High-spectral resolution infrared sounders on board satellites can measure atmospheric trace gases confined to the planetary boundary layer (PBL). However, their sensitivity to the PBL depends on the temperature difference between the surface and the atmosphere, the so-called thermal contrast (TC). After reviewing the physical aspects of TC and how it drives measurement sensitivity, we characterize the global and temporal behavior of TC in clear-sky conditions. Combining land surface temperatures from the Copernicus Global Land Services dataset with air temperatures from the European Centre for Medium-Range Weather Forecasts reanalysis v5, we obtain global monthly averages of TC at high spatial (31 km) and temporal (1 h) resolution. TCs are analyzed as a function of time of the day, time of the year, location and land cover. Daytime maxima are observed from 1130 to 1330 local time, from 5–10 K in winter to 10–30 K in summer. A large dependency on land cover type is observed, both in the magnitude of the daily variations, and in the seasonality. For bare soils, shrublands, sparse and herbaceous vegetation, a maximum is seen in summer with daily TC amplitudes over 30 K. In contrast, for forests, wetlands, and croplands, the seasonal maximum occurs in spring, with daily variations below 15 K. Nighttime TCs typically range between −5 and −10 K. Occasionally, very favorable nighttime measurement conditions occur during winter and autumn due to large temperature inversions. Throughout the paper, we illustrate important concepts by means of satellite observations of NH3 over the Po Valley (Italy).