In certain circumstances, the central pressure inside an area of low pressure can fall at a very rapid rate. These are known as ‘bombs’ and violent winds can develop around the system.
What is a weather bomb?
A ‘weather bomb’ is an unofficial term for a low pressure system whose central pressure falls 24 millibars in 24 hours in a process known as explosive cyclogenesis.
Rapid acceleration of air caused by the jet stream high up in the atmosphere can remove air from the column, reducing its weight so causing pressure to fall at sea level. This in turn sucks in air which converges from surrounding regions resulting in faster and faster rotation of the circulation, in the same way that ice skaters spin faster by drawing their arms in. The resulting winds peak over a period of a few hours and can be strong enough to bring down trees and cause structural damage.
A Blizzard or a Winter Storm?
Of course, if you have an either/or choice – opt for a winter storm!
The word “blizzard” connotes a lengthy snowstorm with strong winds and reduced visibility. The term was into common use around the 1880s on the prairie landscape of the Midwest United States. It may be a creation based on other words like blast, blow, bluster plus the German word “blitz” for lightning.
The blizzards of that era were formidable in part because warnings, communications and transportation were very limited. The deadly Schoolhouse Blizzard extended from Texas to South Dakota is a notable disaster. On January 12, 1988 school children left one-room schoolhouses for the (often long) walk home. Mild temperatures quickly changed to heavy snow and gusty winds that created whiteout conditions. More than 300 people died, most of them children. Some were not found until spring.
Blizzards on the plains and prairies can be insidious. In very cold and dry conditions the result can be very fine snow that has caused suffocation on occasion.
Blizzards are associated with low pressure systems in the cold season that are connected to slow moving mid-atmospheric wind flow. Differences in air pressure are responsible for the winds and similar conditions can persist for many hours, occasionally days.
The criteria for defining blizzards has changed over the decades. Environment Canada and the American Weather Service now warn of blizzard conditions “when winds of 40 km/hr or greater are expected to cause widespread reductions in visibility to 1 km or less, due to blowing snow, or blowing snow in combination with falling snow, for at least 4 hours”.
The older definitions included a persistence period of 6 or more hours with winds of 50 km/hr or more, visibility of 400 metres or less, windchill values of -25 C or lower. Sometimes qualifying adjectives such as occasional or intermittent were included.
The western shoreline of Lake Superior has been immune to blizzard conditions in recent years whatever definition we use. The last formidable storm took place on December 1 and 2, 2007. It was of Colorado low. It originated there and in textbook fashion picked up additional moisture from the Gulf of Mexico. It headed north and Then it slowed and became a massive low pressure system, bringing freezing rain, sleet and/or heavy snow as the storm crawled over Iowa, Minnesota, Wisconsin and Michigan.
Snow began in Thunder Bay on December 1 at 8 a.m. and continued for 42 hours. Wind flow over Lake Superior picked up more moisture and snowfalls of 30 to 50 cm were common around the City of Thunder Bay and along the North Shore. The wind threshold was reached using the newer criteria, the visibility and windchill using the old and no one wanted to quibble over whether this amount of snow qualified.
The storm continued east with considerable enhancement by the relatively warm Great Lakes. Rain, freezing rain and snow caused driving chaos in Toronto – no, the army was not called for this one. Ottawa reported a snowfall rate of 10 cm per hour. The storm continued through Montreal, Quebec and Atlantic Canada, resulting in many road and airport closures and other disruptions.
Blizzard conditions in the vicinity of the Great Lakes are very likely quite different from the events that take place on the Prairies to the west and the plains to the south. There is usually more moisture and heavier snow results. Another fundamental difference is lake effect – the lakes contribute to the consequences – whether a winter storm or blizzard.
Blizzards around the world
Blizzard became a popular term worldwide to describe a fierce storm with strong winds and . . . snow? In Australia, a blizzard can happen when “snow that has been raised from the ground”; another way of saying “blowing snow” as we do in this part of the world.
In the British Isles the term used to be “West of England storm” in another era. “Blizzard” is the proper term now.
Very little is known about the fierce blizzard that struck Iran in February of 1972. The death toll is estimated to be more than 4,000, one of the deadliest snowstorms in history. Whole communities were wiped out within the city of Ardakan and outlying villages were hit the hardest. No survivors were found in Kakkan or Kumar. Near the border with Turkey, the village of Sheklab and its 100 inhabitants were buried by up to eight metres of snow.
On Christmas Eve Environment Canada had a list of weather warnings from coast to coast to coast. A frequent warning was for blizzard conditions. The criteria for defining blizzards has changed over the decades. To recap briefly from the previous Weather Whys, Environment Canada and the American Weather Service currently warn of blizzard conditions “when winds of 40 km/hr or greater are expected to cause widespread reductions in visibility to 1 km or less, due to blowing snow, or blowing snow in combination with falling snow, for at least 4 hours and temperatures colder than 0° C.”.
Blizzards are associated with slow-moving low-pressure systems, gale force winds and snow conditions that can persist for many hours, occasionally days.
Typically, the severity of blizzard conditions peaks and declines during the storm. The strongest winds took place on Christmas Eve when wind gusts of 85 km/h were recorded at the Airport and 116 km/h over the Welcome Islands.
Persistent moderate and occasionally gale-force northwest winds for three days created several effects on the Great Lakes. The average temperature of Lake Superior cooled by 0.26° C down to 3.9° C. This is about twice the usual decline over three days. Water is most dense at 4 degrees and reaching this threshold technically completed the fall overturn. This persistent northwest wind had considerably more effect on other lakes in the Great Lakes system. Lake Huron cooled by 0.6° C, Lake Erie by 1.1° C and Lake St. Clair by 3° C to just above the freezing point.
Persistent winds across a larger lake cause water to “set-up” at the downwind end and lower water levels at the upwind end. In nautical language this process is known as a seiche. The water level imbalance takes a couple of days to subside.
Massive amounts of evaporation took place from all lakes with open water. Major snowfall accumulations and some onshore flooding took place to the east and south of the Great Lakes. It is too early to assess all the records set and a total of the damage caused by the “Christmas Blizzard of 2022”.