Relative permittivity of air

Air is very nearly a vacuum electrically. Its relative permittivity exceeds 1 by only six parts in ten thousand, which is why the free-space formulas for capacitance, wave impedance and the speed of light can be used in ordinary rooms without correction, and why an air-spaced capacitor is the closest practical approach to an ideal one. The tiny excess scales with density, so it falls with altitude and rises with pressure, and the water-vapour content matters more than anything else because the water molecule's permanent dipole is a far stronger contributor than nitrogen or oxygen.

That small residue is not always negligible. Radio and radar refraction, GPS tropospheric delay and surveying with electronic distance meters all live on it: the refractive index of air, n = √εᵣ ≈ 1.000293 at optical frequencies, is the reason a laser rangefinder must be corrected for temperature, pressure and humidity to reach millimetre accuracy over a kilometre.