the thermal radiation of the earth’s surface. Since the surface of the earth has a comparatively low temperature, it radiates electromagnetic waves with wavelengths of 3–80 microns, which fall into the infrared portion of the spectrum, which is invisible to the eye. The surface of the earth cools down because of its intrinsic radiation. The flow of intrinsic radiation of the surface of the earth is directed upward and is absorbed almost entirely by the atmosphere, thus heating it. The atmosphere, in turn, sends counter-radiation toward the surface of the earth (atmospheric counter-radiation) at approximately the same wavelengths, which partially compensates for the loss of heat by the surface of the earth as a result of its intrinsic radiation. The difference between the radiation of the surface of the earth and the counter-radiation is called the effective radiation. On clear nights the counter-radiation decreases and the effective radiation increases; therefore, the surface of the earth is cooled abruptly and the lower layers of the air are cooled by it. In the process fog or dew may occur, and in the spring and fall, there may be frosts. On cloudy nights, on the other hand, the counter-radiation increases because of the radiation of the clouds, and the effective radiation and cooling of the surface of the earth are reduced. During the day the surface of the earth also receives solar radiation in addition to the counter-radiation. Together they exceed terrestrial radiation for most of the day (during the warm part of the year in moderate latitudes), and the surface of the earth heats up. Terrestrial radiation is one of the most important factors determining the thermal conditions of the surface of the earth and of the atmosphere.