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How an ‘atmospheric stream’ drenched British Columbia leading to flooding and mudslides
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How an ‘atmospheric stream’ drenched British Columbia leading to flooding and mudslides

An infographic showing the flow of condensed water vapour from the ocean onto land.


The West Coast of Canada is known for its wet autumn weather, but the storm that British Columbia’s Fraser Valley experienced over the weekend was one for the record books.

A weather system called an “atmospheric river” flowed across the southwest corner of the province and, over a period of two days, brought strong winds and near-record amounts of rain, which caused widespread flooding and landslides. One person has already died.

Hope, Merritt, Princeton, which were especially hard hit, received 100-200 mm (or more) in rain. All of the highways linking Vancouver to the rest were closed due to washouts or landslides. This isolated Vancouver from the rest Canada, at least by road.

What is an atmospheric stream?

An atmospheric river is a band containing warm, moist air that runs hundreds of kilometres along the border of a large cyclonic low pressure system.

This term is used to describe bands of moisture-associated weather system that move inland from Pacific Ocean. An atmospheric river can reach the coast anywhere in the region between Southeast Alaska and Northern California.

These weather systems carry a lot of moisture, so the term has been used as a metaphor for a river on the ground. Atmospheric rivers, however, are not carried in a channel as a real river and can drop large amounts of rain over large areas.

An infographic showing the flow of condensed water vapour from the ocean onto land.

The science behind atmospheric river.

It was a record-breaking rainstorm.

The weekend storm was notable for its duration as well as its intensity. It rained continuously for more than 24hrs over a large storm track. The rate was also higher than normal autumn rains.

Both floods were caused by rapid runoff that exceeded the carrying capacities of rivers and streams, leading to them spilling onto floodplains. Additionally, areas flat like the Sumas Prairie region between Abbotsford, Chilliwack, had been saturated already with October and early November rains and were unable to drain the water away. They were thus flooded.

The warm rainfall also caused melting snow at high altitudes, which further exacerbated the flooding. The region saw record rainfall on Nov. 14, setting new records. Abbotsford, for example, recorded 100 millimetres rain that day, surpassing the previous record of 49 millimetres, set in 1998. Hope had 174 millimetres, five-fold the amount in record year of 2018.

Are summer wildfires a contributing factor to the current situation?

B.C. B.C. had its second worst wildfire season in 2021 with more than 1,600 fires charring nearly 8,700 square kilometersIt is found mainly in the south of the province. Only 2017 saw more forest lands burned.

Soils in forested landscapes are hydrophobic after severe fires — they repel water. Super-heated air during a wildfire disperses waxy compoundsThey are found in the forest litter layer’s uppermost layer. Hydrophobic compounds are added to the soil to coat mineral grains. The water-repelling layer can be found at least a few centimetres below ground surface. It is often covered with a layer of burned dirt or ash.

Many areas that were affected by the wildfires in 2021, such Merritt, were submerged in rain over the weekend of November 13. It is possible that the hydrophobicity and speed of runoff from these burnt areas made it more likely.

Is this related?

Scientists are not usually willing to attribute single extreme weather events as a result of climate change. However, the exceptional events of recent years are shifting opinion.

These are just a few examples of exceptional events flooding in western Germanyand eastern Belgium, and in Henan Province ChinaBoth in July 2021 extreme heat and wildfires in Siberia2020 and 2021; and “heat dome” over western North America at the end of June 2021.

A row of people holding shovels stand on the edge of a dirt road, surrounded by orange smoke.
Volunteers take a moment to pause while they control a wildfire that has erupted near Kyuyorelayakh, Russia, in August 2021.
(AP Photo/Ivan Nikiforov)

These events are so far “off scale” in comparison to past historic extreme events that climate modellers assert that they would not have happened or would not have been as severe were it not for climate change.

These events are consistent in predictions made by atmospheric scientists. weather extremes will become more frequent and more severe as Earth’s climate continues to warm.


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