Thermographic techniques are presented that directly measure the temperature difference across the thermal boundary layer at the sea surface, the probability density function of surface renewal, the net heat flux, and the heat transfer velocity during nighttime. The techniques are based on a model of surface renewal. Through the use of digital image processing techniques, temporally and spatially highly resolved measurements are feasible, limited only by the thermal imager. We present laboratory measurements from the Heidelberg Aeolotron and field measurements from the GasExII cruise taken at a spatial resolution of 3 mm and temporal resolution of 10 ms. The net heat flux estimates of the thermographic techniques and micrometeorological methods agree with an error less than 5% for conditions in which the surface renewal model is applicable. Experimental evidence is presented for the probability density function of surface renewal to be best described by a logarithmic normal distribution. At moderate and high wind speeds when the influence of surface films is not significant, surface renewal seems to be an adequate model for air-water heat exchange.