surface containing points of equal potential temperature. |
| ISOBAR |
A line
of equal barometric pressure on a weather map. |
| ISODROSOTHERM |
A line
of equal dew point temperature. |
| ISOHYET |
A line
of equal precipitation amounts. |
| ISOPLETH |
General
term for a line of equal value of some quantity. Isobars, isotherms,
etc. all are examples of isopleths. |
| ISOTACH |
A line
of equal wind speed. |
| ISOTHERM |
A line
of equal temperature on a weather map. |
| J |
|
| JET
STREAK |
A local
wind speed maximum within a 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 |
|
| KELVIN
TEMPERATURE SCALE |
A temperature
scale in which 0 degrees is the point at which all molecular motion
ceases (absolute zero). |
| KNOT |
One nautical
mile per hour (1.15 mph). |
| L |
|
| LAKE
EFFECT |
The effect
of a lake (usually a large one) in modifying the weather near the
shore and down wind. It is often refers to the enhanced rain or snow
that falls downwind from the lake. This effect can also result in
enhanced snowfall along the east coast of New England in winter. |
| LAMINAR |
Smooth,
non-turbulent. Often used to describe cloud formations which appear
to be shaped by a smooth flow of air traveling in parallel layers
or sheets. |
| LA
NINA |
A cooling
of the equatorial waters in the Pacific Ocean. |
| LANDSPOUT |
A tornado
that does not arise from organized storm-scale rotation and therefore
is not associated with a wall cloud (visually) or a mesocyclone (on
radar). Landspouts typically are observed beneath Cbs or towering
cumulus clouds (often as no more than a dust whirl), and essentially
are the land-based equivalents of waterspouts. |
| 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. |
| LAYER |
An array
of clouds and/or obscurations whose bases are at approximately the
same level. |
LEFT
FRONT QUADRANT
(or LEFT EXIT REGION) |
The area
downstream from and to the left of an upper-level jet max (as would
be viewed looking along the direction of flow). Upward motion and
severe thunderstorm potential sometimes are increased in this area
relative to the wind speed maximum. |
| LEFT
MOVER |
A thunderstorm
which moves to the left relative to the steering winds, and to other
nearby thunderstorms; often the northern part of a splitting storm. |
| LIFTED
INDEX (or LI) |
A common
measure of atmospheric instability. Its value is obtained by computing
the temperature that air near the ground would have if it were lifted
to some higher level (around 18,000 feet, usually) and comparing that
temperature to the actual temperature at that level. Negative values
indicate instability - the more negative, the more unstable the air
is, and if thunderstorms develop they are more likely to be stronger. |
| LIFTING |
The forcing
of air in a vertical direction by an upslope in terrain or by the
movement of a denser air mass. |
| LIFTING
CONDENSATION LEVEL |
The level
in the atmosphere where a lifted air parcel reaches its saturation
point, and as a result, the water vapor within condenses into water
droplets. |
| LIGHTNING |
An electrical
discharge from a thunderstorm. |
| LIKELY |
In probability
of precipitation statements, the equivalent of a 60 or 70 percent
chance. |
| LOADED
GUN (SOUNDING)
| A sounding
characterized by extreme instability but containing a cap, such that
explosive thunderstorm development can be expected if the cap can
be weakened or the air below it heated sufficiently to overcome it. |
| LONGWAVE
TROUGH |
A trough
in the prevailing westerly flow aloft which is characterized by large
length and (usually) long duration. Generally, there are no more than
about five longwave troughs around the Northern Hemisphere at any
given time. Their position and intensity govern general weather patterns
(e.g., hot/cold, wet/dry) over periods of days, weeks, or months. |
| 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. |
| LOW-LEVEL
JET |
A region
of relatively strong winds in the lower part of the atmosphere. |
| M |
|
| MACROBURST |
Large
thunderstorm downbursts with a 2.5 mile diameter or greater outflow
of damaging winds lasting 5 to 20 minutes. |
| MEAN
TEMPERATURE |
The average
of a series of temperatures taken over a period of time, such as a
day or a month. |
| MEDIUM
RANGE |
In forecasting,
(generally) three to seven days in advance. |
| MEASURABLE |
Precipitation
of 0.01" or more. |
| MEDIUM
RANGE |
In forecasting,
(generally) three to seven days in advance. |
| MERIDIONAL
FLOW |
A type
of atmospheric circulation pattern in which the north and south component
of motion is unusually pronounced. Opposite of zonal flow. |
| MESOCYCLONE |
The rotating
updraft in a supercell thunderstorm. |
| MESOHIGH |
A mesoscale
high pressure area, usually associated with MCSs or their remnants. |
| MESOLOW
(or SUB-SYNOPTIC LOW) |
A mesoscale
low-pressure center. Severe weather potential often increases in the
area near and just ahead of a mesolow. |
| MESONET |
A regional
network of observing stations (usually surface stations) designed
to diagnose mesoscale weather features and their associated processes. |
| MESOSCALE |
Size
scale referring to weather systems smaller than synoptic-scale systems
but larger than single storm clouds. Horizontal dimensions generally
range from around 50 miles to several hundred miles. Squall lines
are an example of mesoscale weather systems. |
MESOSCALE
CONVECTIVE COMPLEX
(MCC) |
A large
mesoscale convective system, generally round or oval-shaped, which
normally reaches peak intensity at night. The formal definition includes
specific minimum criteria for size, duration, and eccentricity (i.e.,
"roundness"), based on the cloud shield as seen on infrared satellite
photographs. |
MESOSCALE
CONVECTIVE SYSTEM
(MCS) |
A complex
of thunderstorms which becomes organized on a scale larger than the
individual thunderstorms, and normally persists for several hours
or more. MCSs may be round or linear in shape, and include systems
such as tropical cyclones, squall lines, and MCCs (among others).
