modal verbs - “It can be very cold…” vs “It may be very cold…”

It can be very cold in winters.
It may be very cold in winters.

What is the difference between these two sentences?

People say when may is used then it means that it may be cold in winters or it may not be cold in winters. In other words, may means 'not sure'.

So does that mean if can is used then it is 100% sure that it will be cold in winters?