Ottawa Public Health has issued a frostbite advisory for Monday and New Year's Eve.
The temperature is expected to hover around -16 C with a windchill of close to -30 C, Environment Canada predicts.
As of 2:30 p.m. ET Monday it was -17 C with a windchill of -27 C.
Skies are expected to clear throughout the morning with west winds of about 30 kilometres per hour, gusting at times to 50 km/h. The winds should die down in the later afternoon.
Overnight, the weather agency expects winds up to 15 km/h with a low of -23 C and a wind chill of -27 C.
On Tuesday, the high is once again expected to hit -16 C with periods of snow beginning early in the morning and ending early in the evening.
At -15 C, hypothermia becomes an increasing concern and when the wind chill reaches -35 C or colder, exposed skin can freeze in as little as 10 minutes, Ottawa Public Health said in a notice posted on the city's website.
It said elderly people, children, infants, the homeless, newcomers to Canada, outdoor workers and sport enthusiasts are the most susceptible.
Outreach groups busy
The Ottawa Mission is one place where the city's homeless population of more than 7,000 can come in from the cold, but space is limited. It's able to accommodate around 260 people.
"A lot of our people suffer from mental health issues and they just don't like crowds as it is," said the Mission's executive director Peter Tilley.
"Right now we're trying to ask people to stay inside and not walk the streets."
Alex Marinos said he used the Mission during the nearly two years he lived on Ottawa's streets.
"I have seen people under bridges in this temperature, it's unbelievable," he said.
Experts say about 100 people die in Canada every year from exposure to the cold, most of them homeless.
"They are no doubt the the people that are affected by these very cold conditions, and it's not just the deaths," said David Phillips, a spokesman for Environment Canada.
"People can suffer from frostbite or hypothermia."
The Ottawa Mission said it's asking for donations of coats, gloves and boots to help.
'Frost quakes' rumble through southern, eastern Ontario
Another side effect of the quick drop in temperatures is what is known as "frost quakes."
Environment Canada says that when the ground becomes saturated with water and then there's a warm spell followed immediately by a cold snap, the pressure can cause rocks or soil to crack, causing a loud boom.
Phillips said frost quakes were reported from Kitchener to Ottawa on Monday.
"They're what we call frost quakes or 'cryoseisms' in the technical language, but they're really just large pieces of ice below the surface that when extra cold, will crack," he said.
"It provides a sound and maybe a little bit of movement, I certainly heard them but I didn't feel them."
Phillips said frost quakes won't cause any real damage — they're just loud.