The #uksnow Map is interesting. It's uses Crowd Sourcing and Twitter to build up a map based on people's reports. It seems to work.
Tim, as you are a scientist, its worth debating the validity of crowdsourcing in this situation.
Surely there will be a whole lot of really serious snow missed out on maps like that - as virtually no-one lives in vast tracts of the Highlands for instance? Conversely, if there are a few flakes in London then that part of the map will be dense with reports.
I'm prepared to be proved wrong here - just trying to provoke a sensible debate.
I totally agree with you, and it clearly shouldn't be used for any rigorous measurement of the conditions. In an ideal world, you could normalise the data to a degree using population density maps, but I suspect that would need a much greater number of tweets to have a chance of working
On the whole though, it does give some additional idea of the snow currently falling. Maps like the one Wendy linked to are not always freely available, and I suspect the radars aren't necessarily easily capable of distinguishing between snow and rain (or possibly don't even detect snow).
It's just another source of data, and like any source, has to be used with some degree of care.