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MET 101
Spring 2008 UH Manoa
359
Meteorology
Undergraduate 1
05/01/2008

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Term

ATMOSHPHERE

Definition
THE ENVELOPE OF GASES THAT SURROUND A PLANET AND ARE HELD TO IT BY THE PLANET'S GRAVIATIONAL ATTRACTION.  THE EARTH'S ATMOSPHERE IS MAINLY NITROGEN AND OXYGEN.
Term
NITROGEN
Definition
(N2) A COLORLESS AND ODORLESS GAS THAT OCCUPIES ABOUT 78 PERCENT OF DRY AIR IN THE LOWER ATMOSPHERE.
Term
OXYGEN
Definition
(O2) A COLORLESS, ODORLESS GAS THAT OCCUPIES AOBUT 21 PERCENT OF DRY AIR IN THE LOWER ATMOSPHERE.
Term
WATER VAPOR
Definition
WATER IN A VAPOR (GASEOUS) FORM.  ALSO CALLED MOISTURE.
Term
CARBON DIOXIDE
Definition
(CO2) A COLORLESS, ODORLESS GAS WHOSE CONCENTRATION IS ABOUT 0.038 PERCENT (380 PPM) IN A VOLUME OF AIR NEAR SEA LEVEL.  IT IS A SELECTIVE ABSORBER OF INFRARED RADIATION AND, CONSEQUENTLY, IT IS IMPORTANT IN THE EARTH'S ATMOSPHERIC GREENHOUSE EFFECT.  SOLID CO2 IS CALLED DRY ICE.
Term
OZONE
Definition
(O3) AN ALMOST COLORLESS GASEOUS FORM OF OXYGEN WITH AN ODOR SIMILAR TO WEAK CHLORINE.  THE HIGHEST NATURAL CONCENTRATION IS FOUND IN THE STRATOSPHERE WHERE IT IS KNOWN AS STRATOSPHERIC OZONE.  IT ALSO FORMS IN POLLUTED AIR NEAR THE SURFACE WHERE IT IS THE MAIN INGREDIENT OF PHOTOCHEMICAL SMOG.  HERE, IT IS CALLED TROPOSPHERIC OZONE.
Term
AEROSOLS
Definition
TINY SUSPENDED SOLID PARTICLES (DUST, SMOKE, ETC.) OR LIQUID DROPLETS THAT ENTER THE ATMOSPHERE FROM EITHER NATURAL OR HUMAN (ANTHROPOGENIC) SOURCES, SUCH AS THE BURNING OF FOSSIL FUELS.  SULFUR-CONTAINING FOSSIL FUELS, SUCH AS COAL, PRODUCE SULFATE AEROSOLS.
Term
POLLUTANTS
Definition
ANY GASEOUS, CHEMICAL, OR ORGANIC MATTER THAT CONTAMINATES THE ATMOSPHERE, SOIL, OR WATER.
Term
OUTGASSING
Definition
THE RELEASE OF GASES DISSOLVED IN HOT, MOLTEN ROCK.
Term
AIR DENSITY
Definition
SEE DENSITY.
Term
AIR PRESSURE
Definition
(ATMOSPHERIC PRESSURE) THE PRESSURE EXERTED BY THE MASS OF AIR ABOVE A GIVEN POINT, USUALLY EXPRESSED IN MILLIBARS (MB), INCHES OF MERCURY (HG) OR IN HECTOPASCALS (HPA).
Term
LAPSE RATE
Definition
THE RATE AT WHICH AN ATMOSPHERIC VARIABLE (USUALLY TEMPERATURE) DECREASES WITH HEIGHT.  (SEE ENVIRONMENTAL LAPSE RATE.)
Term
TEMPERATURE INVERSION
Definition
AN INCREASE IN AIR TEMPERATURE WITH HEIGHT, OFTEN SIMPLY CALLED AN INVERSION.
Term
RADIOSONDE
Definition
A BALLOON-BORNE INSTRUMENT THAT MEASURES AND TRANSMITS PRESSURE, TEMPERATURE, AND HUMIDITY TO A GROUND-BASED RECEIVING STATION.
Term
TROPOSPHERE
Definition
THE LAYER OF ATMOSPHERE EXTENDING FROM THE EARTH'S SURFACE UP TO THE TROPOPAUSE (ABOUT 10 KM ABOVE THE GROUND).
Term
STRATOSPHERE
Definition
THE LAYER OF ATMOSPHERE ABOVE THE TROPOSPHERE AND BELOW THE MESOSPHERE (BETWEEN 10 KM AND 50 KM), GENERALLY CHARACTERIZED BY AN INCREASE IN TEMPERATURE WITH HEIGHT.
Term
TROPOPAUSE
Definition
THE BOUNDARY BETWEEN THE TROPOSPHERE AND THE STRATOSPHERE.
Term
MESOSPHERE
Definition
THE ATMOSPHERIC LAYER BETWEEN THE STRATOSPHERE AND THE THERMOSPHERE.  LOCATED AT AN AVERAGE ELEVATION BETWEEN 50 AND 80 KM ABOVE THE EARTH'S SURFACE.
Term
THERMOSPHERE
Definition
THE ATMOSPHERIC LAYER ABOVE THE MESOSPHERE (ABOVE ABOUT 85 KM) WHERE THE TEMPERATURE INCREASES RAPIDLY WITH HEIGHT.
Term
IONOSPHERE
Definition
AN ELECTRIFIED REGION OF THE UPPER ATMOSPHERE WHERE FAIRLY LARGE CONCENTRATIONS OF IONS AND FREE ELECTRONS EXIST.
Term
WEATHER
Definition
THE CONDITION OF THE ATMOSPHERE AT ANY PARTICULAR TIME AND PLACE.
Term
WEATHER ELEMENTS
Definition
THE ELEMENTS OF AIR TEMPERATURE, AIR PRESSURE, HUMIDITY, CLOUDS, PRECIPITATION, VISIBILITY, AND WIND THAT DETERMINE THE PRESENT STATE OF THE ATMOSPHERE-THE WEATHER.
Term
CLIMATE
Definition
THE ACCUMULATION OF DAILY AND SEASONAL WEATHER EVENTS OVER A LONG PERIOD OF TIME.
Term
METEOROLOGY
Definition
THE STUDY OF THE ATMOSPHERE AND ATMOSPHERIC PHENOMENA AS WELL AS THE ATMOSPHERE'S INTERACTION WITH THE EARTH'S SURFACE, OCEANS, AND LIFE IN GENERAL.
Term
MIDDLE LATITUDES
Definition
THE REGION OF THE WORLD TYPICALLY DESCRIBED AS BEING BETWEEN 30 DEGREES AND 50 DEGREES LATITUDE.
Term
MIDDLE-LATITUDE CYCLONIC STORM
Definition
SEE EXTRATROPICAL CYCLONE.
Term
HURRICANE
Definition
A TROPICAL CYCLONE HAVING WINDS IN EXCESS OF 64 KNOTS (74 MI/HR).
Term
THUNDERSTORM
Definition
A LOCAL STORM PRODUCED BY CUMULONIMBUS CLOUDS.  ALWAYS ACCOMPANIED BY LIGHTNING AND THUNDER.
Term
TORNADO
Definition
AN INTENSE, ROTATING COLUMN OF AIR THAT OFTEN PROTRUDES FROM A CUMULONIMBUS CLOUD IN THE SHAPE OF A FUNNEL OR A ROPE WHOSE CIRCULATION IS PRESENT ON THE GROUND.  (SEE FUNNEL CLOUD.)
Term
WIND
Definition
AIR IN MOTION RELATIVE TO THE EARTH'S SURFACE.
Term
WIND DIRECTION
Definition
THE DIRECTION FROM WHICH THE WIND IS BLOWING.
Term
FRONT
Definition
THE TRANSITION ZONE BETWEEN TWO DISTINCT AIR MASSES.
Term
KINETIC ENERGY
Definition
THE ENERGY WITHIN A BODY THAT IS A RESULT OF ITS MOTION.
Term
TEMPERATURE
Definition
THE DEGREE OF HOTNESS OR COLDNESS OF A SUBSTANCE AS MEASURED BY A THERMOMETER.  IT IS ALSO A MEASURE OF THE AVERAGE SPEED OR KINETIC ENERGY OF THE ATOMS AND MOLECULES IN A SUBSTANCE.
Term
ABSOLUTE ZERO
Definition
A TEMPERATURE READING OF -273 DEGREES C, -460 DEGREES F, OR 0 K.  THEORETICALLY, THERE IS NO MOLECULAR MOTION AT THIS TEMPERATURE.
Term
HEAT
Definition
A FORM OF ENERGY TRANSFERRED BETWEEN SYSTEMS BY VIRTURE OF THEIR TEMPERATURE DIFFERENCES.
Term
KELVIN SCALE
Definition
A TEMPERATURE SCALE WITH ZERO DEGREES EQUAL TO THE THEORETICAL TEMPERATURE AT WHICH ALL MOLECULAR MOTION CEASES.  ALSO CALLED THE ABSOLUTE SCALE.  THE UNITS ARE SOMETIMES CALLED "DEGREES KELVIN"; HOWEVER, THE CORRECT SI TERMINOLOGY IS "KELVINS," ABBREVIATED K.
Term
FAHRENHEIT SCALE
Definition
A TEMPERATURE SCALE WHERE 32 IS ASSIGNED TO THE TEMPERATURE AT WHICH WATER FREEZES AND 212 TO THE TEMPERATURE AT WHICH WATER BOILS (AT SEA LEVEL.)
Term
CELSIUS SCALE
Definition
A TEMPERATURE SCALE WHERE ZERO IS ASSIGNED TO THE TEMPERATURE WHERE WATER FREEZES AND 100 TO THE TEMPERATURE WHERE WATER BOILS (AT SEA LEVEL).
Term
LATENT HEAT
Definition
THE HEAT THAT IS EITHER RELEASED OR ABSORBED BYA UNIT MASS OF A SUBSTANCE WHEN IT UNDERGOES A CHANGE OF STATE, SUCH AS DURING EVAPORATION, CONDENSATION, OR SUBLIMATION.
Term
SENSIBLE HEAT
Definition
THE HEAT WE CAN FEEL AND MEASURE WITH A THERMOMETER.
Term
THERMALS
Definition
A SMALL, RISING PARCEL OF WARM AIR PRODUCED WHEN THE EARTH'S SURFACE IS HEATED UNEVENLY.
Term
ADVECTION
Definition
THE HORIZONTAL TRANSFER OF ANY ATMOSPHERIC PROPERTY BY THE WIND.
Term
RADIANT ENERGY (RADIATION)
Definition
(RADIATION) ENERGY PROPOGATED IN THE FORM OF ELECTROMAGNETIC WAVES.  THESE WAVES DO NOT NEED MOLECULES TO PROPGATE THEM, AND IN A VACUUM THEY TRAVEL AT NEARLY 300,000 KM PER SEC (186,000 MI PER SEC).
Term
ELECTROMAGNETIC WAVES
Definition
SEE RADIANT ENERGY.
Term
MICROMETER
Definition
A UNIT OF LENGTH EQUAL TO ONE-MILLIONTH OF A METER.
Term
PHOTONS
Definition
A DISCRETE UNIT OF ENERGY THAT CAN BE THOUGHT OF AS A PACKET OF ELECTROMAGNETIC RADIATION TRAVELING AT THE SPEED OF LIGHT.
Term
VISIBLE REGION
Definition
SEE VISIBLE RADIATION.
Term
ULTRAVIOLET RADIATION (UV)
Definition
ELECTROMAGNETIC RADIATION WITH WAVELENGTHS LONGER THAN X-RAYS BUT SHORTER THAN VISIBLE LIGHT.
Term
INFRARED RADIATION (IR)
Definition
ELECTROMAGNETIC RADIATION WITH WAVELENGTHS BETWEEN ABOUT 0.7 AND 1000 MICROMETERS.  THIS RADIATION IS LONGER THAN VISIBLE RADIATION BUT SHORTER THAN MICROWAVE RADIATION.
Term
BLACKBODY
Definition
A HYPOTHETICAL OBJECT THAT ABSORBS ALL OF THE RADIATION THAT STRIKES IT.  IT ALSO EMITS RADIATION AT A MAXIMUM RATE FOR ITS GIVEN TEMPERATURE.
Term
RADIATIVE EQUILIBRIUM TEMPERATURE
Definition
THE TEMPERATURE ACHIEVED WHEN AN OBJECT, BEHAVING AS A BLACKBODY, IS ABSORBING AND EMITTING RADIATION AT EQUAL RATES.
Term
SELECTIVE ABSORBERS
Definition
SUBSTANCES SUCH AS WATER VAPOR, CARBON DIOXIDE, CLOUDS, AND SNOW THAT ABSORB RADIATION ONLY AT PARTICULAR WAVELENGTHS.
Term
GREENHOUSE EFFECT
Definition
SEE ATMOSPHERIC GREENHOUSE EFFECT.
Term
ATMOSPHERIC WINDOW
Definition
THE WAVELENGTH RANGE BETWEEN 8 AND 11 MICROMETERS IN WHICH LITTLE ABSORPTION OF INFRARED RADIATION TAKES PLACE.
Term
SOLAR CONSTANT
Definition
THE RATE AT WHICH SOLAR ENERGY IS RECEIVED ON A SURFACE AT THE OUTER EDGE OF THE ATMOSPHERE PERPENDICULAR TO THE SUN'S RAYS WHEN THE EARTH IS AT A MEAN DISTANCE FROM THE SUN.  THE VALUE OF THE SOLAR CONSTANT IS ABOUT TWO CALORIES PER SQUARE CENTIMETER PER MINUTE OR ABOUT 1376 W/M2 IN THE SI SYSTEM OF MEASUREMENT. 
Term
SCATTERING
Definition
THE PROCESS BY WHICH SMALL PARTICLES IN THE ATMOSPHERE DEFLECT RADIATION FROM ITS PATH INTO DIFFERENT DIRECTIONS.
Term

