A weather surveillance radar site sends out a beam of energy which reflects off objects (precipitation, clouds, trees, buildings, birds, etc.). Echoes from surface targets appear in almost all radar reflectivity images. "Ground clutter" generally appears within a radius of 20 nm.

The amount of energy which is received back at the radar dish is measured in units of DBZ (decibels of Z - reflectivity). The higher the DBZ value the larger the object, i.e. large raindrops and hail produce high DBZ values. In general, DBZ values greater than 20 indicate areas where precipitation is reaching the ground.