MCS often is used to describe a cluster of thunderstorms that does
not satisfy the size, shape, or duration criteria of an MCC. |
| METAR |
A weather
observation near ground level. It may include date and time, wind,
visibility, weather and obstructions to vision, sky condition, temperature
and dew point, sea level pressure, precipitation amount and other
data used for aircraft operations. |
| METEOROLOGY |
The study
of the atmosphere and atmospheric phenomena. |
| MICROBURST |
A strong
localized downdraft less than 2.5 miles in diameter from a thunderstorm.
Peak gusts last from 2 to 5 minutes. |
| MILLIBAR |
A unit
of atmospheric pressure. 1 mb = 100 Pa (pascal). Normal surface pressure
is approximately 1013 millibars. |
| MIST |
Consists
of microscopic water droplets suspended in the air which produce a
thin grayish veil over the landscape. It reduces visibility to a lesser
extent than fog. |
| MIXING |
Air movements
(usually vertical) that make the properties of the air with a parcel
homogeneous. It may result in a lapse rate approaching the moist or
dry adiabatic rate. |
| MODEL |
A mathematical
representation of a process, system, or object developed to understand
its behavior or to make predictions. The representation always involves
certain simplifications and assumptions. |
MODERATE
RISK
(of severe thunderstorms) |
Severe
thunderstorms are expected to affect between 5 and 10 percent of the
area. |
| MOISTURE
ADVECTION |
Transport
of moisture by horizontal winds. |
| MOISTURE
CONVERGENCE |
A measure
of the degree to which moist air is converging into a given area,
taking into account the effect of converging winds and moisture advection.
Areas of persistent moisture convergence are favored regions for thunderstorm
development, if other factors (e.g., instability) are favorable. |
| MOS |
Model
Output Statistics. |
| MRF |
Medium
Range Forecast model generated every 12 hours by NCEP. |
| MSL |
Mean
sea level. |
| MSLP |
Mean
sea level pressure. |
| MUGGY |
Colloquially
descriptive of warm and especially humid weather. |
| MULTICELL
CLUSTER THUNDERSTORM |
A thunderstorm
consisting of two or more cells, of which most or all are often visible
at a given time as distinct domes or towers in various stages of development. |
| MULTIVORTEX
TORNADO |
A tornado
in which two or more condensation funnels or debris clouds are present
at the same time, often rotating about a common center or about each
other. Multiple-vortex tornadoes can be especially damaging. |
| N |
|
| NCDC |
National
Climatic Data Center. Located in Asheville, North
Carolina, the agency that archives climatic and forecast data from
the National Weather Service. |
| NCEP |
National
Centers for Environmental Prediction. Central
computer and communications facility of the National Weather Service;
located in Washington, DC. |
| NEGATIVE
TILT TROUGH |
An upper
level system which is tilted to the west with increasing latitude
(i.e., with an axis from southeast to northwest). A negative-tilt
trough often is a sign of a developing or intensifying system. |
| NEXRAD |
NEXt
Generation RADar. A NWS network of about 160 Doppler radars
being installed nationwide. |
| 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 U.S. Department of Commerce, NOAA is the parent organization
of the National Weather Service. |
| NOAA
WEATHER WIRE (NWWS) |
A computer
dissemination network that sends National Weather Service products
to the media and public. |
| 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. |
| NOCTURNAL |
Related
to nighttime, or occurring at night. |
| NOR'EASTER |
A low-pressure
disturbance forming along the South Atlantic coast and moving northeast
along the Middle Atlantic and New England coasts to the Atlantic Provinces
of Canada. It usually causes strong northeast winds with rain or snow.