REFLECTED (LIGHT)

Definition
SEE REFLECTION.
Term
ALBEDO
Definition
THE PERCENT OF RADIATION RETURNING FROM A SURFACE COMPARED TO THAT WHICH STRIKES IT.
Term
AURORA
Definition
GLOWING LIGHT DISPLAY IN THE NIGHT SKY CAUSED BY EXCITED GASES IN THE UPPER ATMOSPHERE GIVING OFF LIGHT.  IN THE NORTHERN HEMISPHERE IT IS CALLED THE AURORA BOREALIS (NORTHERN LIGHTS); IN THE SOUTHERN HEMISPHERE, THE AURORA AUSTRALIS (SOUTHERN LIGHTS).
Term
SUMMER SOLSTICE
Definition
APPROXIMATELY JUNE 21 IN THE NORTHERN HEMISPHERE WHEN THE SUN IS HIGHEST IN THE SKY AND DIRECTLY OVERHEAD AT LATITUDE 23 1/2 DEGREES N, THE TROPIC OF CANCER.
Term
AUTMNAL EQUINOX
Definition
THE EQUINOX AT WHICH THE SUN APPROACHES THE SOUTHERN HEMISPHERE AND PASSES DIRECTLY OVER THE EQUATOR.  OCCURS AROUND SEPTEMBER 23.
Term
INDIAN SUMMER
Definition
AN UNSEASONABLY WARM SPELL WITH CLEAR SKIES NEAR THE MIDDLE OF AUTUMN.  USUALLY FOLLOWS A SUBSTANTIAL PERIOD OF COOL WEATHER.
Term
WINTER SOLSTICE
Definition
APPROXIMATELY DECEMBER 21 IN THE NORTHERN HEMISPHERE WHEN THE SUN IS LOWEST IN THE SKY AND DIRECTLY OVERHEAD AT LATITUDE 23 1/2 DEGREES S, THE TROPIC OF CAPRICORN.
Term
VERNAL EQUINOX
Definition
THE EQUINOX AT WHICH THE SUN APPROACHES THE NORTHERN HEMISPHERE AND PASSES DIRECTLY OVER THE EQUATOR.  OCCURS AROUND MARCH 20.
Term
RADIATIONAL COOLING
Definition
THE PROCESS BY WHICH THE EARTH'S SURFACE AND ADJACENT AIR COOL BY EMITTING INFRARED RADIATION.
Term
RADIATION INVERSION
Definition
AN INCREASE IN TEMPERATURE WITH HEIGHT DUE TO RADIATIONAL COOLING OF THE EARTH'S SURFACE.  ALSO CALLED A NOCTURNAL INVERSION.
Term
THERMAL BELT
Definition
HORIZONTAL ZONES OF VEGETATION FOUND ALONG HILLSIDES THAT ARE PRIMARILY THE RESULT OF VERTICAL TEMPERATURE VARIATIONS.
Term
ORCHARD HEATER
Definition
OIL HEATERS PLACED IN ORCHARDS THAT GENERATE HEAT AND PROMOTE CONVECTIVE CIRCULATIONS TO PROTECT FRUIT TREES FROM DAMAGING LOW TEMPERATURES.  ALSO CALLED SMUDGE POTS.
Term
WIND MACHINE
Definition
FANS PLACED IN ORCHARDS FOR THE PURPOSE OF MIXING COLD SURFACE AIR WITH WARMER AIR ABOVE.
Term
FREEZE
Definition
A CONDITION OCCURING OVER A WIDESPREAD AREA WHEN THE SURFACE AIR TEMPERATURE REMAINS BELOW FREEZING FOR A SUFFICIENT TIME TO DAMAGE CERTAIN AGRICULTURAL CROPS.  A FREEZE MOST OFTEN OCCURS AS COLD AIR IS ADVECTED INTO A REGION, CAUSING FREEZING CONDITIONS TO EXIST IN A DEEP LAYER OF SURFACE AIR.  ALSO CALLED ADVECTION FROST.
Term
CONTROLS OF TEMPERATURE
Definition
THE MAIN FACTORS THAT CAUSE VARIATIONS IN TEMPERATURE FROM ONE PLACE TO ANOTHER.
Term
ISOTHERM
Definition
A LINE CONNECTING POINTS OF EQUAL TEMPERATURE.
Term
SPECIFIC HEAT
Definition
THE RATIO OF THE HEAT ABSORBED (OR RELEASED) BY THE UNIT MASS OF THE SYSTEM TO THE CORRESPONDING TEMPERATURE RISE (OR FALL).
Term
DAILY (DIURNAL) RANGE OF TEMPERATURE
Definition
THE DIFFERENCE BETWEEN THE MAXIMUM AND MINIMUM TEMPERATURES FOR ANY GIVEN DAY.
Term
MEAN (AVERAGE) DAILY TEMPERATURE
Definition
THE AVERAGE OF THE HIGHEST AND LOWEST TEMPERATURE FOR A 24-HOUR PERIOD.
Term
ANNUAL RANGE OF TEMPERATURE
Definition
THE DIFFERENCE BETWEEN THE WARMEST AND COLDEST MONTHS AT ANY GIVEN LOCATION.
Term
MEAN (AVERAGE) ANNUAL TEMPERATURE
Definition
THE AVERAGE TEMPERATURE AT ANY GIVEN LOCATION FOR THE ENTIRE YEAR.
Term
HEATING DEGREE-DAY
Definition
A FORM OF THE DEGREE-DAY USED AS AN INDEX FOR FUEL CONSUMPTION.
Term
COOLING DEGREE-DAY
Definition
A FORM OF DEGREE-DAY USED IN ESTIMATING THE AMOUNT OF ENERGY NECESSARY TO REDUCE THE EFFECTIVE TEMPERATURE OF WARM AIR.  A COOLING DEGREE-DAY IS A DAY ON WHICH THE AVERAGE TEMPERATURE IS ONE DEGREE ABOVE A DESIRED BASE TEMPERATURE.
Term
GROWING DEGREE-DAY
Definition
A FORM OF THE DEGREE-DAY USED AS A GUIDE FOR CROP PLANTING AND FOR ESTIMATING CROP MATURITY DATES.
Term
SENSIBLE TEMPERATURE
Definition
THE SENSATION OF TEMPERATURE THAT THE HUMAN BODY FEELS IN CONTRAST TO THE ACTUAL TEMPERATURE OF THE ENVIRONMENT AS MEASURED WITH A THERMOMETER.
Term
WIND-CHILL INDEX
Definition
THE COOLING EFFECT OF ANY COMBINATION OF TEMPERATURE AND WIN, EXPRESSED AS THE LOSS OF BODY HEAT.  ALSO CALLED WIND-CHILL FACTOR.
Term
FROSTBITE
Definition
THE PARTIAL FREEZING OF EXPOSED PARTS OF THE BODY, CAUSING INJURY TO THE SKIN AND SOMETIMES TO DEEPER TISSUES.
Term
HYPOTHERMIA
Definition

THE DETERIORATION IN ONE'S MENTAL AND PHYSICAL CONDITION BROUGHT ON BY A RAPID LOWERING OF HUMAN BODY TEMPERATURE.

