We use across as a preposition (prep) and an adverb (adv). Across means on the other side of something, or from one side to the other of something which has sides or limits such as a city, road or riverThe temperature on mountains becomes colder the higher the altitude gets.Mountains tend to have much wetter climates than the surrounding flat land.Mountain weather conditions can change dramatically from one hour to the next. For example, in just a few minutes a thunder storm can roll in when the sky was perfectly clear, and in just a few hours the temperatures can drop from extremely hot temperatures to temperatures that are below freezing.