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