Term
LIQUID-IN-GLASS THERMOMETER
Definition
SEE THERMOMETER.
Term
MAXIMUM THERMOMETER
Definition
A THERMOMETER WITH A SMALL CONSTRICTION JUST ABOVE THE BULB.  IT IS DESIGNED TO MEASURE THE MAXIMUM AIR TEMPERATURE.
Term
MINIMUM THERMOMETER
Definition
A THERMOMETER DESIGNED TO MEASURE THE MINIMUM TEMPERATURE DURING A DESIRED TIME PERIOD.
Term
ELECTRICAL THERMOMETER
Definition
THERMOMETERS THAT USE ELEMENTS THAT CONVERT ENERGY FROM ONE FORM TO ANOTHER (TRANSDUCERS).  COMMON ELECTRICAL THERMOMETERS INCLUDE THE ELECTRICAL RESISTANCE THERMOMETER, THERMOCOUPLE, AND THERMISTOR.
Term
RADIOMETER
Definition
SEE INFRARED RADIOMETER.
Term
BIMETALLIC THERMOMETER
Definition
A TEMPERATURE-MEASURING DEVICE USUALLY CONSISTING OF TWO DISSIMILAR METALS THAT EXPAND AND CONTRACT DIFFERENTIALLY AS THE TEMPERATURE CAHNGES.
Term
THERMOGRAPH
Definition
AN INSTRUMENT THAT MEASURES AND RECORDS AIR TEMPERATURE.
Term
INSTRUMENT SHELTER
Definition
A BOXLIKE, OFTEN WOODEN, STRUCTURE DESIGNED TO PROTECT WEATHER INSTRUMENTS FROM DIRECT SUNSHINE AND PRECIPITATION.
Term
EVAPORATION
Definition
THE PROCESS BY WHICH A LIQUID CHANGES INTO A GAS.
Term
CONDENSATION
Definition
THE PROCESS BY WHICH WATER VAPOR BECOMES A LIQUID.
Term
PRECIPITATION
Definition
ANY FORM OF WATER PARTICLES-LIQUID OR SOLID-THAT FALLS FROM THE ATMOSPHERE AND REACHES THE GROUND.
Term
HYDROLOGIC CYCLE
Definition
A MODEL THAT ILLUSTRATES THE MOVEMENT AND EXCHANGE OF WATER AMONG THE EARTH, ATMOSPHERE, AND OCEANS.
Term
SATURATED AIR
Definition
AN ATMOSPHERIC CONDITION WHEREBY THE LEVEL OF WATER VAPOR IS THE MAXIMUM POSSIBLE AT THE EXISTING TEMEPRATURE AND PRESSURE.
Term
CONDENSATION NUCLEI
Definition
ALSO CALLED CLOUD CONDENSATION NUCLEI.  TINY PARTICLES UPON WHOSE SURFACES CONDENSATION OF WATER VAPOR BEGINS IN THE ATMOSPHERE.
Term
HUMIDITY
Definition
A GENERAL TERM THAT REFER'S TO THE AIR'S WATER VAPOR CONTENT.  (SEE RELATIVE HUMIDITY.)
Term
ACTUAL VAPOR PRESSURE
Definition
SEE VAPOR PRESSURE.
Term
SATURATION VAPOR PRESSURE
Definition
THE MAXIMUM AMOUNT OF WATER VAPOR NECESASRY TO KEEP MOIST AIR IN EQUILIBRIUM WITH A SURFACE OF PURE WATER OR ICE.  IT REPRESENTS THE MAXIMUM AMOUNT OF WATER VAPOR THAT THE AIR CAN HOLD AT ANY GIVEN TEMPERATURE AND PRESSURE.  (SEE EQUILIBRIUM VAPOR PRESSURE.)
Term
RELATIVE HUMIDITY
Definition
THE RATIO OF THE AMOUNT OF WATER VAPOR IN THE AIR COMPARED TO THE AMOUNT REQUIRED FOR SATURATION (AT A PARTICULAR TEMPERATURE AND PRESSURE).  THE RATIO OF THE AIR'S ACTUAL VAPOR PRESSURE TO ITS SATURATION VAPOR PRESSURE.
Term
SUPERSATURATED AIR
Definition
A CONDITION WHEREBY THE ATMOSPHERE CONTAINS MORE WATER VAPOR THAN IS NEEDED TO PRODUCE SATURATION WITH RESPECT TO A FLAT SURFACE OF PURE WATER OR ICE, AND THE RELATIVE HUMIDITY IS GREATER THAN 100 PERCENT.
Term
DEW-POINT TEMPERATURE (DEW POINT)
Definition

THE TEMPERATURE TO WHICH THE AIR MUST BE COOLED (AT CONSTANT PRESSURE AND CONSTANT WATER VAPOR CONTENT) FOR SATURATION TO OCCUR.

Term
WET-BULB TEMPERATURE
Definition
THE LOWEST TEMPERAURE THAT CAN BE OBTAINED BY EVAPORATING WATER INTO THE AIR.
Term
HEAT INDEX (HI)
Definition
AN INDEX THAT COMBINES AIR TEMPERATURE AND RELATIVE HUMIDITY TO DETERMINE AN APPARENT TEMPERATURE-HOW HOT IT ACTUALLY FEELS.
Term
APPARENT TEMPERATURE
Definition
WHAT THE AIR TEMPERATURE "FEELS LIKE" FOR VARIOUS COMBINATIONS OF AIR TEMPERATURE AND RELATIVE HUMIDITY.
Term
PSYCHROMETER
Definition
AN INSTRUMENT USED TO MEASURE THE WATER VAPOR CONTENT OF THE AIR.  IT CONSISTS OF TWO THERMOMETERS (DRY BULB AND WET BULB).  AFTER WHIRLING THE INSTRUMENT, THE DEW POINT AND RELATIVE HUMIDITY CAN BE OBTAINED WITH THE AID OF TABLES.
Term
HYGROMETER
Definition
AN INSTRUMENT DESIGNED TO MEASURE THE AIR'S WATER VAPOR CONTENT.  THE SENSING PART OF THE INSTRUMENT CAN BE HAIR (HAIR HYGROMETER), A PLATE COATED WITH CARBON (ELECTRICAL HYGROMETER), OR AN INFRARED SENSOR (INFRARED HYGROMETER).
Term
DEW
Definition
WATER THAT HAS CONDENSED ONTO OBJECTS NEAR THE GROUND WHEN THEIR TEMPERATURES HAVE FALLEN BELOW THE DEW POINT OF THE SURFACE AIR.
Term
FROST
Definition
(ALSO CALLED HOARFROST)  A COVERING OF ICE PRODUCED BY DEPOSITION ON EXPOSED SURFACES WHEN THE AIR TEMPERATURE FALLS BELOW THE FROST POINT.
Term
HAZE
Definition
FINE DRY OR WET DUST OR SALT PARTICLES DISPERSED THROUGH A PORTION OF THE ATMOSPHERE.  INDIVIDUALLY THESE ARE NOT VISIBLE BUT CUMULATIVELY THEY WILL DIMINISH VISIBILITY.  DRY HAZE PARTICLES ARE VERY SMALL, ON THE ORDER OF 0.1 MICROMETERS.  WET HAZE PARTICLES ARE LARGER.
Term
FOG
Definition
A CLOUD WITH ITS BASE AT THE EARTH'S SURFACE.
Term
RADIATION FOG
Definition
FOG PRODUCED OVER LAND WHEN RADIATIONAL COOLING REDUCES THE AIR TEMPERATURE TO OR BELOW ITS DEW POINT.  IT IS ALSO KNOWN AS GROUND FOG AND VALLEY FOG.
Term
ADVECTION FOG
Definition
OCCURS WHEN WARM, MOIST AIR MOVES OVER A COLD SURFACE AND THE AIR COOLS TO BELOW ITS DEW POINT.
Term
UPSLOPE FOG
Definition
FOG FORMED AS MOIST, STABLE AIR FLOWS UPWARD OVER A TOPOGRAPHIC BARRIER.
Term
EVAPORATION (MIXING) FOG
Definition
FOG PRODUCED WHEN SUFFICIENT WATER VAPOR IS ADDED TO THE AIR BY EVAPORATION, AND THE MOIST AIR MIXES WITH RELATIVELY DRIER AIR.  THE TWO COMMON TYPES ARE STEAM FOG, WHICH FORMS WHEN COLD AIR MOVE OVER WARM WATER, AND FRONTAL FOG, WHICH FORMS AS WARM RAINDROPS EVAPORATE IN A COOL AIR MASS.
Term
CIRRUS CLOUDS
Definition
A HIGH CLOUD COMPOSED OF ICE CRYSTALS IN THE FORM OF THIN, WHITE, FEATHERLIKE CLOUDS IN PATCHES, FILAMENTS, OR NARROW BANDS.
Term
CIRROCUMULUS CLOUDS
Definition
A HIGH CLOUD THAT APPEARS AS A WHITE PATCH OF CLOUDS WITHOUT SHADOWS.  IT CONSISTS OF VERY SMALL ELEMENTS IN THE FORM OF GRAINS OR RIPPLES.
Term
CIRROSTRATUS CLOUDS
Definition
HIGH, THIN, SHEETLIKE CLOUD, COMPOSED OF ICE CRYSTALS. THEY FREQUENTLY COVER THE ENTIRE SKY AND OFTEN PRODUCE A HALO.
Term
ALTOCUMULUS CLOUDS
Definition
A MIDDLE CLOUD, USUALLY WHITE OR GRAY.  OFTEN OCCURS IN LAYERS OR PATCHES WITH WAVY, ROUNDED MASSES OR ROLLS.
Term
ALTOSTRATUS CLOUDS
Definition
A MIDDLE CLOUD COMPOSED OF GRAY OR BLUISH SHEETS OR LAYERS OF UNIFORM APPEARANCE.  IN THE THINNER REGIONS, THE SUN OR MOON USUALLY APPEARS DIMLY VISIBLE.
Term
NIMBOSTRATUS CLOUDS
Definition
A DARK, GRAY CLOUD CHARACTERIZED BY MORE OR LESS CONTINUOUSLY FALLING PRECIPITATION.  IT IS RARELY ACCOMPANIED BY LIGHTNING, THUNDER, OR HAIL.
Term
STRATOCUMULUS CLOUDS
Definition
A LOW CLOUD, PREDOMINANTLY STRATIFORM, WITH LOW, LUMPY, ROUNDED MASSES, OFTEN WITH BLUE SKY BETWEEN THEM.
Term
STRATUS CLOUDS
Definition
A LOW, GRAY CLOUD LAYER WITH A RATHER UNIFORM BASE WHOSE PRECIPITATION IS MOST COMMONLY DRIZZLE.
Term
CUMULUS CLOUDS
Definition
A CLOUD IN THE FORM OF INDIVIDUAL, DETACHED DOMES OR TOWERS THAT ARE USUALLY DENSE AND WELL DEFINED.  IT HAS A FLAT BASE WITH A BULGING UPPER PART THAT OFTEN RESEMBLES CAULIFLOWER.  CUMULUS CLOUDS OF FAIR WEATHER ARE CALLED CUMULUS HUMILIS.  THOSE THAT EXHIBIT MUCH VERTICAL GROWTH ARE CALLED CUMULUS CONGESTUS OR TOWERING CUMULUS.
Term
CUMULONIMBUS CLOUDS
Definition
AN EXCEPTIONALLY DENSE AND VERTICALLY DEVELOPED CLOUD, OFTEN WITH A TOP IN THE SHAPE OF AN ANVIL.  THE CLOUD IS FREQUENTLY ACCOMPANIED BY HEAVY SHOWERS, LIGHTNING, THUNDER, AND SOMETIMES HAIL.  IT IS ALSO KNOWN AS A THUNDERSTORM CLOUD.
Term
LENTICULAR CLOUDS
Definition
A CLOUD IN THE SHAPE OF A LENS.
Term
PILEUS CLOUDS
Definition
A SMOOTH CLOUD IN THE FORM OF A CAP.  OCCURS ABOVE, OR IS ATTACHED TO, THE TOP OF A CUMULIFORM CLOUD.  ALSO CALLED A CAP CLOUD.
Term
MAMMATUS CLOUDS
Definition
CLOUDS THAT LOOK LIKE POUCHES HANGING FROM THE UNDERSIDE OF A CLOUD.
Term
CONTRAIL
Definition
(CONDENSATION TRAIL)  A CLOUDLIKE STREAMER FREQUENTLY SEEN FORMING BEHIND AIRCRAFT FLYING IN CLEAR, COLD, HUMID AIR.
Term
NACREOUS CLOUDS
Definition
CLOUDS OF UNKNOWN COMPOSITION THAT HAVE A SOFT, PEARLY LUSTER AND THAT FORM AT ALTITUDES ABOUT 25 TO 30 KM ABOVE THE EARTH'S SURFACE.  THEY ARE ALSO CALLED MOTHER-OF-PEARL CLOUDS.
Term
NOCTILUCENT CLOUDS
Definition
WAVY, THIN, BLUISH-WHITE CLOUDS THAT ARE BEST SEEN AT TWILIGHT IN POLAR LATITUDES.  THEY FORM AT ALTITUDES ABOUT 80 TO 90 KM ABOVE THE SURFACE.
Term
ADIABATIC PROCESS
Definition
A PROCESS THAT TAKES PLACE WITHOUT A TRANSFER OF HEAT BETWEEN THE SYSTEM (SUCH AS AN AIR PARCEL) AND ITS SURROUNDINGS.  IN AN ADIABATIC PROCESS, COMPRESSION ALWAYS RESULTS IN WARMING, AND EXPANSION RESULTS IN COOLING.
Term
DRY ADIABATIC RATE
Definition
THE RATE OF CHANGE OF TEMPERATURE IN A RISING OR DESCENDING UNSATURATED AIR PARCEL.  THE RATE OF ADIABATIC COOLING OR WARMING IS ABOUT 10 DEGREES C PER 1000 M (5.5 DEGREES F PER 1000 FT).
Term
MOIST ADIABATIC RATE
Definition
THE RATE OF CHANGE OF TEMPERATURE IN A RISING OR DESCENDING SATURATED AIR PARCEL.  THE RATE OF COOLING OR WARMING VARIES BUT A COMMON VALUE OF 6 DEGREES C PER 1000 M (3.3 DEGREES F PER 1000 FT) IS USED.
Term
ENVIRONMENTAL LAPSE RATE
Definition
THE RATE OF DECREASE OF AIR TEMPERATURE WITH ELEVATION.  IT IS MOST OFTEN MEASURED WITH A RADIOSONDE.
Term
ABSOLUTELY STABLE ATMOSPHERE
Definition
AN ATMOSPHERIC CONDITION THAT EXISTS WHEN THE ENVIRONMENTAL LAPSE RATE IS LESS THAN THE MOIST ADIABATIC RATE.  THIS RESULTS IN A LIFTED PARCEL OF AIR BEING COLDER THAN THE AIR AROUND IT.
Term
ABSOLUTELY UNSTABLE ATMOSPHERE
Definition
AN ATMOSPHERIC CONDITION THAT EXISTS WHEN THE ENVIRONMENTAL LAPSE RATE IS GREATER THAN THE DRY ADIABATIC RATE.  THIS RESULTS IN A LIFTED PARCEL OF AIR BEING WARMER THAN THE AIR AROUND IT.
Term
CONDENSATION LEVEL
Definition
THE LEVEL ABOVE THE SURFACE MARKING THE BASE OF A CUMULIFORM CLOUD.
Term
CONDITIONALLY UNSTABLE ATMOSPHERE
Definition
AN ATMOSPHERIC CONDITION THAT EXISTS WHEN THE ENVIRONMENTAL LAPSE RATE IS LESS THAN THE DRY ADIABATIC RATE BUT GREATER THAN THE MOIST ADIABATIC RATE.  ALSO CALLED CONDITIONAL INSTABILITY.
Term
OROGRAPHIC UPLIFT
Definition
THE LIFTING OF AIR OVER A TOPOGRAPHIC BARRIER.  CLOUDS THAT FORM IN THIS LIFTING PROCESS ARE CALLED OROGRAPHIC CLOUDS.
Term
RAIN SHADOW
Definition
THE REGION ON THE LEESIDE OF A MOUNTAIN WHERE THE PRECIPITATION IS NOTICEABLY LESS THAN ON THE WINDWARD SIDE.
Term
PRECIPITATION
Definition
ANY FORM OF WATER PARTICLES-LIQUID OR SOLID-THAT FALLS FROM THE ATMOSPHERE AND REACHES THE GROUND.
Term
COLLISION-COALESCENCE PROCESS
Definition
THE PROCESS OF PRODUCING PRECIPITATION BY LIQUID PARTICLES (CLOUD DROPLETS AND RAINDROPS) COLLIDING AND JOINING (COALESCING).
Term
COALESCENCE
Definition
THE MERGING OF CLOUD DROPLETS INTO A SINGLE LARGER DROPLET.
Term
ICE-CRYSTAL (BERGERON) PROCESS
Definition

