用英语的回答：SunlightreachesEarth'satmosphereandisscatteredinalldirectionsbyallthegasesandparticlesintheair.BluelightisscatteredinalldirectionsbythetinymoleculesofairinEarth'satmosphere.Blueisscatteredmorethanothercolorsbecauseittravelsasshorter,smallerwaves.Thisiswhyweseeablueskymostofthetime. 　　（更深一步的回答：） 　　skyappearsbluebecausebluecolorhaveminimumwavelengthduetowhichitscattersmost(seediffraction)ascomparedtoothercolorsredscattersleastthatswhyitisusedinmakingsignals,neonadvertisements.thoughothersalsocontributetothecoloroftheskytosomeextent. 　　Intheeveningwhensunisathorizononlyredandotherhighwavelengthlightsmanagetoreachoureyes,mostofthebluescattersinthewayandtheskyappearreddish. 　　这应该足够来回答了