Completely off-topic help needed :)

One of my 8 year old son's class assignment questions reads, "How can
you look at the clouds and know if it is about to snow?". Never having
seen snow, I hadn't the foggiest idea. Asked a few friends and none
knows either! A quick search on the internet but came up blank,
probably because I haven't given the keywords that might elicit the
correct answer. Wonder who here (Dubai, UAE) came up with such an
irrelevant question!

Any Canadians here who can help? Or anyone near Chicago (snows there,
right?)?

Thanks much in advance and apologies to the rest,

Ramapriya