 A PROCESS THAT PRODUCES PRECIPITATION.  THE PROCESS INVOLVES TINY ICE CRYSTALS IN A SUPERCOOLED CLOUD GROWING LARGER AT THE EXPENSE OF THE SURROUNDING LIQUID DROPLETS.  ALSO CALLED THE BERGERON PROCESS.

Term
SUPERCOOLED (WATER DROPLET)
Definition
A CLOUD COMPOSED OF LIQUID DROPLETS AT TEMPERATURES BELOW 0 DEGREES C (32 DEGREES F).  WHEN THE CLOUD IS ON THE GROUND IT IS CALLED SUPERCOOLED FOG OR COLD FOG.
Term
ICE NUCLEI
Definition
PARTICLES THAT ACT AS NUCLEI FOR THE FORMATION OF ICE CRYSTALS IN THE ATMOSPHERE.
Term
ACCRETION
Definition
THE GROWTH OF A PRECIPITATION PARTICLE BY THE COLLISION OF AN ICE CRYSTAL OR SNOWFLAKE WITH A SUPERCOOLED LIQUID DROPLET THAT FREEZES UPON IMPACT.
Term
CLOUD SEEDING
Definition
THE INTRODUCTION OF ARTIFICIAL SUBSTANCES (USUALLY SILVER IODIDE OR DRY ICE) INTO A CLOUD FOR THE PURPOSE OF EITHER MODIFYING ITS DEVELOPMENT OR INCREASING ITS PRECIPITATION.
Term
RAIN
Definition
PRECIPITATION IN THE FORM OF LIQUID WATER DROPS THAT HAVE DIAMETERS GREATER THAN THAT OF DRIZZLE.
Term
DRIZZLE
Definition
SMALL WATER DROPS BETWEEN 0.2 AND 0.5 MM IN DIAMETER THAT FALL SLOWLY AND REDUCE VISIBILITY MORE THAN LIGHT RAIN.
Term
VIRGA
Definition
PRECIPITATION THAT FALLS FROM A CLOUD BUT EVAPORATES BEFORE REACHING THE GROUND.  (SEE FALLSTREAKS.)
Term
SHOWER (RAIN)
Definition
INTERMITTENT PRECIPITATION FROM A CUMULIFORM CLOUD, USUALLY OF SHORT DURATION BUT OFTEN HEAVY.
Term
SNOW
Definition
A SOLID FORM OF PRECIPITATION COMPOSED OF ICE CRYSTALS IN COMPLEX HEXAGONAL FORM.
Term
FALLSTREAKS
Definition
FALLING ICE CRYSTALS THAT EVAPORATE BEFORE REACHING THE GROUND.
Term

FLURRIES (OF SNOW)

Definition
LIGHT SHOWERS OF SNOW THAT FALL INTERMITTENTLY.
Term
SNOW SQUALL
Definition
(SHOWER) AN INTERMITTENT HEAVY SHOWER OF SNOW THAT GREATLY REDUCES VISIBILITY.
Term
BLIZZARD
Definition
A SEVERE WEATHER CONDITION CHARACTERIZED BY LOW TEMPERATURES AND STRONG WINDS (GREATER THAN 35 MI/HR) BEARING A GREAT AMOUNT OF SNOW EITHER FALLING OR BLOWING. WHEN THESE CONDITIONS CONTINUE AFTER THE FALLING SNOW HAS ENDED, IT IS TERMED A GROUND BLIZZARD.
Term

SLEET

Definition
A TYPE OF PRECIPITATION CONSISTING OF TRANSPARENT PELLETS OF ICE 5 MM OR LESS IN DIAMETER.  SAME AS ICE PELLETS.
Term
FREEZING RAIN (GLAZE)
Definition
RAIN OR DRIZZLE THAT FALLS IN LIQUID FORM AND THEN FREEZES UPON STRIKING A COLD OBJECT OR GROUND.  BOTH CAN PRODUCE A COATING OF ICE ON OBJECTS WHICH IS CALLED GLAZE.
Term
RIME
Definition
A WHITE OR MILKY GRANULAR DEPOSIT OF ICE FORMED BY THE RAPID FREEZING OF SUPERCOOLED WATER DROPS AS THEY COME IN CONTACT WITH AN OBJECT IN BELOW-FREEZING AIR.
Term
SNOW GRAINS
Definition
PRECIPITATION IN THE FORM OF VERY SMALL, OPAQUE GRAINS OF ICE.  THE SOLID EQUIVALENT OF DRIZZLE.
Term
SNOW PELLETS
Definition
WHITE, OPAQUE, APPROXIMATELY ROUND ICE PARTICLES BETWEEN 2 AND 5 MM IN DIAMETER THAT FORM IN A CLOUD EITHER FROM THE STICKING TOGETHER OF ICE CRYSTALS OR FROM THE PROCESS OF ACCRETION.  ALSO CALLED GRAUPEL.
Term
HAILSTONES
Definition
TRANSPARENT OR PARTIALLY OPAQUE PARTICLES OF ICE THAT RANGE IN SIZE FROM THAT OF A PEA TO THAT OF GOLF BALLS.
Term
STANDARD RAIN GAUGE
Definition
A NONRECORDING RAIN GAUGE WITH AN 8-INCH-DIAMETER COLLECTOR FUNNEL AND A TUBE THAT AMPLIFIES RAINFALL BY TENFOLD.
Term
TRACE (OF PRECIPITATION)
Definition
AN AMOUNT OF PRECIPITATION LESS THAN 0.01 IN. (0.025 CM).
Term
WATER EQUIVALENT
Definition
THE DEPTH OF WATER THAT WOULD RESULT FROM THE  MELTING OF A SNOW SAMPLE.  TYPICALLY, ABOUT 10 INCHES OF SNOW WILL MELT TO 1 INCH OF WATER, PRODUCING A WATER EQUIAVALENT OF 10 TO 1.
Term
RADAR
Definition
AN ELECTRONIC INSTRUMENT USED TO DETECT OBJECTS (SUCH AS FALLING PRECIPITATION) BHY THEIR ABILITY TO REFLECT AND SCATTER MICROWAVES BACK TO A RECEIVER.  (SEE ALSO DOPPLER RADAR.)
Term
DOPPLER RADAR
Definition
A RADAR THAT DETERMINES THE VELOCITY OF FALLING PRECIPITATION EITHER TOWARD OR AWAY FROM THE RADAR UNIT BY TAKING INTO ACCOUNT THE DOPPLER SHIFT.
Term
AIR PRESSURE
Definition
(ATMOSPHERIC PRESSURE) THE PRESSURE EXERTED BY THE MASS OF AIR ABOVE A GIVEN POINT, USUALLY EXPRESSED IN MILLIBARS (MB), INCHES OF MERCURY (HG) OR IN HECTOPASCALS (HPA).
Term
MILLIBAR
Definition
A UNIT OF EXPRESSING ATMOSPHERIC PRESSURE. SEA-LEVEL PRESSURE IS NORMALLY CLOSE TO 1013 MB.
Term
STANDARD ATMOSPHERIC PRESSURE
Definition
A PRESSURE OF 1013.25 MILLIBARS (MB), 29.92 INCHES OF MERCURY (HG), 760 MILLIMETERS (MM) OF MERCURY, 14.7 POUNDS PER SQUARE INCH (LB/IN.2), OR 1013.25 HECTOPASCALS (HPA).
Term

