right|thumb|A large hailstone, about in diameter Hail forms in storm clouds when supercooled water droplets freeze on contact with condensation nuclei, such as dust or dirt. The storm's updraft blows the hailstones to the upper part of the cloud. The updraft dissipates and the hailstones fall down, back into the updraft, and are lifted up again. Hail has a diameter of or more. Within METAR code, GR is used to indicate larger hail, of a diameter of at least and GS for smaller. Stones of , and are the most frequently reported hail sizes in North America. Hailstones can grow to and weigh more than . In large hailstones, latent heat released by further freezing may melt the outer shell of the hailstone. The hailstone then may undergo 'wet growth', where the liquid outer shell collects other smaller hailstones. The hailstone gains an ice layer and grows increasingly larger with each ascent. Once a hailstone becomes too heavy to be supported by the storm's updraft, it falls from the cloud. thumb|right|Soft hail, or graupel, in Nevada Hail forms in strong thunderstorm clouds, particularly those with intense updrafts, high liquid water content, great vertical extent, large water droplets, and where a good portion of the cloud layer is below freezing . Hail-producing clouds are often identifiable by their green coloration. The growth rate is maximized at about , and becomes vanishingly small much below as supercooled water droplets become rare. For this reason, hail is most common within continental interiors of the mid-latitudes, as hail formation is considerably more likely when the freezing level is below the altitude of . Entrainment of dry air into strong thunderstorms over continents can increase the frequency of hail by promoting evaporative cooling which lowers the freezing level of thunderstorm clouds giving hail a larger volume to grow in. Accordingly, hail is actually less common in the tropics despite a much higher frequency of thunderstorms than in the mid-latitudes because the atmosphere over the tropics tends to be warmer over a much greater depth. Hail in the tropics occurs mainly at higher elevations. thumb|An accumulation of ice pellets Ice pellets (METAR code PL) are a form of precipitation consisting of small, translucent balls of ice, which are usually smaller than hailstones. This form of precipitation is also referred to as "sleet" by the United States National Weather Service. (In British English "sleet" refers to a mixture of rain and snow.) Ice pellets typically form alongside freezing rain, when a wet warm front ends up between colder and drier atmospheric layers. There, raindrops would both freeze and shrink in size due to evaporative cooling. So-called snow pellets, or graupel, form when multiple water droplets freeze onto snowflakes until a soft ball-like shape is formed. So-called "diamond dust", (METAR code IC) also known as ice needles or ice crystals, forms at temperatures approaching due to air with slightly higher moisture from aloft mixing with colder, surface-based air.
RighT|thumB|a LaRge haiLSTONe, about In diameTeR Hail foRmS IN stoRm clOudS WhEN SuperCOoLed waTer DropLEts FReEze oN contACT wIth condensATion nUcleI, such as dust or dIrT. thE storm's updrafT blows the hAiLstoneS to tHe Upper paRt oF tHe cloud. thE upDraFT dissipaTeS and The HAIlstones faLl down, Back Into thE updRaft, and are LIfted uP Again. HAIl HAs A dIameTER OF Or mORe. WiThiN METar Code, gR is used to iNdIcAtE LArgEr haiL, of a dIameTeR of AT leaSt AnD gs for SmAlLER. stOnEs of , AnD ArE THe mosT freqUENtLY rePoRted hail SIzes in nOrtH AMeRiCa. hailstOnEs caN groW to and wEigH More Than . iN lARgE hailstones, LAtenT HeaT reLeASEd BY FurTheR frEezinG mAy meLt tHe OUtER SheLL of The hailstoNE. the hAIlstoNe tHen may undergo 'Wet gRowth', wHEre thE LiquId ouTeR shELL collects other smAlLer hAiLstoNes. tHe hailSTone gains an Ice layer and Grows INCreaSingly LargER wIth EacH asceNT. Once A HaiLsTone becomes toO HEAvy TO Be suppOrtEd by The storm'S updRafT, It falls froM The cLOud. THuMB|Right|soFT haiL, oR GRAupEL, in nevada hAIl fOrMs in StrOng tHunderstoRM clouds, partICularlY ThoSe wItH inTenSe upDraFts, HigH LIquId WateR cONtenT, great veRtical exteNt, larGe water Droplets and Where a good PoRtion of thE clouD lAYer is bELow FreeZing . haIl-ProDuCiNg ClouDs aRe often iDEntifiable By theiR greeN cOlorATiON. the gRoWtH RAte is mAxImIzed at about , and bEComEs VANishiNgLy SmAlL mUcH belOW aS sUperCOoLed WAter DroplETs bEcomE rare. For thiS rEaSon, Hail is moSt coMmon wiThin continentaL InterIOrs of the MiD-laTitudes, as haIl forMation is cOnSidERAblY more lIkely whEN tHe freEziNG lEVeL is below tHE alTiTudE Of . eNtrAinment of DRy air into sTroNG tHundErstormS OVer coNtiNeNTS cAn incrEase the frEquencY of hAil bY proMoTiNG EvaporatiVE coOling WhiCh lOwers ThE fReezInG level of ThUndErstOrm cloUds GIVING hAIl a largeR volUmE to grOw In. ACcorDinGly, HaiL is acTualLy less commoN iN the TROpIcs despite a muCH hIghEr freQUENcy of THUndersToRms tHAn In THE mID-LaTiTuDEs beCause ThE atmOspherE OvER tHE tropics tEnds to be wARmer ovER A mucH grEater DePth. HaiL IN thE tRopIcs occURs mAinLY at HiGHeR eLEVaTiONs. tHUmb|an aCcUMUlaTiOn of IcE pellEtS Ice pELLEts (metar code pl) arE A fOrm oF prEcIPitaTion ConsIsTInG Of Small, trAnslucent baLlS Of ice, whicH arE usUaLly smaLler ThAn haiLStONEs. this form of prECIpitation is AlSO referred To as "sLeet" by the United states NaTionAl weaTher service. (in BRitish EngLish "sLeet" refeRs to A mIXtuRE OF raiN and sNoW.) ice PellEts TypicaLly ForM ALonGsIde freEzINg rain, WHen a WeT wArm froNT eNds uP betWeen colDEr aNd drier AtmOsPhERic LAyeRS thEre, rAInDRopS would both FrEeZE and shrInk in SiZE due to EvapoRATive CoOlINg. sO-Called snOW pellets, or GRaupel, ForM wHeN muLtipLE wAter drOpLETs freeze ONTO snowFLaKes UNtil a SOft ball-lIKE shape is FORmed. So-cAlled "dIamond dusT", (metAr COde ic) AlSo known as icE neEdles or iCe crYstalS, foRmS at TemperaturEs APproachiNG DuE to AiR wIth sLightly HighER MoIsture FROm alOFt mixiNg wiTH coldeR, SuRfACE-bAsEd aIr.