 The atmospheric pressure on Earth 's surface averages 101 @.@ 325 kPa , with a scale height of about 8 @.@ 5 km . It has a composition of 78 % nitrogen and 21 % oxygen , with trace amounts of water vapor , carbon dioxide and other gaseous molecules . The height of the troposphere varies with latitude , ranging between 8 km at the poles to 17 km at the equator , with some variation resulting from weather and seasonal factors . 
