SUPERSATURATION

When the air over a cloud droplet is supersaturated (relative humidity greater than 100 percent), the vapor pressure of the air is greater than the saturation vapor pressure on the droplet's surface. In such an environment, water droplets grow by vapor deposition; more water vapor molecules are condensing to the droplet than are evaporating from it. Greater supersaturation occurs in stronger storm updrafts because the process of vapor deposition cannot keep up with the decompressional cooling with ascent. Ice crystals in the presence of abundant cloud droplets are said to be supersaturated with respect to ice even though relative humidity may only be 100 percent. This is because the saturation vapor pressure over ice is less than that over water, allowing vapor deposition onto the ice.