Since mid-December, my inbox has been flooded with emails about Sunday storms. This one came from Danielle Pellerine: “It seems that here in Atlantic Canada we have one particularly stormy day each winter. This year on Sundays. Is there a scientific reason behind it or is it just coincidence?”
It is common for the weather to become locked in a repeating pattern. The hemispheric weather patterns are governed by mid-latitude westerly winds which move in large, wavy patterns. Known as planetary waves, these long waves are also called Rossby waves, named after Carl Rossby who discovered them in the 1930s.
These waves become locked in position for many days or weeks in a row. When this happens, the pockets of vorticity – or surface weather systems – start to track through in predictable patterns.
systems that develop between longwave troughs move through in about 36 hours. Between each one of these, there’s an area of fair weather (ridge of high pressure). Typically, in the winter, these hold for 48 hours. So, in three to three-and-a-half days, you’ll have experienced a low and a high. Another three-and-a-half or so, and the next low will move in. There’s your pattern. This year we seem to be experiencing a Wednesday/Sunday cycle.
The Sunday factor is an interesting one. For decades, researchers in the UK have been examining the high rate of wet systems on Sunday. It appears that this could be more than a myth.
New research shows weekends are wetter. The cause, scientists suspect, is the build-up of pollution during the week, resulting in rain or snow by Sunday. Records from monitoring stations showed that levels of two urban pollutants, ozone and carbon monoxide, rose as the weekend approached.
The region just off the heavily populated east coast of the U.S. was soaked on weekends; on Saturdays, there was about 22 per cent more rain than on Mondays.
An examination of five decades of data on hurricanes and tropical storms in the Atlantic showed a similar cyclical pattern.
The scientists at Arizona State University suspect the link between human activity and the weather is pollution. Writing in the journal Nature, they said: “Although our statistical findings limit the identification of cause-effect relationships, we advance the hypothesis that the thermal influence of pollution-derived aerosols on storms may drive these weekly climate cycles.”
I’ll use Halifax as an example: there has been significant snow or rain five of the last seven Sundays. Tomorrow will make it six in eight, or 75 per cent of the time.
It’s not carved in stone, or snow, but I would choose Saturday over Sunday for my next winter outing.
- Want more weather information? Visit your weather page.
- Have a weather question, photo or drawing to share with Cindy Day? Email [email protected]
Cindy Day is the chief meteorologist for SaltWire Network
