An extreme cold weather alert has been issued by the City of Toronto as an Arctic air mass moved in over the GTA.

Weather forecasters predict that the mercury will drop over the next four to five days to sub-zero levels.

Extreme cold weather alerts are issued when Environment Canada says temperatures will drop below -15C without wind chill factored in.

The alert causes city services to spring into action, opening up 80 additional emergency sleeping spaces for homeless people and providing enhanced street outreach services.

Emergency TTC tickets will also be issued through drop-in centres, allowing homeless people to get a free ride to shelters.

The public can also call a 24-hour toll-free number if they see a homeless person sleeping outside in the cold weather. The Street Helpline can be reached at 1-866-392-3777.

This is the third cold weather alert issued by the city in January. The first was issued on Jan. 15. A second alert was issued the next day as temperatures remained low.

Toronto is expected to experienced -13C over night Wednesday and stay there through Thursday.

Temperatures are expected to stay in the same range into the weekend.