英语翻译
Whatmakesitrain?RainfallsfromcloudsforthesamereasonasanythingfallstotheEarth.TheEarth'sgravitypullsit.Buteverycloudismadeofwaterdroplets(微滴)oricecrystals(结晶).Whydoesn'trainorsnowfallcontinuouslyfromallclouds?Thedropletsoricecrystalsincloudsareverysmall.Theeffectofgravityonthemistiny.
Dropletsandicecrystalsactsomewhatlikedustintheairwhichismadevisible(可见的)intheraysofsunlight.Tothecarelessobserver,dustseemstoactinatotallydisorderly(混乱的)waymovingaboutfreelywithoutfixeddirection.Butinfactdustparticlesaremuchlargerthanwaterdropletsandtheyfinallyfall.
Theclouddropletofordinarysizeisonly1/2500inchindiameter(直径).Itissosmallthatitwouldtakesixteenhourstofallhalfamileinperfectlystillair,anditdoesnotfalloutofmovingairatall.Onlywhenthedropletgrowstoadiameterof1/125inchorlargercanitfallfromthecloud.Theordinaryraindrophasamilliontimesasmuchwaterasatinyclouddroplet.Thegrowthofaclouddroplettoasizelargeenoughtofalloutisthecauseofrain.