Usually, the surroundings of objects located in open atmospheric space consists of two surfaces: the hypothetical surface of the sky and the surface of the ground. Radiative and convective heat transfer occurs between such objects and the environment. The purpose of thermovision examination of objects placed in open atmospheric space, especially buildings, is to determine the value of heat loss or to assess the quality parameters of insulation of walls, roofs and other partitions. The examined surfaces, including elements of the outer shell of buildings, are usually arranged at different angles to the ground surface and the sky. For the needs of thermovision examination of considered objects as well as calculations regarding radiation heat exchanging, knowledge concerning the radiation parameters of the sky is necessary. These parameters include the equivalent radiative ambient temperature needed for thermovision measurements (including the sky and ground temperature) and the radiative ambient temperature that decides about the radiative heat loss from the surface being tested. This paper contains a description of the method for the determination of sky thermal radiation intensity on the basis of sky temperature measurement with the use of long-wave infrared camera. Quite often the sky temperature is very low and is outside the measuring range of the infrared camera. In such cases, a specially developed measurement method was used. To verify the developed method, the results obtained with the use of the long-wave infrared camera and measurement results obtained by means of a pyrgeometer have been compared. The verification included a comparison of the radiative flux of thermal emission of the sky calculated on the basis of thermovision measurements and the energy flux measured using the pyrgeometer. The verification result is quite satisfactory.