# The mean sea- level pressure of the atmosphere decreases by half at the altitude of ? A) 6 Km. B) 8 Km. C) 10 Km. D) 12 Km.

The atmospheric pressure depends by the weight of the air over us. air has a density of about $0 , 9 \frac{k g}{m} ^ 3$ that decreases with altitude starting from about $1 , 2 \frac{k g}{m} ^ 3$ at ground level to $0 , 6 \frac{k g}{m} ^ 3$ at about 6000 m). therefore an height of air of 6000 m wiht a m^2 of surface weighs about $P = d \times V \times g = 0 , 9 \frac{k g}{m} ^ 3 \times 6000 {m}^{3} \times 10 \frac{m}{s} ^ 2 = 54000 N$ that give on a surface of a m^2 a pressure of 54000 Pascal , about 0,5 atm.