BAROMETER

Definition
AN INTSRUMENT THAT MEASURES ATMOSPHERIC PRESSURE. THE TWO MOST COMMON BAROMETERS ARE THE MERCURY BAROMETER AND THE ANEROID BAROMETER.
Term
MERCURY BAROMETER
Definition
A TYPE OF BAROMETER THAT USES MERCURY TO MEASURE ATMOSPHERIC PRESSURE. THE HEIGHT OF THE MERCURY COLUMN IS A MEASURE OF ATMOSPHERIC PRESSURE.
Term
ANEROID BAROMETER
Definition
AN INSTRUMENT DESIGNED TO MEASURE ATMOSPHERIC PRESSURE. IT CONTAINS NO LIQUID.
Term
STATION PRESSURE
Definition
THE ACTUAL AIR PRESSURE COMPUTED AT THE OBSERVING STATION.
Term
SEA-LEVEL PRESSURE
Definition
THE ATMOSPHERIC PRESSURE AT MEAN SEA LEVEL.
Term
ISOBAR
Definition
A LINE CONNECTING POINTS OF EQUAL PRESSURE.
Term
SURFACE MAP
Definition
A MAP THAT SHOWS THE DISTRIBUTION OF SEA-LEVEL PRESSURE WITH ISOBARS AND WEATHER PHENOMENA. ALSO CALLED A SURFACE CHART.
Term
ANTICYCLONE
Definition
AN AREA OF HIGH ATMOSPHERIC PRESSURE AROUND WHICH THE WIND BLOWS CLOCKWISE IN THE NORTHERN HEMISPHERE AND COUNTERCLOCKWISE IN THE SOUTHERN HEMISPHERE. ALSO CALLED A HIGH.
Term
MID-LATITUDE CYCLONE
Definition
SEE EXTRATROPICAL CYCLONE.
Term
ISOBARIC MAP
Definition
(CHART) SEE CONSTANT-PRESSURE CHART.
Term
CONTOUR LINE
Definition
A LINE THAT CONNECTS POINTS OF EQUAL ELEVATION ABOVE A REFERENCE LEVEL, MOST OFTEN SEA LEVEL.
Term
RIDGE
Definition
AN ELONGATED AREA OF HIGH ATMOSPHERIC PRESSURE.
Term
TROUGH
Definition
AN ELONGATED AREA OF LOW ATMOSPHERIC PRESSURE.
Term
PRESSURE GRADIENT
Definition
THE RATE OF DECREASE OF PRESSURE PER UNIT OF HORIZONTAL DISTANCE. ON THE SAME CHART, WHEN THE ISOBARS ARE CLOSE TOGETHER, THE PRESSURE GRADIENT IS STEEP. WHEN THE ISOBARS ARE FAR APART, THE PRESSURE GRADIENT IS WEAK.
Term
PRESSURE GRADIENT FORCE
Definition
(PGF) THE FORCE DUE TO DIFFERENCES IN PRESSURE WITHIN THE ATMOSPHERE THAT CAUSES AIR TO MOVE AND, HENCE, THE WIND TO BLOW. IT IS DIRECTLY PROPORTIONAL TO THE PRESSURE GRADIENT.
Term
CORIOLIS FORCE
Definition
AN APPARENT FORCE OBSERVED ON ANY FREEMOVING OBJECT IN A ROTATING SYSTEM. ON THE EARTH, THIS DEFLECTIVE FORCE RESULTS FROM THE EARTH'S ROTATION AND CAUSES MOVING PARTICLES (INCLUDING THE WIND) TO DEFLECT TO THE RIGHT IN THE NORTHERN HEMISPHERE AND TO THE LEFT IN THE SOUTHERN HEMISPHERE.
Term
GEOSTROPHIC WIND
Definition
A THEORETICAL HORIZONTAL WIND BLOWING IN A STRAIGHT PATH, HORIZONTAL TO THE ISOBARS OR CONTOURS, AT A CONSTANT SPEED. THE GEOSTROPHIC WIND RESULTS WHEN THE CORIOLIS FORCE EXACTLY BALANCES THE HORIZONTAL PRESSURE GRADIENT FORCE.
Term
GRADIENT WIND
Definition
A THEORETICAL WIND THAT BLOWS PARALLEL TO CURVED ISOBARS OR CONTOURS.
Term
CENTRIPETAL FORCE
Definition
THE RADIAL FORCE REQUIRED TO KEEP AN OBJECT MOVING IN A CIRCULAR PATH. IT IS DIRECTED TOWARD THE CENTER OF THAT CURVED PATH.
Term
MERIDIONAL (WIND FLOW)
Definition
A TYPE OF ATMOSPHERIC CIRCULATION PATTERN IN WHICH THE NORTH-SOUTH COMPONENT OF THE WIND IS PRONOUNCED.
Term

ZONAL FLOW

Definition
A WIND THAT HAS A PREDOMINANT WEST-TO-EAST COMPONENT.
Term
FRICTION LAYER
Definition
THE ATMOSPHERIC LAYER NEAR THE SURFACE USUALLY EXTENDING UP TO ABOUT 1 KM (3300 FT) WHERE THE WIND IS INFLUENCED BY FRICTION OF THE EARTH'S SURFACE AND OBJECTS ON IT. ALSO CALLED THE ATMOSPHERIC BOUNDARY LAYER AND PLANETARY BOUNDARY LAYER.
Term
HYDROSTATIC EQUILIBRIUM
Definition
THE STATE OF THE ATMOSPHERE WHEN THERE IS A BALANCE BETWEEN THE VERTICAL PRESSURE GRADIENT FORCE AND THE DOWNWARD PULL OF GRAVITY.
Term
ONSHORE WIND
Definition
A BREEZE THAT BLOWS FROM THE WATER ONTO THE LAND. OPPOSITE OF AN OFFSHORE WIND.
Term
OFFSHORE WIND
Definition
A BREEZE THAT BLOWS FROM THE LAND OUT OVER THE WATER. OPPOSITE OF AN ONSHORE WIND.
Term
PREVAILING WIND
Definition
THE WIND DIRECTION MOST FREQUENTLY OBSERVED DURING A GIVEN PERIOD.
Term
WIND ROSE
Definition
A DIAGRAM THAT SHOWS THE PERCENT OF TIME THAT THE WIND BLOWS FROM DIFFERENT DIRECTIONS AT A GIVEN LOCATION OVER A GIVEN TIME.
Term
WIND VANE
Definition
AN INSTRUMENT USED TO INDICATE WIND DIRECTION.
Term
ANEMOMETER
Definition
AN INSTRUMENT DESIGNED TO MEASURE WIND SPEED.
Term
AEROVANE
Definition
A WIND INSTRUMENT THAT INDICATES OR RECORDS BOTH WIND SPEED AND WIND DIRECTION.  ALSO CALLED A SKYVANE.
Term
WIND PROFILER
Definition
A DOPPLER RADAR CAPABLE OF MEASURING THE TURBULENT EDDIES THAT MOVE WITH THE WIND.  BECAUSE OF THIS, IT IS ABLE TO PROVIDE A VERTICAL PICTURE OF WIND SPEED AND WIND DIRECTION.
Term
SCALES OF MOTION
Definition
THE HIERARCHY OF ATMOSPHERIC CIRCULATIONS FROM TINY GUSTS TO GIANT STORMS.
Term
MICROSCALE
Definition
THE SMALLEST SCALE OF ATMOSPHERIC MOTIONS.
Term
MESOSCALE
Definition
THE SCALE OF METEORLOGICAL PHENOMENA THAT RANGE IN SIZE FROM A FEW KM TO ABOUT 100 KM.  IT INCLUDES LOCAL WINDS, THUNDERSTORMS, AND TORNADOES.
Term
SYNOPTIC SCALE
Definition
THE TYPICAL WEATHER MAP SCALE THAT SHOWS FEATURES SUCH AS HIGH- AND LOW-PRESSURE AREAS AND FRONTS OVER A DISTANCE SPANNING A CONTINENT.  ALSO CALLED THE CYCLONIC SCALE.
Term
PLANETARY SCALE
Definition
THE LARGEST SCALE OF ATMOSPHERIC MOTION.  SOMETIMES CALLED THE GLOBAL SCALE.
Term
ROTOR
Definition
TURBULENT EDDIES THAT FORM DOWNWIND OF A MOUNTAIN CHAIN, CREATING HAZARDOUS FLYING CONDITIONS.
Term
WIND SHEAR
Definition
THE RATE OF CHANGE OF WIND SPEED OR WIND DIRECTION OVER A GIVEN DISTANCE.
Term
CLEAR AIR TURBULENCE (CAT)
Definition
TURBULENCE ENCOUNTERED BY AIRCRAFT FLYING THROUGH CLOUDLESS SKIES.  THERMALS, WIND SHEAR, AND JET STREAMS CAN EACH BE A FACTOR IN PRODUCING CAT.
Term
THERMAL CIRCULATION
Definition
AIR FLOW RESULTING PRIMARILY FROM THE HEATING AND COOLING OF AIR.
Term
SEA BREEZE
Definition
A COASTAL LOCAL WIND THAT BLOWS FROM THE OCEAN ONTO THE LAND.  THE LEADING EDGE OF THE BREEZE IS TERMED A SEA BREEZE FRONT.
Term
LAND BREEZE
Definition
A COASTAL BREEZE THAT BLOWS FROM LAND TO SEA, USUALLY AT NIGHT.
Term
MONSOON WIND SYSTEM
Definition
A WIND SYSTEM THAT REVERSES DIRECTION BETWEEN WINTER AND SUMMER.  USUALLY THE WIND BLOWS FROM LAND TO SEA IN WINTER AND FROM SEA TO LAND IN SUMMER.
Term
VALLEY BREEZE
Definition
SEE MOUNTAIN BREEZE.
Term
MOUNTAIN BREEZE
Definition
A LOCAL WIND SYSTEM OF A MOUNTAIN VALLEY THAT BLOWS DOWNHILL (MOUNTAIN BREEZE) AT NIGHT AND UPHILL (VALLEY BREEZE) DURING THE DAY.
Term
KATABATIC WIND
Definition

ANY WIND BLOWING DOWNSLOPE.  IT IS USUALLY COLD.

Term
CHINOOK WIND
Definition
A WARM, DRY WIND ON THE EASTERN SIDE OF THE ROCKY MOUNTAINS.  IN THE ALPS, THE WIND IS CALLED A FOEHN.
Term
SANTA ANA WIND
Definition
A WARM, DRY WIND THAT BLOWS INTO SOUTHERN CALIFORNIA FROM THE EAST OFF THE ELEVATED DESERT PLATEAU.  ITS WARMTH IS DERIVED FROM COMPRESSIONAL HEATING.
Term
HABOOB
Definition
A DUST OR SANDSTORM THAT FORMS AS COLD DOWNDRAFTS FROM A THUNDERSTORM TURBULENTLY LIFT DUST AND SAND INTO THE AIR.
Term
DUST DEVILS (WHIRLWINDS)
Definition

A SMALL BUT RAPIDLY ROTATING WIND MADE VISIBLE BY THE DUST, SAND, AND DEBRIS IT PICKS UP FROM THE SURFACE.  IT DEVELOPS BEST ON CLEAR, DRY, HOT AFTERNOONS.

 

Term
GENERAL CIRCULATION OF THE ATMOSPHERE
Definition
LARGE-SCALE ATMOSPHERIC MOTIONS OVER THE ENTIRE EARTH.
Term
HADLEY CELL
Definition
A THERMAL CIRCULATION PROPOSED BY GEORGE HADLEY TO EXPLAIN THE MOVEMENT OF THE TRADE WINDS.  IT CONSISTS OF RISING AIR NEAR THE EQUATOR AND SINKING AIR NEAR 30 DEGREES LATITUDE.
Term
DOLDRUMS
Definition

THE REGION NEAR THE EQUATOR THAT IS CHARACTERIZED BY LOW PRESSURE AND LIGHT, SHIFTING WINDS.

