These layers of warm and cold air can cause signal degradation or complete loss of signal by bending the ENG signal down into the ground or up into space. Refraction happens The bending of microwave signals passing through thermal layers of air is similar to the bending of light as it passes from air to glass. There is a point, as the angle of the light striking the glass decreases, where all the light is reflected away from the glass. The angle at which this occurs is called the critical angle. As air density decreases, the index of refraction increases.
Air density is affected by temperature.
This may not sound like much to worry about, but ENG signals may be hitting these layers at very shallow angles. A small density change will affect the critical angle, potentially resulting in a complete loss of signal. With microwave signals penetrating thermal layers, the bending is far more gradual and prolonged. The effect is the loss of a signal over a period of seconds or minutes when thermal layers develop. Cities generate considerable heat.