The temperature profile of the troposphere makes a strong contribution to how radar emitted radiation will refract in the troposphere. Superrefraction is the beam bending more toward the earth's surface than in normal tropospheric conditions and subrefraction is the beam bending less toward the earth's surface than in normal tropospheric conditions.
An inversion is a situation in which the temperature increases with height. Thus it is a situation where there is colder air under warmer air. An inversion layer is a layer of stability since cold air under warm air is a stable situation. A common type of inversion is the radiational cooling inversion in which overnight the earth's air near the surface cools by ground surface longwave radiation emission. The optimum conditions for a radiation inversion is a dry, clear and long night. Inversions at and near the earth's surface can also occur due to shallow cold front passages and evaporative cooling in the boundary layer. An inversion promotes superrefraction.
Ground clutter is returns to the radar from radar emitted energy scattering off of objects on and near the earth's surface. Ground clutter is most evident when low tilt angles are used since the radar energy travels close to the earth's surface especially at close ranges to the radar. Since a superrefraction situation causes the radar beam to travel closer to the earth's surface, superrefraction will promote an increase in ground clutter.
Thus, the combination of a low tilt angle and an inversion at and near the earth's surface promotes an abundance of ground clutter. Below is an example radar images using the lowest tilt angle (0.5 degrees) taken in the morning when a radiation inversion was in place.