Thermal image quality depends on properties of hardware, atmosphere, and thermal contrast in the target plane. Weather affects both the modulation transfer function (MTF) of the atmosphere and the thermal contrast in target space. Atmospheric effects are chiefly aerosol light scatter, which causes blurring as well as reduced contrast, and absorption, which reduces contrast. Thermal contrast in the target plane is affected by wind, which tends to equalize temperatures, and by dew, which tends to equalize emissivity. Experiments were carried out over a 2-km line of sight and MTF results and weather parameters processed. Quantitative relationships have been determined relating overall thermal image quality to weather for imaging of passive targets and are suggested as a criterion for forecasting relative quality of thermal imagery according to weather forecast.