Snow was a form of precipitation which occurred when the air temperature was below the freezing point of water. It fell on planets such as Earth, (TV: "The Roof of the World") Gallifrey, (TV: Gridlock) Sto, (TV: Voyage of the Damned) Ood Sphere (TV: Planet of the Ood) and Ribos (during its Icetime season). (TV: The Ribos Operation)
Instances when it snowed Edit
On Christmas Eve 1892, it was snowing in London although there were no clouds. (WC: Vastra Investigates) The Great Intelligence used animated snowmen to try to take over the Earth, which could form by themselves, due to them being made up of low-level telepathic snow. This meant that if someone caught in the telepathic field was thinking about the snowmen, then more would have been formed. If that persons thought about them melting, they melted. In the early hours of Christmas Day, the Intelligence was overpowered by the tears of an entire family crying for the loss of Clara Oswald, who had just been invited to be a companion of the Doctor's before she fell to her death. The snow that had been falling was turned into salty rain, the tears of a certain family. (TV: The Snowmen)
Fake snow Edit
The Doctor has ran into 'fake snow' several times.
During Christmas Day 2006, the "snow" falling on London was in fact ashes from the remnants of the Sycorax space ship that had been destroyed by the Torchwood Institute, on Harriet Jones' order. (TV: The Christmas Invasion)
During Christmas 2007, the tenth incarnation of the Doctor used his TARDIS to excite the atmospheric conditions around London to create snow. Although presumably "real" in that it was a form of water, it wasn't "naturally occurring" snow. (TV: The Runaway Bride)
Behind the scenes Edit
One of the deleted scenes for The Waters of Mars showed the Tenth Doctor expressing his love for snow, only to have Yuri Kerenski disappoint him, by telling him that it's the "carbon wash" cleaning the atmosphere.