"Interesting related point; my friend Clarence Dykstra, claims that his POI is 2-1/2 MOA higher at Raton NM. than it is in southern Mississippi because of the higher elevation, thinner air, etc." To help understand: The density of air, ρ (Greek: rho) (air density), is the mass per unit volume of Earth's atmosphere, and is a useful value in aeronautics and other sciences. Air density decreases with increasing altitude, as does air pressure. It also changes with variances in temperature or humidity. At sea level and 20 °C, air has a density of approximately 1.2 kg/m3. The density of dry air can be calculated using the ideal gas law, expressed as a function of temperature and pressure: \rho = \frac{p}{R \cdot T} \, where ρ is the air density, p is absolute pressure, R is the specific gas constant for dry air, and T is absolute temperature. The specific gas constant for dry air is 287.05 J/(kg·K) in SI units, and 53.35 (ft·lbf)/(lbm·R) in United States customary and Imperial units. Therefore: * At IUPAC standard temperature and pressure (0 °C and 100 kPa), dry air has a density of 1.2754 kg/m3. * At 20 °C and 101.325 kPa, dry air has a density of 1.2041 kg/m3. * At 70 °F and 14.696 psia, dry air has a density of 0.074887 lbm/ft3. The addition of water vapor to air (making the air humid) reduces the density of the air, which may at first appear contrary to logic. This occurs because the molecular mass of water (18) is less than the molecular mass of air (around 29). For any gas, at a given temperature and pressure, the number of molecules present is constant for a particular volume (see Avogadro's Law). So when water molecules (vapor) are introduced to the air, the number of air molecules must reduce by the same number in a given volume, without the pressure or temperature increasing. Hence the mass per unit volume of the gas (its density) decreases.
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