Also called a Northeaster or Coastal Storm. |
| NORMAL |
The long-term
average value of a meteorological element for a certain area. For
example, "temperatures are normal for this time of year" Usually averaged
over 30 years. |
| NOWCAST |
A short-term
weather forecast, generally out to six hours or less. |
| NSSL |
The National
Severe Storms Laboratory. |
| NUCLEUS |
A particle
of any nature upon which molecules of water or ice accumulate. |
| NUMERICAL
FORECASTING |
Forecasting
the weather through digital computations carried out by supercomputers. |
| NWP |
Numerical
Weather Prediction. |
| 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. |
| 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. |
| 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. |
| OVERRUNNING |
A condition
that exists when a relatively warm air mass moves up and over a colder
and denser air mass on the surface. The result is usually low clouds,
fog and steady, light precipitation. |
| OVERCAST |
Sky condition
when 9/10 or 10/10 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 |
|
| PARTLY
CLOUDY |
Sky condition
when between 3/10 and 7/10 of the sky is covered. Used more frequently
at night.
| PARTLY
SUNNY |
Similar
to partly cloudy. Used to emphasize daytime sunshine. |
| PATCHES |
Used
with fog to denote random occurrence over relatively small areas. |
| PENDANT
ECHO |
Radar
signature generally similar to a hook echo, except that the hook shape
is not as well defined. |
| POLAR
VORTEX |
A circumpolar
wind circulation which isolates the Antarctic continent during the
cold Southern Hemisphere winter, heightening ozone depletion. |
| POP |
Probability
Of Precipitation. Probability forecasts are subjective
estimates of the chances of encountering measurable precipitation
at some time during the forecast period. |
| POSITIVE
AREA |
The area
on a sounding representing the layer in which a lifted parcel would
be warmer than the environment; thus, the area between the environmental
temperature profile and the path of the lifted parcel. |
| POSITIVE-TILE
TROUGH |
An upper
level system which is tilted to the east with increasing latitude
(i.e., from southwest to northeast). A positive-tilt trough often
is a sign of a weakening weather system, and generally is less likely
to result in severe weather than a negative-tilt trough if all other
factors are equal. |
| POTENTIAL
TEMPERATURE |
The temperature
a parcel of dry air would have if brought adiabatically (i.e., without
transfer of heat or mass) to a standard pressure level of 1000 mb. |
| PRECIPITATION |
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. |
| PRESSURE
CHANGE |
The net
difference between pressure readings at the beginning and ending of
a specified interval of time. |
| PRESSURE
FALLING RAPIDLY |
A decrease
in station pressure at a rate of 0.06 inch of mercury or more per
hour which totals 0.02 inch or more. |
| PRESSURE
GRADIENT |
The rate
of decrease of pressure with distance at a fixed level. |
| PRESSURE
GRADIENT FORCE |
Force
acting on air that causes it to move from areas of higher pressure
to areas of lower pressure. |
| PRESSURE
RISING RAPDILY |
An increase
in station pressure at a rate of 0.06 inch of mercury or more per
hour which totals 0.02 inch or more. |
| PRESSURE
TENDENCY |
The character
and amount of atmospheric pressure change during a specified period
of time, usually the 3-hour period preceding an observation. |
| PRESSURE
UNSTEADY |
A pressure
that fluctuates by 0.03 inch of mercury or more from the mean pressure
during the period of measurement. |
| PREVAILING
WESTERLIES |
Winds
in the middle latitudes (approximately 30 degrees to 60 degrees) that
generally blow from west to east. |
| PREVAILING
WIND |
The direction
from which the wind blows most frequently in any location. |
| PROFILER |
An instrument
designed to measure horizontal winds directly above its location,
and thus measure the vertical wind profile. Profilers operate on the
same principles as Doppler radar. |
| PSYCHROMETER |
An instrument
used for measuring the water vapor content of the atmosphere. It consists
of two thermometers, one of which is an ordinary glass thermometer,
while the other has its bulb covered with a jacket of clean muslin
which is saturated with distilled water prior to use. |
| PULSE
STORM |
A thunderstorm
within which a brief period (pulse) of strong updraft occurs, during
and immediately after which the storm produces a short episode of
severe weather. These storms generally are not tornado producers,
but often produce large hail and/or damaging winds. See overshooting
top, cyclic storm. |
| PVA |
Positive
Vorticity Advection. Advection of higher values of vorticity
into an area, which often is associated with upward motion (lifting)
of the air. PVA typically is found in advance of disturbances aloft
(i.e., shortwaves), and is a property which often enhances the potential
precipitation. |
| Q |
|
| QPF |
Quantitative
Precipitation Forecast |
| R |
|
| RADAR |
An instrument
used to detect precipitation by measuring the strength of the electromagnetic
signal reflected back. RADAR = RAdio Detection And Ranging. |
| RADIATION |
Energy
emitted in the form of electromagnetic waves. Radiation has differing
characteristics depending upon the wavelength. Radiation from the
Sun has a short wavelength (ultra-violet) while energy re-radiated
from the Earth's surface and the atmosphere has a long wavelength
(infra-red). |
| RADIATION
FOG |
Fog produced
over the land by the cooling of the lower atmosphere as it comes in
contact with the ground. Also known as ground fog. |
| RADIATIONAL
COOLING |
Cooling
process of the Earth's surface and adjacent air, which occurs when
infrared (heat) energy radiates from the surface of the Earth upward
through the atmosphere into space. Air near the surface transfers
its thermal energy to the nearby ground through cond | |
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