For most of the lowest 90 miles or so of the atmosphere, the primary tendency of the temperature of the air is as follows: the higher the elevation above the surface of the earth, the lower the temperature. Layers of air that don't follow this trend are called inversion layers. They usually form near the ground.

Temperature inversions are common. They occur on most calm clear nights and, less frequently, under certain daytime conditions. A principal mechanism whereby they form at night is the movement of heat away from the earth's surface by infrared radiation. These radiation inversions dissipate rapidly when the sun comes up, but during the winter the radiation inversions that form during the night are intense. Calm clear conditions are also likely to bring about a temperature inversion within hours of sunset. Usually the most intense ground-based inversions form during seasonably cold nights, during which there is also likely to be a heavy dewfall or frost.

The acoustical significance of such layers comes from the fact that the speed of sound is higher at higher temperatures. That means that within inversion layers the speed of sound is highest at the top of the layer and lowest at the bottom of the layer.

The consequence of this deflection of skyward-bound sound toward the ground is that the noise levels on the ground can be greater than they would otherwise be if only the direct path along the ground between the source and receptor were available to the sound. One could almost say, by way of an analogy with optics, that the source of the sound is magnified by the temperature inversion.

The reader who has never before taken note of this phenomenon might now try to see if it's 'real'. Just listen to the background noises from distant trains, distant aircraft (just during takeoff runs) and vehicles on distant roadway segments, preferably after dark or shortly before or after dawn, and note the atmospheric conditions. Disregard windy conditions, but take note of the loudness of such noises under all other atmospheric conditions. It should become apparent that the noise levels from distant sources are clearly higher on some days than on others, even though the observations were made at approximately the same time each day. With some effort it should also be apparent that the calm and clear conditions that are said here to give rise to temperature inversions are in effect when the distant sources seem loudest.