It is the distance from the center of the earth, usually referenced to sea level. At different elevations plants and animals respond differently. Air at higher elevations is rarer and makes it difficult to breath. Also the atmosphere has wind currents that affect weather patterns and temperatures at different altitudes.

Higher elevations are cooler because the air is thinner. More mass is needed to hold more heat, and the smaller amount of air density at higher altitudes is not able to hold as much heat, and is thus cooler than near sea level where the air is more dense.

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the higher up mountains, the colder it gets. this is why there is snow on top of mountains.

vegetation is also different.

I don't know

dunno