Radar scatter by large hail

Radars with wavelengths of 3·3 and 4·67 cm have been used to measure the back‐scatter cross‐sections of individual artificial hailstones and their variations as melting occurs. The measurements confirm the cross‐sections computed theoretically. An ice sphere is a better scatterer than a liquid water sphere of equal size when its diameter exceeds 0·8 wavelengths; when the diameter lies between 1·2 and 2·3 wavelengths it is better by a factor of 10 or more. This behaviour of ice is supported by scatter measurements on a set of plexiglass (perspex) spheres with refractive index close to that of ice. When melting begins, the 3·3 cm cross‐sections drop sharply toward the all‐liquid values; those at 4·67 cm decrease gradually, and sometimes not at all. The rate of decrease of the cross‐sections during melting is also more variable at 4·67 cm than at 3·3 cm. Most of the 3·3 cm change appears to occur during the initial growth of the liquid coat to a maximum thickness of about 0·01 cm, while the major drop in 4·67 cm cross‐sections occurs well beyond this stage of melting. The implications are that: (1) a water coat of 0·01 cm thickness has a major effect at 3·3 cm and a minor one at 4·67 cm; (2) liquid continues to collect in subsurface cracks and cavities in the ice and causes a further decrease in the cross‐sections after the maximum thickness of water coat has formed. The final cross‐sections are generally within 2 db of the all‐liquid value after 7 to 10 minutes of melting at 3·3 cm and after 15 to 20 minutes at 4·67 cm. Possible reasons for these results are discussed.