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Watch: Breaking down the science behind horrible Hurricane Harvey

Meteorologist Shawn Milrad calls Harvey “an extremely unusual event, unprecedented in modern times.” We asked him why it got so bad.

NASA

WATCH: Tune in Live to the Metro Canada Facebook page on Friday Sept. 1 at 12 EST to see Genna Buck, who writes our citizen scientist column, talk about the week's science stories and answer questions from our readers.

DECODED: Why Hurricane Harvey was so horrible

Hurricane Harvey slammed southeastern Texas and Louisiana, hovering over Houston for days. It claimed lives, wreaked havoc that will take years to clean up, and dumped a record amount of rain for a single storm. Meteorologist Shawn Milrad calls Harvey “an extremely unusual event, unprecedented in modern times.” We asked him why it got so bad.

Warmer waters: The warm, moist air over the Gulf of Mexico near Houston is the ultimate source of hurricane fuel. The Gulf’s water temperature is also 2-3 C warmer than normal. That’s why Harvey was able to ratchet up from a tropical depression to a Category 4 hurricane in a little over a day.

Jammed jet stream: Normally the polar jet stream picks up hurricanes and blows them northeast, simultaneously weakening them as the storm is dragged over land. But for days during Harvey, the jet stream was stuck in central Canada, blocked in by a high-pressure system over the Rockies and a low-pressure one over the Great Lakes. This left the storm with nowhere to go.

Drastic deluge: Harvey’s rate of rain, seven to 13 cm an hour, is normal for a hurricane, but it’s unprecedented for such a system to spin in place for five days, meandering around at just 3-5 km/h (16 to 24 km/h is the norm). More than a metre accumulated in some places.

Vicious cycle: Hurricanes intensify over water but weaken over land, thanks to friction. With so much of Houston underwater, “To the hurricane, the flooded area is not different than an open ocean,” Milrad said. This could help make the storm self-sustaining, though the importance of this “brown ocean effect” still needs to be measured.

Mediocre models: Meteorologists are hundreds of times better at predicting the paths of hurricanes than they were during Hurricane Katrina in 2005. But computer modelling of hurricanes’ intensity is still a developing science. The level of spatial detail still isn’t fine enough to make reliable predictions, and the “internal dynamics” of hurricanes – exactly what goes on in the eye and surrounding wall – are poorly understood.

What’s climate change got to do with it?

It’s complicated. It’s not possible to link any single storm to climate change. However, the warming planet is creating conditions for stronger, but not necessarily more frequent, hurricanes. Warmer oceans provide fertile ground for storms to rapidly escalate in intensity. Warmer air creates higher average amounts of water vapour, which also fuels hurricanes. And there’s some evidence, not yet confirmed, that warming temperatures in the Arctic are causing the jet stream, the broom that sweeps hurricanes away, to become wavier, weaker and more stagnant.

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FINDINGS: Your week in science

American pika on rock with grass in mouth.

Moose Henderson / iStock

American pika on rock with grass in mouth.

Pika boo-hoo The super-cute American pika, which loves the cold and also lives in the Canadian Rockies, has died out in 165 square kilometres of its core habitat in California's Sierra Nevada mountains, according to a new study in PLOS One. As climate change has caused average temperatures to climb, so too have the fur-coated pikas. They've moved to higher and higher elevations to try to escape the heat, until there was nowhere left to go and they died.

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SOUND SMART: Your vocabulary word for the week

Andres Plana

The word: Wind shear

Definition: Wind shear is a change in wind speed and direction over a relatively short distance in the atmosphere. Low wind shear encourages hurricanes. When a swirling hurricane or tropical storm hits wind shear, it tilts over and weakens, getting torn apart.

Use it in a sentence:

Hold on to your hats: The plane just encountered wind shear and turbulence is coming.

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