Aspects of the present disclosure describe rapid temperature measurement by wavelength modulation spectroscopy (WMS) that determines gas temperature from 2ƒ signals from two absorption lines by WMS methodologies even when the gas concentration is sufficiently high to saturate optical absorptions. In sharp contrast to the prior art, rapid temperature measurement by WMS according to aspects of the present disclosure employs both a 2ƒ signal ratio and gas concentration determined from the 2ƒ signal.