An article in the Wisconsin State Journal reported that on November 26, 2024, a factory near Menomonie, Wisconsin contributed to a narrow band of snow that extended for nearly 100 miles. This can occur only with certain atmospheric conditions.
A cloud deck that contained supercooled water droplets was present over the area. Supercooled water droplets are liquid water that are at temperatures below freezing. This frequently occurs in cloud decks.
Relative humidity and dew point are common ways of reporting the amount of water vapor in the atmosphere. Another way is vapor pressure. Gas molecules exert a pressure when they collide with objects. The atmosphere is a mixture of gas molecules and each type contributes its part to the total atmospheric pressure. The pressure that water molecules exert is called the vapor pressure. When the relative humidity is close to 100%, the vapor pressure is close to the saturation vapor pressure.
The bonding forces of the molecules in ice are much stronger than those in water. As a result, the saturation vapor pressure over ice is much lower than that over liquid water when at the same temperature. If the air is saturated with respect to water droplets, it is supersaturated with respect to the ice crystals. Water vapor molecules will deposit onto the crystals, lowering the relative humidity of the air. In response, water molecules evaporate from the water droplets, supplying more water molecules to the air that then deposit onto the crystals. Emissions from the factory likely contained some particles that collided with the supercooled cloud droplets, and then the droplets converted to ice. This increased the ratio of ice to liquid and the ice crystals grew as the water droplets evaporated. At some point, the ice crystals were large enough to fall out of the cloud as snowfall.
Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.
