Laboratory experiments measuring the charge transferred when individual 100-μm ice spheres impact upon various metal targets show that the charge transferred depends upon the work function of the metal. If ice is assigned a ''work function'' of 4.3 eV, then the contact potential difference between the ice and the metal accounts for the observed charging. It may be possible to explain the generation of charge within thunderstorms in terms of a contact potential difference between colliding vapor-grown crystals and riming hailstones. Although many experiments measuring the charging of ice from collisions can be explained in terms of contact potentials, we report one that cannot. A layer of ice estimated to be 1 μm thick deposited from the vapor (''frost'') on the target is sufficient to cause it to charge positively, and conversely, if a similar thickness is allowed to evaporate, the target reverts to negative charging.