Urea was observed to have extraordinary ice nucleating properties in laboratory and natural clouds. Ice nucleation was induced in laboratory clouds at temperatures as warm as +6C. The nucleation mechanism involves the high endothermic heat of solution and high solubility of urea. Three field releases of urea in supercooled stratus resulted in marked visual transformation of the cloud and substantial snow showers. It is concluded that urea is an effective seeding reagent for supercooled clouds. In these experiments urea appeared to be equally effective as dry ice in causing shower formation.