Snow
From Memory Alpha, the free Star Trek reference.
Snow is the formation of frozen water droplets high in a planet's atmosphere. It is white in color, and requires a temperature below 0°C to hold its form. Snow is extremely common on Andoria, (ENT: "The Aenar") and is fairly common in the high mountains of Earth.
In 2364 Wesley Crusher and a friend joined a holodeck program, which recreated the Denubian Alps. They went to the program for skiing lessons and later had a snowball battle. One snowball hits Captain Picard. (TNG: "Angel One")
In Data's recreation of the holonovel A Christmas Carol from Charles Dickens it is snowing outside the window. (TNG: "Devil's Due")
In 2369, when the thoughts of the Deep Space 9 crew were becoming reality, snow appeared on the station's Promenade. Odo predicted that there would be five or six centimeters of snow. (DS9: "If Wishes Were Horses")
