Operation Manual

16.10Glossaryofweatherterms
TermDenition
ColdfrontTheboundarybetweentwodifferentairmasseswherecoldairpusheswarmairoutofthewayandbringscolderweather.
CycloneAlargeareaoflowatmosphericpressure,characterizedbyinwardspirallingwinds.A“low”alsocalleda“depression”.Alsothename
usedforahurricaneintheIndianOceanandWesternPacic.
Depression
Anareaoflowpressure.Alsocalledacyclone.
Dryline
Aregionwherethereisastronggradientindewpointtemperatures.Itisoftenfoundinaregionwherestrongthunderstormsdevelop.
Forecast
Somethingthattellsuswhattheweatherisprobablygoingtobelike.
Front
Theboundarybetweentwomassesofairwithdifferenttemperatures(i.e.amassofcoldairandamassofwarmair).
High
Alsoknownasan'anticyclone'anareaofhighatmosphericpressurewithasystemofwindsrotatingoutwards.Thisusuallymeansdry
weather.Itistheoppositeofa'low'.
HighPressure
AmassofairthatpressesdownstronglyonthesurfaceoftheEarthbecauseitisbeingcooledandisthereforemoredense.
Hurricane
Aviolent,spirallingstormthatformsovertheAtlanticOcean,withwindsover120kph.Suchstormsusuallyhavealifespanofseveraldays.
Alsoknownasatyphoonortropicalcyclone.Thereare5levelsofhurricane:
•Category1—Winds74–95mph(64–82ktor119–153km/hr).Stormsurgegenerally4–5ftabovenormal.Norealdamagetobuilding
structures.Damageprimarilytounanchoredmobilehomes,shrubbery,andtrees.Somedamagetopoorlyconstructedsigns.Also,
somecoastalroadoodingandminorpierdamage.
•Category2—Winds96–110mph(83–95ktor154–177km/hr).Stormsurgegenerally6–8feetabovenormal.Someroongmaterial,
door,andwindowdamageofbuildings.Considerabledamagetoshrubberyandtreeswithsometreesblowndown.Considerabledamage
tomobilehomes,poorlyconstructedsigns,andpiers.Coastalandlowlyingescaperoutesood2–4hoursbeforearrivalofthehurricane
centreSmallcraftinunprotectedanchoragesbreakmoorings.
•Category3—Winds111–130mph(96–113ktor178–209km/hr).Stormsurgegenerally9–12ftabovenormal.Somestructuraldamage
tosmallresidencesandutilitybuildingswithaminoramountofcurtainwallfailures.Damagetoshrubberyandtreeswithfoliageblownoff
treesandlargetreesblowndown.Mobilehomesandpoorlyconstructedsignsaredestroyed.Lowlyingescaperoutesarecutbyrising
water3–5hoursbeforearrivalofthecentreofthehurricane.Floodingnearthecoastdestroyssmallerstructureswithlargerstructures
damagedbybatteringfromoatingdebris.Terraincontinuouslylowerthan5ftabovemeansealevelmaybeoodedinland8miles(13
km)ormore.Evacuationoflowlyingresidenceswithseveralblocksoftheshorelinemayberequired.
•Category4—Winds131–155mph(114–135ktor210–249km/hr).Stormsurgegenerally13–18ftabovenormal.Moreextensivecurtain
wallfailureswithsomecompleteroofstructurefailuresonsmallresidences.Shrubs,trees,andallsignsareblowndown.Complete
destructionofmobilehomes.Extensivedamagetodoorsandwindows.Lowlyingescaperoutesmaybecutbyrisingwater3–5hours
beforearrivalofthecentreofthehurricane.Majordamagetoloweroorsofstructuresneartheshore.Terrainlowerthan10ftabovesea
levelmaybeoodedrequiringmassiveevacuationofresidentialareasasfarinlandas6miles(10km).
•Category5—Windsgreaterthan155mph(135ktor249km/hr).Stormsurgegenerallygreaterthan18ftabovenormal.Completeroof
failureonmanyresidencesandindustrialbuildings.Somecompletebuildingfailureswithsmallutilitybuildingsblownoveroraway.All
shrubs,trees,andsignsblowndown.Completedestructionofmobilehomes.Severeandextensivewindowanddoordamage.Lowlying
escaperoutesarecutbyrisingwater3–5hoursbeforearrivalofthecentreofthehurricane.Majordamagetoloweroorsofallstructures
locatedlessthan15ftabovesealevelandwithin500yardsoftheshoreline.Massiveevacuationofresidentialareasonlowground
within5–10miles(8–16km)oftheshorelinemayberequired.
IsobarAlineonaweathermaplinkingareaswithequalairpressure.
Lightning
Dischargeofstaticelectricityintheatmosphere,usuallybetweenthegroundandastormcloud.
Low
Alsocalleda'depression'thisregionoflowpressurecanmeanwetweather.
LowPressure
AmassofairthatpressesdownonlyweaklyonthesurfaceoftheEarth’ssurfaceasitiswarmedanditthereforelessdense.
MillibarAunitusedtomeasureatmosphericpressure.
OccludedFrontAnareawherewarmairispushedupwardsasacoldfrontovertakesawarmfrontandpushesunderneathit.
Precipitation
Moisturethatisreleasedfromtheatmosphereasrain,drizzle,hail,sleetorsnow,aswellasdewandfog.
PressureCentreAregionofhighorlowpressure.
SqualllineAnon-frontalband,orline,ofthunderstorms.
SupertyphoonAtyphoonthatreachesmaximumsustained1minutesurfacewindsofatleast65m/s(130kt,150mph).Thisistheequivalentofastrong
category4or5hurricaneintheAtlanticbasinoracategory5severetropicalcycloneintheAustralianbasin.
Tornado
Afunnelshapedwhirlwindwhichextendstothegroundfromstormclouds.
Tropicalcyclone
Alowpressuresystemthatgenerallyformsinthetropics.Thecycloneisaccompaniedbythunderstormsand,intheNorthernHemisphere,a
counterclockwisecirculationofwindsneartheearth'ssurface.
Tropicaldepression
Anorganizedsystemofcloudsandthunderstormswithadenedsurfacecirculationandmaximumsustainedwindsof38mph(33kt)orless.
Tropicalstorm
Anorganizedsystemofstrongthunderstormswithadenedsurfacecirculationandmaximumsustainedwindsof3973mph(3463kt).
Tropics
AnareaontheEarth'ssurfacethatliesbetween30ºnorthand30ºsouthoftheequator.
Trough
Anelongatedareaofrelativelylowatmosphericpressure,usuallyextendingfromthecentreofalowpressureregion.
Typhoon
ThenameforatropicalstormoriginatinginthePacicOcean,usuallytheChinaSea.Theyarebasicallythesameasthehurricanesofthe
AtlanticOceanandthecyclonesoftheBayofBengal.
192NewcSeries/NeweSeries