The earth is enveloped by layers of air, known as the atmosphere. It is pulled down toward the surface by gravity, and exerts a pressure known as atmospheric pressure. Atmospheric pressure changes with height and can be given in terms of the weight or density of air. However, from the ideal gas law, density also depends on pressure. Assuming that the air temperature and the acceleration due to gravity remain constant with height and by substituting the expression for density and rearranging the equation, the rate of change of pressure with height can be obtained. Here, the constant quantities inside the parentheses are termed alpha. The equation is then integrated from sea level and solved to obtain the relation for pressure with height—the Barometric formula. By considering nitrogen molecules at 300 kelvin, one can obtain an approximate value of one over alpha, a characteristic length known as pressure scale height. The atmospheric pressure drops exponentially by a factor of one over e for every 8,800 meters above sea level.