 In the warm season , lee troughs , breezes , outflow boundaries and occlusions can lead to convection if enough moisture is available . Orographic precipitation is precipitation created through the lifting action of air moving over terrain such as mountains and hills , which is most common behind cold fronts that move into mountainous areas . It may sometimes occur in advance of warm fronts moving northward to the east of mountainous terrain . However , precipitation along warm fronts is relatively steady , as in rain or drizzle . Fog , sometimes extensive and dense , often occurs in pre @-@ warm @-@ frontal areas . Although , not all fronts produce precipitation or even clouds because moisture must be present in the air mass which is being lifted . 
