When it Snows on Twitter is an animated map of yesterday's Twitter messages mentioning snow in the USA. If you want to you can compare this CartoDB map to the NOAA map of actual snowfall. Comparing the two maps it appears that Twitter users can actually tell when it is snowing.
In the UK you can check out the famous #uksnow Map, which maps Tweets mentioning snow. The map is actually showing a bit of action now, in Scotland, for the first time this year.