A wind band in excess of 200 km/h is responsible for most of the weather we experience on a daily basis.
Jet streams are one of the true wonders of the world. There isn’t just one jet stream – both the northern and southern hemispheres each have multiple jet streams that determine general conditions around the globe.
But there’s a catch: they’re not consistent year-round. Jet streams ebb and flow with the seasons, growing stronger in winter and weaker in summer.
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Jet streams push and pull enormous amounts of air
A jet stream affects our weather by pushing and pulling the air beneath it.
Winds don’t rush around troughs and ridges in a straight line—they fan out (diverge) as they whiz through the base of a trough and collide (converge) as they slow as they round the top of a ridge.
The divergence creates a cavity that pulls air up from the surface, resulting in extensive low-pressure systems that can wreak havoc on the ground. Convergence forces the air to descend toward the ground, resulting in those clear, comfortable days we cherish between storms.
Jet streams change with the seasons
This cycle is not consistent throughout the year.
General weather patterns depend on the seasonal strength of the jet stream, which is why we see scenic lows swirl like atmospheric cinnamon rolls over Canada in winter, but rarely see these giant systems arrive in the heart of summer.
The jet stream itself exists because of temperature differences between the tropics and the poles.
Warm air rising from lower latitudes flows toward higher latitudes in nature’s constant effort to balance the atmosphere. As the air descends, the Coriolis force causes the air to spin horizontally and blow parallel to the ground, creating these relatively narrow wind bands.
There are several of these large-scale circulations in each hemisphere, creating several distinct jet streams. The subtropical jet stream typically extends across the southern United States, while the polar jet stream is typically at home over Canada and the northern United States
WATCH: How jet stream movement is affecting Canada’s weather
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Seasonal fluctuations in the jet stream are driven by seasonal temperature differences here on the ground.
In summer there isn’t a dramatic difference in temperature between the tropics and, say, southern Ontario – after all, Miami and Toronto could be 30°C on the same day.
But in the middle of winter there are often gigantic temperature gradients between the northern and southern parts of North America, sometimes on the order of 70 degrees or more.
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As a result, summer jet streams are much weaker than the jet streams we see during the colder months.
Smaller features determine much of our summer weather – think sea breeze thunderstorms, squalls and pop-up showers produced by daytime heating. The greatest impact of these upper-level patterns on our summer weather is when mammoth crests promote unbearable heat and humidity ranges.
Compare that to so much of the weather we see in winter that is directly driven by the jet stream.
Colorado lows and Texas lows rushing north of the border can hit half of Canada at once, which is no small feat given the country’s massive size. Some of the most memorable storms in recent history have been northeast winds that roared up the east coast.
