The reduced target returns from the L-band radar indicated that the reflecting areas are formed by atmospheric disturbances or discontinuities rather than by some form of ionization. If the cause were ionization, it would be expected that the lower frequency of the L-band equipment would increase the susceptibility of the radar energy to reflection or refraction effects. An example of this trend is that of ionospheric layers which produce no appreciable reflection of ultra-high-frequency energy but cause strong skip propagation of the lower radio frequencies.