ASK TOM: Airborne ice crystals

Dear Tom,

At 5 a.m. on the morning of Jan. 22 here in Elgin, the temperature was 3 three below zero and the sky was clear. Under the beam of floodlights in our yard, I observed ice crystals floating in the air. With the air so dry, what explanation can you provide for this phenomenon?

–Dave Kolosowski, Elgin


Dear Dave,


Those were indeed ice crystals, but the air wasn’t was not dry; it was at or near saturation— ; that is, its relative humidity was near 100 percent. Very little moisture is required to saturate bitterly cold air — only 0.001 ounce of water per 1 one cubic foot of air at zero degrees will do it, versus 22 times as much at 80 degrees.
Airborne ice crystals form when warmer air aloft with containing more moisture mixes into colder air near the ground, saturating the colder air so that moisture begins to condense as ice into ice crystals (the process is called deposition).