mass per air molecule, is the acceleration due to gravity, is height from reference point 0, is the Boltzmann constant, is the temperature in kelvins. Therefore, instead of pressure being a linear function of height as one might expect from the more simple formula given in the "basic formula" section, it is more accurately represented as an exponential function of height. Note that in this simplification, the temperature is treated as constant, even though temperature also varies with height. However, the temperature variation within the lower layers of the atmosphere (troposphere, stratosphere) is only in the dozens of degrees, as opposed to their thermodynamic temperature, which is in the hundreds, so the temperature variation is reasonably small and is thus ignored. For smaller height differences, including those from top to bottom of even the tallest of buildings, (like the CN tower) or for mountains of comparable size, the temperature variation will easily be within the single-digits. (See also lapse rate.) An alternative derivation, shown by the Portland State Aerospace Society, is used to give height as a function of pressure instead. This may seem counter-intuitive, as pressure results from height rather than vice versa, but such a formula can be useful in finding height based on pressure difference when one knows the latter and not the former. Different formulas are presented for different kinds of approximations; for comparison with the previous formula, the first referenced from the article will be the one applying the same constant-temperature approximation; in which case: where (with values used in the article) is the elevation in meters, is the specific gas constant = is the absolute temperature in kelvins = at sea level, is the acceleration due to gravity = at sea level, is the pressure at a given point at elevation in Pascals, and is pressure at the reference point = at sea level. A more general formula derived in the same article accounts for a linear change in temperature as a function of height (lapse rate), and reduces to above when the temperature is constant: where is the atmospheric lapse rate (change in temperature divided by distance) = , and is the temperature at the same reference point for which and the other quantities are the same as those above. This is the recommended formula to use. See also Barometer Hypsometric equation Pascal's barrel Ruina montium Pressure gradient Siphon References Pressure Vertical position