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.
This can cause a bubble of warmer air to form over the city, with much cooler temperatures in the air surrounding the countryside. In the same manner, a large body of cold water can produce a layer of cooler air over a lake and the surrounding area for several miles inland. In the area around the Great Lakes, there can be springtime air temperatures of up to 60 degrees inland while there is still a layer of ice and 32-degree water on the lake. This can cause a very sharp thermocline as the colder, lake-blown air drifts inland. The same effect can occur near the ocean on a warm spring day.