Last week, the temperatures rarely got above zero so when we went out geocaching, the kids were delighted to find a patch of frost in the grass - it was the nearest we've got to snow so they had fun flinging it about and melting it in their hands. I thought it was a bit strange that there was only one white patch in amongst the green grass, but I'd also noticed that there was just one tree covered in white in our local park too. I assumed it was some weird climatic phenomenon.
However, last week was also a big air pollution peak, with public transport free for four days in an attempt to try to reduce the numbers of cars of the roads and therefore pollution levels. It turns out that the two were linked. What we had wasn't natural snow, it was "pollution snow", also called "urban snow" or "industrial snow", which is created when subzero temperatures combine with air humidity (fog or low level cloud) and polluted air. The particles of pollution become coated in a layer of ice and fall as snow. This article explains it very well.
If you look closely, you can see that the ice particles are chunkier than normal, like long needles rather than regular hexagons. Fortunately the kids were none the wiser and still had fun but it makes you think ... I'm dreaming of a white Christmas ? Maybe not then !