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The lapse-rate, or the rate at which it gets colder the higher up in the atmosphere that you travel is around 5 degrees Celsius per kilometer.

Up in the mountains it is colder than at sea level, even though it is closer to the Sun, because air holds heat. The thinner the air, the less heat it can hold. This is why on a 25 degrees Celsius (77 degrees Fahrenheit) day in the city, a neighboring mountain that is 3000 meters high will be 15 degrees colder at the summit, a chilly 10 degrees Celsius (50 degrees Fahrenheit).