Term
SUBTROPICAL HIGHS
Definition

A SEMIPERMANENT HIGH IN THE SUBTROPICAL HIGH-PRESSURE BELT CENTERED NEAR 30 DEGREES LATITUDE.  THE BERMUDA HIGH IS LOCATED OVER THE ATLANTIC OCEAN OFF THE EAST COAST OF NORTH AMERICA.  THE PACIFIC HIGH IS LOCATED OFF THE WEST COAST OF NORTH AMERICA.

Term
TRADE WINDS
Definition
THE WINDS THAT OCCUPY MOST OF THE TROPICS AND BLOW FROM THE SUBTROPICAL HIGHS TO THE EQUATORIAL LOW.
Term
INTERTROPICAL CONVERGENCE ZONE (ITCZ)
Definition
THE BOUNDARY ZONE SEPARATING THE NORTHEAST TRADE WINDS OF THE NORTHERN HEMISPHERE FROM SOUTHEAST TRADE WINDS OF THE SOUTHERN HEMISPHERE.
Term
WESTERLIES
Definition
THE DOMINANT WESTERLY WINDS THAT BLOW IN THE MIDDLE LATITUDES ON THE POLEWARD SIDE OF THE SUBTROPICAL HIGH-PRESSURE AREAS.
Term

POLAR FRONT

Definition
A SEMIPERMANENT, SEMICONTINUOUS FRONT THAT SEPARATES TROPICAL AIR MASSES FROM POLAR AIR MASSES.
Term
SUBPOLAR LOW
Definition
A BELT OF LOW PRESSURE LOCATED BETWEEN 50 DEGREES AND 70 DEGREES LATITUDE.  IN THE NORTHERN HEMISPHERE, THIS "BELT" CONSISTS OF THE ALEUTIAN LOW IN THE NORTH PACIFIC AND THE ICELANDIC LOW IN THE NORH ATLANTIC.  IN THE SOUTHERN HEMISPHERE, IT EXISTS AROUND THE PERIPHERY OF THE ANTARCTIC CONTINENT.
Term
POLAR EASTERLIES
Definition
A SHALLOW BODY OF EASTERLY WINDS LOCATED AT HIGH LATITUDES POLEWARD OF THE SUBPOLAR LOW.
Term
BERMUDA HIGH
Definition
SEE SUBTROPICAL HIGH.
Term
PACIFIC HIGH
Definition
SEE SUBTROPICAL HIGH.
Term
ICELANDIC LOW
Definition
THE SUBPOLAR LOW-PRESSURE AREA THAT IS CENTERED NEAR ICELAND ON CHARTS THAT SHOW MEAN SEA-LEVEL PRESSURE.
Term
ALEUTIAN LOW
Definition

THE SUBPOLAR LOW-PRESSURE AREA THAT IS CENTERED NEAR THE ALEUTIAN ISLANDS ON CHARTS THAT SHOW MEAN SEA-LEVEL PRESSURE.

Term
SIBERIAN HIGH
Definition
A STRONG, SHALLOW AREA OF HIGH PRESSURE THAT FORMS OVER SIBERIA IN WINTER.
Term

JET STREAM

Definition
RELATIVELY STRONG WINDS CONCENTRATED WITHIN A NARROW BAND IN THE ATMOSPHERE.
Term
SUBTROPICAL JET STREAM
Definition
THE JET STREAM TYPICALLY FOUND BETWEEN 20 DEGREES AND 30 DEGREES LATITUDE AT ALTITUDES BETWEEN 12 AND 14 KM.
Term

POLAR FRONT JET STREAM

Definition
(POLAR JET)  THE JET STREAM THAT IS ASSOICATED WITH THE POLAR FRONT IN MIDDLE AND HIGH LATITUDES.  IT IS USUALLY LOCATED AT ALTITUDES BETWEEN 9 AND 12 KM.
Term
UPWELLING
Definition
THE RISING OF WATER (USUALLY COLD) TOWARD THE SURFACE FROM THE DEEPER REGIONS OF A BODY OF WATER.
Term
EL NINO
Definition

AN EXTENSIVE OCEAN WARMING THAT BEGINS ALONG THE COAST OF PERU AND ECUADOR AND EXTENDS WESTWARD OVER THE TROPICAL PACIFIC.  MAJOR EL NINO EVENTS, OR STRONG EL NINOS, OCCUR ONCE EVERY 2 TO 7 YEARS AS A CURRENT OF NUTRIENT-POOR TROPICAL WATER MOVES SOUTHWARD ALONG THE WEST COAST OF SOUTH AMERICA.

Term
SOUTHERN OSCILLATION
Definition
THE REVERSAL OF SURFACE AIR PRESSURE AT OPPOSITE ENDS OF THE TROPICAL PACIFIC OCEAN THAT OCCURS DURING MAJOR EL NINO EVENTS.
Term
ENSO
Definition
(EL NINO/SOUTHERN OSCIALLATION)  A CONDITION IN THE TROPICAL PACIFIC WHEREBY THE REVERSAL OF SURFACE AIR PRESSURE AT OPPOSITE ENDS OF THE PACIFIC OCEAN INDUCES WESTERLY WINDS, A STRENGTHENING OF THE EQUATORIAL COUNTERCURRENT, AND EXTENSIVE OCEAN WARMING.
Term
LA NINA
Definition
A CONDITION WHERE THE CENTRAL AND EASTERN TROPICAL PACIFIC OCEAN TURNS COOLER THAN NORMAL.
Term
TELECONNECTIONS
Definition
A LINKAGE BETWEEN WEATHER CHANGES OCCURING IN WIDELY SEPARATED REGIONS OF THE WORLD.
Term
NORTH ATLANTIC OSCILLATION (NAO)
Definition
A REVERSAL OF ATMOSPHERIC PRESSURE OVER THE ATLANTIC OCEAN THAT INFLUENCES THE WEATHER OVER EUROPE AND OVER EASTERN NORTH AMERICA.
Term
ARCTIC OSCILLATION (AO)
Definition
A REVERSAL OF ATMOSPHERIC PRESSURE OVER THE ARCTIC THAT PRODUCES CHANGES IN THE UPPER-LEVEL WESTERLY WINDS OVER NORTHERN LATITUDES.  THESE CHANGES IN UPPER-LEVEL WINDS INFLUENCE WINTER WEATHER PATTERNS OVER NORTH AMERICA, GREENLAND, AND EUROPE.
Term
PACIFIC DECADAL OSCIALLTION (PDO)
Definition
A REVERSAL IN OCEAN SURFACE TEMPERATURES THAT OCCURS EVERY 20 TO 30 YEARS OVER THE NORTHERN PACIFIC OCEAN.
Term
AIR MASS
Definition
A LARGE BODY OF AIR THAT HAS SIMILAR HORIZONTAL TEMPERATURE AND MOISTURE CHARACTERISTICS.
Term

SOURCE REGION (FOR AIR MASSES)

Definition
REGIONS WHERE AIR MASSES ORIGINATE AND ACQUIRE THEIR PROPERTIES OF TEMPERATURE AND MOISTURE.
Term
CONTINENTAL POLAR (AIR MASS)
Definition
AN AIR MASS CHARACTERIZED BY LOW TEMPERATURES AND DRY AIR.  NOT AS COLD AS ARCTIC AIR MASSES.
Term
CONTINENTAL ARCTIC (AIR MASS)
Definition
AN AIR MASS CHARACTERIZED BY EXTREMELY LOW TEMPERATURES AND VERY DRY AIR.
Term
LAKE-EFFECT SNOWS
Definition
LOCALIZED SNOWSTORMS THAT FORM ON THE DOWNWIND SIDE OF A LAKE.  SUCH STORMS ARE COMMON IN LATE FALL AND EARLY WINTER NEAR THE GREAT LAKES AS COLD, DRY AIR PICKS UP MOISTURE AND WARMTH FROM THE UNFROZEN BODIES OF WATER.
Term
MARITIME POLAR (AIR MASS)
Definition
AN AIR MASS CHARACTERIZED BY LOW TEMPERATURES AND HIGH HUMIDITY.
Term
MARITIME TROPICAL (AIR MASS)
Definition
AN AIR MASS CHARACTERIZED BY HIGH TEMPERATURES AND HIGH HUMIDITY.
Term
CONTINENTAL TROPICAL (AIR MASS)
Definition
AN AIR MASS CHARACTERIZED BY HIGH TEMPERATURES AND LOW HUMIDITY.
Term
FRONT
Definition
THE TRANSITION ZONE BETWEEN TWO DISTINCT AIR MASSES.
Term
STATIONARY FRONT
Definition
A FRONT THAT IS NEARLY STATIONARY WITH WINDS BLOWING ALMOST PARALLEL AND FROM OPPOSITE DIRECTIONS ON EACH SIDE OF THE FRONT.
Term
COLD FRONT
Definition
A TRANSITION ZONE WHERE A COLD AIR MASS ADVANCES AND REPLACES A WARM AIR MASS.
Term
BACK DOOR COLD FRONT
Definition
A COLD FRONT MOVING SOUTH OR SOUTHWEST ALONG THE ATLANTIC SEABOARD OF THE UNITED STATES.
Term
WARM FRONT
Definition
A FRONT THAT MOVES IN SUCH A WAY THAT WARM AIR REPLACES COLD AIR.
Term

OVERRUNNING

Definition
A CONDITION THAT OCCURS WHEN AIR MOVES UP AND OVER ANOTHER LAYER OF AIR.
Term
OCCLUDED FRONT (OCCLUSION)
Definition

A COMPLEX FRONTAL SYSTEM THAT IDEALLY FORMS WHEN A COLD FRONT OVERTAKES A WARM FRONT.  WHEN THE AIR BEHIND THE FRONT IS COLDER THAN THE AIR AHEAD OF IT, THE FRONT IS CALLED A COLD OCCLUSION.  WHEN THE AIR BEHIND THE FRONT IS MILDER THAN THE AIR AHEAD OF IT, IT IS CALLED A WARM OCCLUSION.

