Radiation Inversions in the Troposphere

Radiation inversions are probably the most common and wide- spread of the various weather conditions that affect propagation. Radiation inversions form only over land after sunset as a result of progressive cooling of the air near the Earth’s surface. As the Earth cools by radiating heat into space, the air just above the ground is cooled in turn. At higher altitudes, the air remains rela- tively warmer, thus creating the inversion. A typical radiation- inversion temperature profile is shown in Fig 21.25.

The cooling process may continue through the evening and pre- dawn hours, creating inversions that extend as high as 500 m (1500 feet). Deep radiation inversions are most common during clear, calm, summer evenings. They are more distinct in dry climates, in valleys and over open ground. Their formation is inhibited by wind, wet ground and cloud cover. Although radiation inversions are common and widespread, they are rarely strong enough to cause true ducting. The enhanced conditions so often observed after sunset during the summer are usually a result of this mild kind of inversion.