Abstract
The ice forming ability of solid particles was tested at −2 to −3C after first cooling them to −35 to −78C. During the cooling process, the water vapor pressure over the solid particles was kept below ice saturation, in order to avoid any ice formation on them. All of the treated samples of nine test materials showed ice-forming ability at −2 or −3C which is much warmer than their ordinary threshold temperatures. This ability was lost after warming above 0C.