Shower Formation in Large Cumulus

A distinction is drawn between the appearance of evanescent fibrous streaks in decaying cumulus towers (fibrillation), and the glaciation of cumulus. A technique of measuring cumulus is described, and used to show that fibrillation, which marks the onset of shower formation, occurs when cumulus towers reach a fibrillation zone or a fibrillation level; towers which rise appreciably above this zone glaciate if supercooled. The fibrillation level varies from occasion to occasion, apparently according to the strength of the cloud updraughts, and in a manner which indicates the coalescence process to be the general shower-forming agency in clouds observed in Sweden, although the temperatures at the summit-level of incipient showers may be as low as — 20° C.