Temperature inversion is a meteorological phenomenon where a deviation occurs from normal change of an atmospheric property with altitude. It usually refers to the inversion of the thermal lapse rate. The lapse rate is the rate at which temperature falls with altitude. The temperature drops as you gain altitude due to the lowering of pressure, following the ideal gas law. An inversion would mean that the temperature would be higher at a higher altitude than it is in a lower altitude. This traps air pollution like smog closer to the ground. Inversion can occur when a warm less-dense air mass moves over a more dense cold air mass. This in turn can cut off any convection in the region stopping the air form circulating. As a consequence of inversion, it is seen that radio waves can be refracted making it possible to receive radio signals from long distances.