Term
POLAR FRONT THEORY
Definition
A THEORY DEVELOPED BY A GROUP OF SCANDANAVIAN METEOROLOGISTS THAT EXPLAINS THE FORMATION, DEVELOPMENT, AND OVERALL LIFE HISTORY OF CYCLONIC STORMS THAT FORM ALONG THE POLAR FRONT.
Term
WAVE CYCLONE
Definition
AN EXTRATROPICAL CYCLONE THAT FORMS AND MOVES ALONG A FRONT.  THE CIRCULATION OF WINDS ABOUT THE CYCLONE TENDS TO PRODUCE A WAVELIKE DEFORMATION ON THE FRONT.
Term
FRONTAL WAVE
Definition
A WAVELIKE DEFORMATION ALONG A FRONT IN THE LOWER LEVELS OF THE ATMOSPHERE.  THOSE THAT DEVELOP INTO STORMS ARE TERMED UNSTABLE WAVES, WHILE THOSE THAT DO NOT ARE CALLED STABLE WAVES.
Term
OPEN WAVE
Definition
THE STAGE OF DEVELOPMENT OF A WAVE CYCLONE (MID-LATITUDE CYCLONIC STORM) WHERE A COLD FRONT AND A WARM FRONT EXIST, BUT NO OCCLUDED FRONT.  THE CENTER OF LOWEST PRESSURE IN THE WAVE IS LOCATED AT THE JUNCTION OF THE TWO FRONTS.
Term
CYCLOGENESIS
Definition
THE DEVELOPMENT OR STRENGTHENING OF MIDDLE-LATITUDE (EXTRATROPICAL) CYCLONES.
Term
LEE-SIDE LOW
Definition
STORM SYSTEMS (EXTRATROPICAL CYCLONES) THAT FORM ON THE DOWNWIND (LEE) SIDE OF A MOUNTAIN CHAIN.  IN THE UNITED STATES LEE-SIDE LOWS FREQUENTLY FORM ON THE EASTERN SIDE OF THE ROCKIES AND SIERRA NEVADA MOUNTAINS.
Term
NORTHEASTER
Definition
A NAME GIVEN TO A STRONG, STEADY WIND FROM THE NORTHEAST THAT IS ACCOMPANIED BY RAIN AND INCLEMENT WEATHER.  IT OFTEN DEVELOPS WHEN A STORM SYSTEM MOVES NORTHEASTWARD ALONG THE COAST OF NORTH AMERICA.  ALSO CALLED NOR'EASTER.
Term
CONVERGENCE
Definition
AN ATMOSPHERIC CONDITION THAT EXISTS WHEN THE WINDS CAUSE A HORIZONTAL NET INFLOW OF AIR INTO A SPECIFIED REGION.
Term
DIVERGENCE
Definition
AN ATMOSPHERIC CONDITION THAT EXISTS WHEN THE WINDS CAUSE A HORIZONTAL NET OUTFLOW OF AIR FROM A SPECIFIC REGION.
Term
JET STREAK
Definition
A REGION OF HIGH WIND SPEED THAT MOVES THROUGH THE AXIS OF A JET STREAM.  ALSO CALLED JET MAXIMUM.
Term

CONDUCTION

Definition
THE TRANSFER OF HEAT BY MOLECULAR ACITIVITY FROM ONE SUBSTANCE TO ANOTHER, OR THROUGH A SUBSTANCE.  TRANSFER IS ALWAYS FROM WARMER TO COLDER REGIONS.
Term
CONVECTION
Definition
MOTIONS IN A FLUID THAT RESULT IN THE TRANSPORT AND MIXING OF THE FLUID'S PROPERTIES.  IN METEOROLOGY, CONVECTION USUALLY REFERES TO ATMOSPHERIC MOTIONS THAT ARE PREDOMINANTLY VERTICAL, SUCH AS RISING AIR CURRENTS DUE TO SURFACE HEATING.  THE RISING OF HEATED SURFACE AIR AND THE SINKING OF COOLER AIR ALOFT IS OFTEN CALLED FREE CONVECTION.  (COMPARED WITH FORCED CONVECTION.)
Term
SEVERE THUNDERSTORMS
Definition
INTENSE THUNDERSTORMS CAPABLE OF PRODUCING HEAVY SHOWERS, FLASH FLOODS, HAIL, STRONG AND GUSTY SURFACE WINDS, AND TORNADOES.
Term
ORDINARY CELL THUNDERSTORMS
Definition
(FORMERLY CALLED AIR-MASS THUNDERSTORM)  A THUNDERSTORM PRODUCED BY LOCAL CONVECTION WITHIN A CONDITIONALLY UNSTABLE AIR MASS.  IT OFTEN FORMS IN THE AFTERNOON AND DOES NOT REACH THE INTENSITY OF A SEVERE THUNDERSTORM.
Term
CUMULUS STAGE
Definition
THE INITIAL STAGE IN THE DEVELOPMENT OF AN ORDINARY CELL THUNDERSTORM IN WHICH RISING, WARM, HUMID AIR DEVELOPS INTO A CUMULUS CLOUD.
Term
MATURE STAGE
Definition
THE SECOND STAGE IN THE THREE-STAGE CYCLE OF AN ORDINARY THUNDERSTORM.  THIS MATURE STAGE IS CHARACTERIZED BY HEAVY SHOWERS, LIGHTNING, THUNDER, AND VIOLENT VERTICAL MOTIONS INSIDE CUMULONIMBUS CLOUDS.
Term
DISSIPATING STAGE
Definition
THE FINAL STAGE OF THE DEVELOPMENT OF AN ORDINARY CELL THUNDERSTORM WHEN DOWNDRAFTS EXIST THROUGHOUT THE CUMULONIMBUS CLOUD.
Term
MULTICELL STORM
Definition
THUNDERSTORMS OFTEN IN A LINE, EACH OF WHICH MAY BE IN A DIFFERENT STAGE OF ITS LIFE CYCLE.
Term
SUPERCELL
Definition
AN ENORMOUS SEVERE THUNDERSTORM THAT CONSISTS PRIMARILY OF A SINGLE ROTATING UPDRAFT.  ITS ORGANIZED INTERNAL STRUCTURE ALLOWS THAT STORM TO MAINTAIN ITSELF FOR SEVERAL HOURS.  THE STORM CAN PRODUCE LARGE HAIL AND DANGEROUS TORNADOES.
Term
SQUALL LINE
Definition
A LINE OF THUNDERSTORMS THAT FORM ALONG A COLD FRONT OR OUT AHEAD OF IT.
Term
MESOSCALE CONVECTIVE COMPLEXES (MCCS)
Definition
A LARGE ORGANIZED CONVECTIVE WEATHER SYSTEM COMPRISED OF A NUMBER OF INDIVIDUAL THUNDERSTORMS.  THE SIZE OF AN MCC CAN BE 1000 TIMES LARGER THAN AN INDIVIDUAL ORDINARY THUNDERSTORM.
Term
DRYLINE
Definition
A BOUNDARY THAT SEPARATES WARM, DRY AIR FROM WARM, MOIST AIR.  IT USUALLY REPRESENTS A ZONE OF INSTABILITY ALONG WHICH THUNDERSTORMS FORM.
Term
GUST FRONT
Definition
A BOUNDARY THAT SEPARATES A COLD DOWNDRAFT OF A THUNDERSTORM FROM WARM, HUMID SURFACE AIR.  ON THE SURFACE ITS PASSAGE RESEMBLES THAT OF A COLD FRONT.
Term
SHELF CLOUD
Definition
A DENSE, ARCH-SHAPED, OMINOUS-LOOKING CLOUD THAT OFTEN FORMS ALONG THE LEADING EDGE OF A THUNDERSTORM'S GUST FRONT, ESPECIALLY WHEN STABLE AIR RISES UP AND OVER COOLER AIR AT THE SURFACE.  ALSO CALLED AN ARCUS CLOUD.
Term
ROLL CLOUD
Definition
A DENSE, CYLINDRICAL, ELONGATED CLOUD THAT APPEARS TO SLOWLY SPIN ABOUT A HORIZONTAL AXIS BEHIND THE LEADING EDGE OF A THUNDERSTORM'S GUST FRONT.
Term
OUTFLOW BOUNDARY
Definition

A SURFACE BOUNDARY SEPARATING COOLER MORE-DENSE AIR FROM WARMER LESS-DENSE AIR.  OUTFLOW BOUNDARIES FORM BY THE HORIZONTAL SPREADING OF COOL AIR THAT ORIGINATED INSIDE A THUNDERSTORM.

Term
DOWNBURST
Definition
A SEVERE LOCALIZED DOWNDRAFT THAT CAN BE EXPERIENCED BENEATH A SEVERE THUNDERSTORM.  (COMPARE MICROBURST AND MACROBURST.)
Term
MICROBURST
Definition
A STRONG LOCALIZED DOWNDRAFT (DOWNBURST) LESS THAN 4 KM WIDE THAT OCCURS BENEATH THUNDERSTORMS.  A STRONG DOWNBURST GREATER THAN 4 KM ACROSS IS CALLED A MACROBURST.
Term
DERECHO
Definition
STRONG, DAMAGING STRAIGHT-LINE WINDS ASSOCIATED WITH A CLUSTER OF SEVERE THUNDERSTORMS THAT MOST OFTEN FORM IN THE EVENING OR AT NIGHT.
Term
BOW ECHO
Definition
A LINE OF THUNDERSTORMS ON A RADAR SCREEN THAT APPEARS IN THE SHAPE OF A BOW.  BOW ECHOES ARE OFTEN ASSOCIATED WITH DAMAGING STRAIGHT-LINE WINDS AND SMALL TORNADOES.
Term
HEAT BURST
Definition
A SUDDEN INCREASE IN SURFACE AIR TEMPERATURE OFTEN ACCOMPANIED BY EXTREME DRYING.  A HEAT BURST IS ASSOICATED WITH THE DOWNDRAFT OF A THUNDERSTORM, OR A CLUSTER OF THUNDERSTORMS.
Term
FLASH FLOODS
Definition
A FLOOD THAT RISES AND FALLS QUITE RAPIDLY WITH LITTLE OR NO ADVANCE WARNING, USUALLY AS THE RESULT OF INTENSE RAINFALL OVER A RELATIVELY SMALL AREA.
Term
LIGHTNING
Definition
A VISIBLE ELECTRICAL DISCHARGE PRODUCED BY THUNDERSTORMS.
Term
THUNDER
Definition
THE SOUND DUE TO RAPIDLY EXPANDING GASES ALONG THE CHANNEL OF A LIGHTNING DISCHARGE.
Term
SONIC BOOM
Definition
A LOUD EXPLOSIVE-LIKE SOUND CAUSED BY A SHOCK WAVE EMANATING FROM AN AIRCRAFT (OR ANY OBJECT) TRAVELING AT OR ABOVE THE SPEED OF SOUND.
Term
STEPPED LEADER
Definition
AN INITIAL DISCHARGE OF ELECTRONS THAT PROCEEDS INTERMITTENTLY TOWARDS THE GROUND IN A SERIES OF STEPS IN A CLOUD-TO-GROUND LIGHTNING STROKE.
Term
RETURN STROKE
Definition
THE LUMINOUSE LIGHTNING STROKE THAT PROPOGATES UPWARD FROM THE EARTH TO THE BASE OF A CLOUD.
Term
DART LEADER
Definition
THE DISCHARGE OF ELECTRONS THAT PROCEEDS INTERMITTENTLY TOWARD THE GROUND ALONG THE SAME IONIZED CHANNEL TAKEN BY THE INITIAL LIGHTNING STROKE.
Term
DRY LIGHTNING
Definition
LIGHTNING THAT OCCURS WITH THUNDERSTORMS THAT PRODUCE LITTLE, IF ANY, APPRECIABLE PRECIPITATION THAT REACHES THE SURFACE.
Term
HEAT LIGHTNING
Definition
DISTANT LIGHTNING THAT ILLUMINATES THE SKY BUT IS TOO FAR AWAY FOR ITS THUNDER TO BE HEARD.
Term
ST. ELMO'S FIRE
Definition
A BRIGHT ELECTRIC DISCHARGE THAT IS PROJECTED FROM OBJECTS (USUALLY POINTED) WHEN THEY ARE IN A STRONG ELECTRIC FIELD, SUCH AS DURING A THUNDERSTORM.
Term
TORNADO
Definition

AN INTENSE, ROTATING COLUMN OF AIR THAT OFTEN PROTRUDES FROM A CUMULONIMBUS CLOUD IN THE SHAPE OF A FUNNEL OR A ROPE WHOSE CIRCULATION IS PRESENT ON THE GROUND.  (SEE FUNNEL CLOUD.)

