▁of ▁g ases ▁that ▁en velop s ▁the ▁Earth ▁is ▁held ▁in ▁place ▁by ▁gravity . ▁Air ▁is ▁mostly ▁nit ro gen , ▁o xygen , ▁water ▁v apor , ▁with ▁much ▁smaller ▁amounts ▁of ▁carbon ▁dio x ide , ▁arg on , ▁etc . ▁The ▁atmos pher ic ▁pressure ▁decl ines ▁stead ily ▁with ▁alt itude . ▁The ▁o zone ▁layer ▁plays ▁an ▁important ▁role ▁in ▁de ple ting ▁the ▁amount ▁of ▁ult rav io let ▁( U V ) ▁radiation ▁that ▁reaches ▁the ▁surface . ▁As ▁DNA ▁is ▁readily ▁dam aged ▁by ▁U V ▁light , ▁this ▁serves ▁to ▁protect ▁life ▁at ▁the ▁surface . ▁The ▁atmosphere ▁also ▁retain s ▁heat ▁during ▁the ▁night , ▁thereby ▁reducing ▁the ▁daily ▁temperature ▁extrem es . ▁ ▁Terr est rial ▁weather ▁occurs ▁almost ▁exclus ively ▁in ▁the ▁lower ▁part ▁of ▁the ▁atmosphere , ▁and ▁serves ▁as ▁a ▁con ve ct ive ▁system ▁for ▁red ist rib uting ▁heat . ▁Ocean ▁curr ents ▁are ▁another ▁important ▁factor ▁in ▁determ ining ▁climate , ▁particularly ▁the ▁major ▁under water ▁therm oh al ine ▁circul ation ▁which ▁dist ributes ▁heat ▁energy ▁from ▁the ▁equ atorial ▁o ce ans ▁to ▁the ▁polar ▁regions . ▁These ▁curr ents ▁help ▁to ▁moder ate ▁the ▁differences ▁in ▁temperature ▁between ▁winter ▁and ▁summer ▁in ▁the ▁temper ate ▁zones . ▁Also , ▁without ▁the ▁red ist ribution s ▁of ▁heat ▁energy ▁by ▁the ▁ocean ▁curr ents ▁and ▁atmosphere , ▁the ▁trop ics ▁would ▁be ▁much ▁hot ter , ▁and ▁the ▁polar ▁regions ▁much ▁col