Ontario experienced a rare weather event on Family Day as most of the province was under an extreme cold warning.

Environment Canada meteorologist Geoff Coulson said what counts as extreme cold in one part of the province often isn't deemed extreme in another region.

The warnings are calculated based on what people are used to, and the conditions to which they can acclimatize, Coulson said.

Environment Canada warning preparedness meteorologist Geoff Coulson says it needs to be 10 degrees colder in Thunder Bay than to Toronto, to trigger an extreme cold warning. (Environment Canada) "What we're trying to do with the extreme cold warning is to notify folks when we're getting into values that we don't see very often, in different parts of the province, " he said.

"Something that is relatively rare"

Environment Canada studies standard temperatures and wind chills for an average winter in a given area "trying to ensure that the criteria we're using for extreme cold is something that's relatively rare in the given area," Coulson said.

Coulson explained the extreme cold warning thresholds in the following regions:

a wind chill or temperature of -30 C, or colder, in south-central and southwestern Ontario

In eastern Ontario, towards the North Bay area, temperature or wind chill value of –35, or colder

temperature or wind chill of –40, or colder, for regions around Thunder Bay, Sault Ste. Marie, Kenora and the north shore of Lake Superior

for the Far North region, north of Pickle Lake to the shoreline of Hudson Bay and James Bay, temperature or wind chill value of –45 or colder

Coulson said it's uncommon for most of the province to be under an extreme cold warning at the same time.

"Significant outbreak of Arctic air"

"Normally as the cold air moves in, it tends to affect different parts of the province at difference times," he said. "But every once in awhile we get a significant outbreak of Arctic air that really affects a huge portion of the province, as well as surrounding provinces, as well as deep into the United States."

Coulson said this blast of Arctic air may trigger several more extreme cold warnings in Ontario in the coming days.