Term
FUNNEL CLOUD
Definition
A TORNADO WHOSE CIRCULATION HAS NOT REACHED THE GROUND.  OFTEN APPEARS AS A ROTATING CONELIKE CLOUD THAT EXTENDS DOWNWARD FROM THE BASE OF A THUNDERSTORM.
Term
TORNADO OUTBREAK
Definition
A SERIES OF TORNADOES THAT FORMS WITHIN A PARTICULAR REGION-A REGION THAT MAY INCLUDE SEVERAL STATES.  OFTEN ASSOCIATED WITH WIDESPREAD DAMAGE AND DESTRUCTION.
Term
SUCTION VORTICES
Definition
SMALL, RAPIDLYD ROTATING WHIRLS PERHAPS 10 M IN DIAMETER THAT ARE FOUND WITHIN LARGE TORNADOES.
Term
TORNADO WATCH
Definition
A FORECAST ISSUED TO ALERT THE PUBLIC THAT TORNADOES MAY DEVELOP WITHIN A SPECIFIED AREA.
Term
TORNADO WARNING
Definition
A WARNING ISSUED WHEN A TORNADO HAS ACTUALLY BEEN OBSERVED EITHER VIRTUALLY OR ON A RADAR SCREEN.  IT IS ALSO ISSUED WHEN THE FORMATION OF TORNADOES IS IMMINENT.
Term
FUJITA SCALE
Definition
A SCALE DEVELOPED BY T. THEODORE FUJITA FOR CLASSIFYING TORNADOES ACCORDING TO THE DAMAGE THEY CAUSE AND THEIR ROTATIONAL WIND SPEED.
Term
SUPERCELL TORNADOES
Definition
TORNADOES THAT OCCUR WITHIN SUPERCELL THUNDERSTORMS THAT CONTAIN WELL-DEVELOPED MID-LEVEL MESOCYCLONES.
Term

MESOCYCLONE

Definition
A VERTICAL COLUMN OF CYCLONICALLY ROTATING AIR WITHIN A SUPERCELL THUNDERSTORM.
Term

HOOK ECHO

Definition
THE SHAPE OF AN ECHO ON A DOPPLER RADAR SCREEN THAT INDICATES THE POSSIBLE PRESENCE OF A TORNADO.
Term
WALL CLOUD
Definition
AN AREA OF ROTATING CLOUDS THAT EXTENDS BENEATH A SUPERCELL THUNDERSTORM AND FROM WHICH A FUNNEL CLOUD MAY APPEAR.  ALSO CALLED A COLLAR CLOUD AND PEDESTAL CLOUD.
Term
NONSUPERCELL TORNADOES
Definition
A TORNADO THAT OCCURS WITH A CLOUD THAT IS OFTEN IN ITS GROWING STAGE AND ONE THAT DOES NOT CONTAIN A MID-LEVEL MESOCYCLONE OR WALL CLOUD.  LANDSPOUTS AND GUSTANADOES ARE EXAMPLES OF NONSUPERCELL TORNADOES.
Term
GUSTNADOES
Definition
A RELATIVELY WEAK TORNADO ASSOCIATED WITH A THUNDERSTORM'S OUTFLOW.  IT MOST OFTEN FORMS ALONG THE GUST FRONT.
Term
LANDSPOUT
Definition
RELATIVELY WEAK NONSUPERCELL TORNADO THAT ORIGINATES WITH A CUMULIFORM CLOUD IN ITS GROWTH STAGE AND WITH A CLOUD THAT DOES NOT CONTAIN A MID-LEVEL MESOCYCLONE.  ITS SPIN ORIGINATES NEAR THE SURFACE.  LANDSPOUTS OFTEN LOOK LIKE WATERSPOUTS OVER LAND.
Term
NEXRAD
Definition
AN ACRONYM FOR NEXT GENERATION WEATHER RADAR.  THE MAIN COMPONENT OF NEXRAD IS THE WSR 88-D, DOPPLER RADAR.
Term
WATERSPOUT
Definition
A COLUMN OF ROTATING WIND OVER WATER THAT HAS CHARACTERISTICS OF A DUST DEVIL AND TORNADO.
Term
STREAMLINES
Definition
A LINE THAT SHOWS THE WIND FLOW PATTERN.
Term
TROPICAL WAVE (EASTERLY WAVE)
Definition
A MIGRATORY WAVELIKE DISTURBANCE IN THE TROPICAL EASTERLIES.  TROPICAL WAVES OCCASIONALLY INTENSIFY INTO TROPICAL CYCLONES.  THEY ARE ALSO CALLED EASTERLY WAVES.
Term
HURRICANE
Definition
A TROPICAL CYCLONE HAVING WINDS IN EXCESS OF 64 KNOTS (74 MI/HR).
Term
TYPHOON
Definition
A HURRICAN THAT FORMS IN THE WESTERN PACIFIC OCEAN.
Term
TROPICAL CYCLONE
Definition
THE GENERAL TERM FOR STORMS (CYCLONES) THAT FORM OVER WARM TROPICAL OCEANS.
Term
EYE (OF HURRICANE)
Definition
A REGION IN THE CENTER OF A HURRICANE (TROPICAL STORM) WHERE THE WINDS ARE LIGHT AND SKIES ARE CLEAR TO PARTLY CLOUDY.
Term
EYEWALL
Definition
A WALL OF DENSE THUNDERSTORMS THAT SURROUNDS THE EYE OF A HURRICANE.
Term
TRADE WIND INVERSION
Definition
A TEMPERATURE INVERSION FREQUENTLY FOUND IN THE SUBTROPICS OVER THE EASTERN PORTIONS OF THE TROPICAL OCEANS.
Term
TROPICAL DEPRESSION
Definition
A MASS OF THUNDERSTORMS AND CLOUDS GENERALLY WITH A CYCLONIC WIND CIRCULATION OF BETWEEN 20 AND 34 KNOTS.
Term
TROPICAL STORM
Definition
ORGANIZED THUNDERSTORMS WITH A CYCLONIC WIND CIRCULATION BETWEEN 35 AND 64 KNOTS.
Term
EKMAN TRANSPORT
Definition
NET SURFACE WATER TRANSPORT DUE TO THE EKMAN SPIRAL.  IN THE NORTHERN HEMISPHERE THE TRANSPORT IS 90 DEGREES TO THE RIGHT OF THE SURFACE WIND DIRECTION.
Term
STORM SURGE
Definition
AN ABNORMAL RISE OF THE SEA ALONG A SHORE; PRIMARILY DUE TO THE WINDS OF A STORM, ESPECIALLY A HURRICANE.
Term
SAFFIR-SIMPSON SCALE
Definition
A SCALE RELATING TO A HURRICANE'S CENTRAL PRESSURE AND WINDS TO THE POSSIBLE DAMAGE IT IS CAPABLE OF INFLICTING.
Term
SUPER-TYPHOON
Definition
A TROPICAL CYCLONE (TYPHOON) IN THE WESTERN PACIFIC THAT HAS SUSTAINED WINDS OF 130 KNOTS OR GREATER.
Term
HURRICANE WATCH
Definition
A HURRICANE WATCH INDICATES THAT A HURRICANE POSES A THREAT TO AN AREA (OFTEN WITHIN SEVERAL DAYS) AND RESIDENTS OF THE WATCH AREA SHOULD BE PREPARED.
Term
HURRICANE WARNING
Definition
A WARNING GIVEN WHEN IT IS LIKELY THAT A HURRICAN WILL STRIKE AN AREA WITHIN 24 HOURS.
Term
REFLECTED LIGHT
Definition
SEE REFLECTION.
Term
SCATTERED LIGHT
Definition
THE PROCESS BY WHICH SMALL PARTICLES IN THE ATMOSPHERE DEFLECT RADIATION FROM ITS PATH INTO DIFFERENT DIRECTIONS.
Term
CREPUSCULAR RAYS
Definition
ALTERNATING LIGHT AND DARK BANDS OF LIGHT THAT APPEAR TO FAN OUT FROM THE SUN'S POSITION, USUALLY AT TWILIGHT.
Term
REFRACTION (OF LIGHT)
Definition
THE BENDING OF LIGHT AS IT PASSES FROM ONE MEDIUM TO ANOTHER.
Term
SCINTILLATION
Definition
THE APPARENT TWINKLING OF A STAR DUE TO ITS LIGHT PASSING THROUGH REGIONS OF DIFFERING AIR DENSITIES IN THE ATMOSPHERE.
Term
TWILIGHT
Definition
THE TIME AT THE BEGINNING OF THE DAY IMMEDIATELY BEFORE SUNRISE AND AT THE END OF THE DAY AFTER SUNSET WHEN THE SKY REMAINS ILLUMINATED.
Term
GREEN FLASH
Definition
A SMALL GREEN COLOR THAT OCCASIONALLY APPEARS ON THE UPPER PART OF THE SUN AS IT RISES OR SETS.
Term

MIRAGE

Definition
A REFRACTION PHENOMENON THAT MAKES AN OBJECT APPEAR TO BE DISPLACED FROM ITS TRUE POSITION.  WHEN AN OBJECT APPEARS HIGHER THAN IT ACTUALLY IS, IT IS CALLED A SUPERIOR MIARAGE.  WHEN AN OBJECT APPEARS LOWER THAN IT ACTUALLY IS, IT IS CALLED AN INFERIOR MIRAGE.
Term
INFERIOR MIRAGE
Definition
SEE MIRAGE.
Term
SUPERIOR MIRAGE
Definition
SEE MIRAGE.
Term
HALO
Definition
RINGS OR ARCS THAT ENCIRCLE THE SUN OR MOON WHEN SEN THROUGH AN ICE CRYSTAL CLOUD OR A SKY FILLED WITH FALLING ICE CRYSTALS.  HALOS ARE PRODUCED BY REFRACDTION OF LIGHT.
Term
TANGENT ARC
Definition
AN ARC OF LIGHT TANGENT TO A HALO.  IT FORMS BY REFRACTION OF LIGHT THROUGH ICE CRYSTALS.
Term
DISPERSION (OF LIGHT)
Definition
THE SEPARATION OF WHITE LIGHT INTO ITS DIFFERENT COMPONENT WAVELENGTHS.
Term
SUNDOG (OR PARHELIA)
Definition
A COLORED LUMINOUS SPOT PRODUCED BY REFRACTION OF LIGHT THROUGH ICE CRYSTALS THAT APPEARS ON EITHER SIDE OF THE SUN.  ALSO CALLED PARHELIA.
Term
SUN PILLARS
Definition
A VERTICAL STREAK OF LIGHT EXTENDING ABOVE (OR BELOW) THE SUN.  IT IS PRODUCED BY THE REFLECTION OF SUNLIGHT OFF ICE CRYSTALS.
Term
RAINBOW
Definition
AN ARC OF CONCENTRIC BANDS THAT SPANS A SECTION OF THE SKY WHEN RAIN IS PRESENT AND THE SUN IS POSITIONED AT THE OBSERVER'S BACK.
Term
CORONA
Definition
(OPTIC)  A SERIES OF COLORED RINGS CONCENTRICALLY SURROUNDING THE DISK OF THE SUN OR MOON.  SMALLER THAN THE HALO, THE CORONA IS OFTEN CAUSED BY THE DIFFRACTION OF LIGHT AROUND SMALL WATER DROPLETS OF UNIFORM SIZE.
Term
DIFFRACTION
Definition
THE BENDING OF LIGHT AROUND OBJECTS, SUCH AS CLOUD AND FOG DROPLETS, PRODUCING FRINGES OF LIGHT AND DARK OR COLORED BANDS.
Term
IRIDESCENCE
Definition
BRILLIANT SPOTS OR BORDERS OF COLORS, MOST OFTEN RED AND GREEN, OBSERVED IN CLOUDS UP TO ABOUT 30 DEGREES FROM THE SUN.
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