Standard Atmosphere & Density Altitude

Two calculators built on the same model. The first gives the properties of the International Standard Atmosphere, identical to the U.S. Standard Atmosphere 1976 from −5 km to 80 km: temperature, pressure, density, speed of sound, dynamic and kinematic viscosity, gravity, and the temperature, pressure and density ratios θ, δ and σ used in aircraft performance work, for either a geometric height or a geopotential one, with the other shown alongside. The second is the pilot's density altitude check: from the field elevation, the altimeter setting (QNH), the outside air temperature and optionally the dew point, it gives the station pressure, the pressure altitude, the deviation from ISA and the density altitude, the height in the standard atmosphere where the air is as thin as it is on the runway now. Humidity is included through the vapour pressure, because moist air is lighter than dry air: a humid 32 °C day at Denver reads a couple of hundred feet higher than a dry one. The values match the ambiance library, itself checked against the 1976 tables, to within a few parts per million. What the model cannot tell you: the real atmosphere on a given day departs from ISA by tens of degrees, so the first calculator describes an average, not today's sky; and above 80 km the gas composition changes, which is why the tool stops there. Density altitude here comes from physics, not the 120-feet-per-degree rule of thumb, so it can differ from a quick E6B estimate by a few hundred feet.