What is an inverson layer?

Normally the higher in the atmosphere you go, the colder it gets. However under certain conditions, a temperature inversion can form close to the Earths surface where the temperature goes up as you get higher.
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.
These temperature inversions have the effect of making a distant sound appear louder that it would otherwise be.
Funny old thing, frost fans rely on temperature inversions to operate effectively (i.e. pull warmer air down from above to distribute it over a crop). So it should come as no surprise that frost fans are noisier at night under the conditions that they normally operate in than during the day.
This is important, as if a frost fan is being tested to see how noisy it is, this testing needs to be done in the same conditions as it operates in. I.e. at night in a frost with an inversion layer. Otherwise the tests will not show an accurate noise level.
For a little more information see here at the frost fans web site and here at Wikipedia.