North America's East Coast is shivering in a record-breaking freeze in the wake of a deadly "bomb cyclone" that dumped snow as far south as Florida.
In parts of US and Canada, temperatures were forecast to fall below -20F (-29C), with wind chill making it feel more like -90F (-67C) on Friday night.
In Canada, high winds have knocked out power for tens of thousands of residents in Nova Scotia.
Thousands of snow ploughs are clearing roads across the US East Coast.
The extreme weather has so far been linked to up to 19 deaths in the US and two more in Canada, according to reports.
Four deaths were reported in traffic accidents in North and South Carolina. Further fatalities occurred in Wisconsin, Kentucky and Texas.
Travellers at New York's JFK airport reported chaos at terminals on Friday and Saturday, and hours-long waits in aircraft stranded waiting for a gate.
One passenger tweeted to say people were calling the police and threatening to break the doors open after hours waiting aboard a flight from Montreal.
Been stuck on tarmac for over 3 hours at JFK Alitalia flight 8604. Multiple passengers seeking medical attention. Staff not communicating. Babies literally crying from hunger and people calling police from the plane. Please RT to get this to the press.— Chris Mendez (@thechrismendez) January 6, 2018
We’ve been on the plane itself for over 8 hours from Montreal (1 hour away). People are beginning to threaten to break open emergency exits and staff is still saying “30 more minutes” they’ve been saying this for hours. Something is up.— Chris Mendez (@thechrismendez) January 6, 2018
Amid the chaos at JFK, two passenger jets collided early on Saturday, according to officials from the Port Authority, which manages the airport.
The right wing of a China Southern plane struck the tail of a Kuwait airliner. Both aircraft were damaged but no injuries were reported.
In a separate incident, two planes collided at Toronto's Pearson airport late on Friday night, sparking a fire on one aircraft and requiring passengers to evacuate via emergency slides in the cold.
The record-breaking cold comes just days after a phenomenon described as a "bomb cyclone" brought heavy snowfall to a wide area along the US East Coast, as well as hurricane-force winds.
The National Weather Service (NWS) said on Saturday it expected dangerously low temperatures to last into Monday, as an arctic surge brought freezing air down from the North North Pole.
"An arctic outbreak will keep temperatures 20F to 30F degrees below average across the north-eastern US," the NWS said, adding that there was a "chance for many daily temperature records to be broken".
New Hampshire's Mount Washington, which boasts of having the "world's worst weather", experienced record low wind-chill temperatures of -90F (-67C) on Friday night.
In Massachusetts, residents of Boston, which received over 1ft (30cm) of snowfall, were clearing the streets with shovels. Boston harbour's tide gauge matched its record at 15.1ft - previously set during the great blizzard of 1978.
Flooding affected the city's newly renovated seaport district and deluged a downtown subway station, and giant waves saw freezing floodwaters inundate parts of the New England coast.
Mayor Marty Walsh blamed climate change. "If anyone wants to question global warming, just see where the flood zones are," he said.
The storm has so far forced hundreds of schools and businesses to close in New York City, Philadelphia, Boston, the Carolinas, Maryland and Virginia.
Panicked New York City residents rushed to shops before the snow came, clearing shelves of milk, eggs, and kale, the New York Times reported.
The weather pattern has already brought snow to the US South as far down as Florida, where the cold snap has caused iguanas to lose their grip and fall from trees.
Natural gas prices in the US north-east have risen to an all-time high, driven by demand for heating fuel.
In Philadelphia, a car was unable to stop at a railway line at the bottom of a steep hill and was hit by a commuter train, killing a passenger in the vehicle.
In Virginia, a girl was fatally struck by a car while sledging, and a 75-year-old man was killed after being hit by a snow plough.
A 13-year-old girl died and 35 others suffered carbon-monoxide poisoning in an apartment building in Perth Amboy, New Jersey, where the temperature averaged 20F (-7C) on Thursday night. Seven of those treated were first responders.