All winter snow storms except lake-effect snow storms are low pressure systems, with a warm front in front and a cold front trailing. All the precipitation from a winter storm falls as snow or ice.
Lake-effect snow storms have a localized effect, and only occur in narrow bands to the lee of a large body of unfrozen water. All other snow storms are large enough that the snowfall associated with the storm covers a fairly large area. Sometimes a snow storm is also windy, but sometimes there’s just a lot of snow drifting peacefully down. If you don’t have to go anywhere and you are dressed for it, that last kind of snow storm is one of the most quiet, peaceful things you will ever experience.
A blizzard is a very strong snow storm which is accompanied by winds strong enough to reduce visibility. The windspeed and the amount by which visibility is reduced are the other two characteristics which define a blizzard.
Canada
Environment Canada issues blizzard warnings when hazardous amounts of snow are accompanied by winds of at least 40 kilometres per hour. The amount of snow which is considered hazardous is different in different parts of the country, depending upon what each place is used to. Where I live, it’s 25 centimetres of snow within 24 hours. Although the warning is issued only after visibility also falls below 400 metres; in most blizzards you can’t see across the street.
A snow storm must meet blizzard conditions for at least four hours to be classified as a blizzard. North of the tree line, conditions have to stay at blizzard level for at least six hours. Winter storm warnings are issued instead for winter storms which have at least 25 cm of snow within 24 hours, plus at least one of freezing rain, strong winds, blowing snow, and extreme windchill.
If it’s only wind without snow, a blowing snow warning is issued. If it’s just a lot of snow without the blowing snow and wind, or if it doesn’t last for four hours, a heavy snowfall warning is issued instead. For lake-effect snow, Environment Canada issues a snow squall warning. Lake-effect snow is too localized to be considered a blizzard, even though all the other criteria are usually met.
United States
The National Weather Service issues blizzard warnings when hazardous amounts of snow are accompanied by winds of at least 56 kilometres per hour, reducing visibility to below 400 metres. As in Canada, the amount varies depending upon where you are. Severe blizzard warnings, which also include temperature and have a higher minimum windspeed, have been discontinued.
A snow storm must meet blizzard conditions for at least three hours to be classified as a blizzard. Winter storm warnings are issued instead for winter storms which have at least two of heavy snow, freezing rain, sleet, or strong winds.
If it’s only wind without snow, a blowing snow advisory is issued. If it’s just a lot of snow without the blowing snow and wind, or if it doesn’t last for four hours, a heavy snow warning is issued instead. For lake-effect snow, the NWS issues a lake-effect snow warning. Lake-effect snow is too localized to be considered a blizzard, even though all the other criteria are usually met.
Although temperature and windchill values are no longer used to classify a blizzard, that’s not to say it won’t be bitterly cold in the wind. The combination of freezing temperatures plus blizzard-level winds always creates windchills of at least -18 degree Celsius, even when the temperature outside the wind is just below freezing. It’s a good idea to stay out of the wind during a blizzard.
