Two principal gases make up Earth’s atmosphere; about
four-fifths is nitrogen and one-fifth is oxygen. There are
also many important minor constituents, such as water
vapor and carbon dioxide. The amounts of atmospheric
water vapor and carbon dioxide are somewhat variable, depending
on global location and season. The composition of
Earth’s atmosphere is relatively uniform on a global scale,
but temperatures can vary widely. Atmospheric temperatures
near Earth’s surface can range from as high as 60°C
in the deserts to as low as –90°C in the polar regions. The
mean global temperature is about 15°C.