Winter tires are recommended when the daily high is at or below 7°C.
Why? Winter does not mean SNOW - it means when temperatures drop below 7°C the tread compound on all-season tires begins to harden, causing loss of grip on cold, wet roads. Winter tires however, remain flexible and can still grip at low temperatures, thanks to a higher percentage of natural rubber.
Winter tire tread patterns, which are much deeper than all-seasons, are designed to dig down and bite into snow and ice. Snow sticks in wider tread grooves, as snow-on-snow traction is better than snow on rubber.
Always install four winter tires! Installing only two will cause inconsistent traction as one end of the vehicle won't react the same as the other, meaning you could fishtail or spin out of control.