Considering the heat convection in a wind field, the infrared radiation statistics of natural terrain is analyzed by the heat balance equation. The first-order approximation is used for the radiation term in the equation. The results show that the heat convection will change the statistical distribution of the radiance in the thermal image of natural terrain. With the increase of wind speed, the most probable and average temperature will trend to the steady air temperature, and the contrast of the thermal image will decrease. The saturation phenomena of the convection effect are pointed out.