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Fort McMurray wildfire and the smoke-trapping inversion which made the fire look calm

WATCH ABOVE: Ongoing video coverage of the Fort McMurray wildfire

Fort McMurray residents, all 70,000 of them, were ordered to evacuate Tuesday due to a raging wildfire threatening the northeastern Alberta community.

But to many on Tuesday morning, it appeared the fires had died down from Monday night. Fire officials, however, warned it was largely an illusion due to an inversion.

READ MORE: Fort McMurray wildfire: Flames erupt along Highway 63 during live Global News broadcast

Listen below: 630 CHED is providing ongoing live coverage of the state of emergency in Fort McMurray.
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“We wake up this morning and we don’t see anything. And people think it’s fine and it’s all gone away… Don’t get into a false sense of security,”
warned Regional Fire Chief of Wood Buffalo Darby Allen.

“We’re in for a rough day. It will wake up and it will come back.”

A few hours later, what Fort McMurray-area residents saw when they peered out the window quickly changed. The inversion had broken, allowing the smoke trapped close to the ground to rise, revealing a resurgent fire.

An inversion explained

Generally, nights and mornings are cool, along with the surface of the earth. And often, warmer air is sitting on top of that cooler air that’s close to the ground, acting much like a lid.

“As you go up in the atmosphere from the surface, temperatures usually decrease with height,” says Mike Flannigan, University of Alberta professor and wildfire expert.

“But at times temperatures increase with height above ground, and that’s called an inversion because it’s inverted from the normal position of it being cooler as you go higher.”

This is common; and as the day heats up the surface of the earth warms and the inversion disappears.

“When we do have inversions, smoke or pollution gets trapped.”

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A recent heat wave has brought scorching temperatures to the region, with thermostats reaching a high of 32 C Tuesday.

READ MORE: Fort McMurray wildfire: a closer look at the community under fire threat

Inversions also keep strong winds from reaching to the earth’s surface, but once it breaks the gusty winds sweep through.

“Once the inversion breaks down then you get these stronger winds from above the earth’s surface coming down to the earth’s surface, coming down to the head of the fire,” Flannigan says.

“So the winds pick up in speed and can be gusty, and that helps the fire grow.”

Inversions happen almost every day, especially where there is a high-pressure system. When there’s rain or wind they are less common.

“In spring in Alberta, just about every day there will be an inversion. That’s fairly normal.”

Trees burn near a road in Fort McMurray, Alberta on Tuesday May 3, 2016 in this image provide by radio station CAOS91.1. At least half of the city of Fort McMurray in northern Alberta was under an evacuation notice Tuesday as a wildfire whipped by winds engulfed homes and sent ash raining down on residents. THE CANADIAN PRESS/HO-CAOS91.1(KAOS).
Trees burn near a road in Fort McMurray, Alberta on Tuesday May 3, 2016 in this image provide by radio station CAOS91.1. At least half of the city of Fort McMurray in northern Alberta was under an evacuation notice Tuesday as a wildfire whipped by winds engulfed homes and sent ash raining down on residents. THE CANADIAN PRESS/HO-CAOS91.1(KAOS).
Fort McMurray wildfire at Highway 63 Tuesday, May 3, 2016.
Fort McMurray wildfire at Highway 63 Tuesday, May 3, 2016. Dean Twardzik, Global News
Fort McMurray wildfire Tuesday, May 3, 2016.
Fort McMurray wildfire Tuesday, May 3, 2016. Dean Twardzik, Global News
Fort McMurray wildfire Tuesday, May 3, 2016.
Fort McMurray wildfire Tuesday, May 3, 2016. Dean Twardzik, Global News
Fort McMurray wildfire Tuesday, May 3, 2016.
Fort McMurray wildfire Tuesday, May 3, 2016. Dean Twardzik, Global News
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Highway 63 at Confederation Way facing west Tuesday, May 3, 2016 at around 5 p.m. MT.
Highway 63 at Confederation Way facing west Tuesday, May 3, 2016 at around 5 p.m. MT. Credit, Alberta government traffic camera
Fire consumes an RV park in Fort McMurray, AB.
Fire consumes an RV park in Fort McMurray, AB. Jame Babcock
CJ Varney posted this picture of the Fort McMurray fire approaching the Shell gas station 3:35PM May 3.
CJ Varney posted this picture of the Fort McMurray fire approaching the Shell gas station 3:35PM May 3. CJ Varney / Twitter
Peter Fortna says that as of 3:30PM on May 3 the Fort McMurray fire is meters from the Waterways overpass.
Peter Fortna says that as of 3:30PM on May 3 the Fort McMurray fire is meters from the Waterways overpass. Peter Fortna / Twitter
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Trevor Norris (@TNorrisYEG on Twitter) posted this photo from Fort McMurray on May 3, 2016 at 3:30PM. Norris says that traffic isn't moving at all.
Trevor Norris (@TNorrisYEG on Twitter) posted this photo from Fort McMurray on May 3, 2016 at 3:30PM. Norris says that traffic isn't moving at all. Trevor Norris / Twitter
Cody McNernie (@CodyMcNernie on Twitter) posted this photo from Fort McMurray on May 3, 2016 at 3:15PM.
Cody McNernie (@CodyMcNernie on Twitter) posted this photo from Fort McMurray on May 3, 2016 at 3:15PM. Cody McNernie / Twitter
A wildfire in Fort McMurray intensified Tuesday, May 3, 2016.
A wildfire in Fort McMurray intensified Tuesday, May 3, 2016. Reid Fiest, Global News
Fort McMurray wildfire flares up Tuesday, May 3, 2016.
Fort McMurray wildfire flares up Tuesday, May 3, 2016. Fletcher Kent, Global News
The Fort McMurray wildfire flared up just before 2 p.m. Tuesday, May 3, 2016, leading to more voluntary evacuation notices being issued.
The Fort McMurray wildfire flared up just before 2 p.m. Tuesday, May 3, 2016, leading to more voluntary evacuation notices being issued. Dean Twardzik, Global News
The Fort McMurray wildfire flared up just before 2 p.m. Tuesday, May 3, 2016, leading to more voluntary evacuation notices being issued.
The Fort McMurray wildfire flared up just before 2 p.m. Tuesday, May 3, 2016, leading to more voluntary evacuation notices being issued. Dean Twardzik, Global News
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The Fort McMurray wildfire flared up just before 2 p.m. Tuesday, May 3, 2016, leading to more voluntary evacuation notices being issued.
The Fort McMurray wildfire flared up just before 2 p.m. Tuesday, May 3, 2016, leading to more voluntary evacuation notices being issued. Dean Twardzik, Global News
The Fort McMurray wildfire flared up just before 2 p.m. Tuesday, May 3, 2016, leading to more voluntary evacuation notices being issued.
The Fort McMurray wildfire flared up just before 2 p.m. Tuesday, May 3, 2016, leading to more voluntary evacuation notices being issued. Dean Twardzik, Global News
The fire in Fort McMurray early Tuesday afternoon, May 3, 2016.
The fire in Fort McMurray early Tuesday afternoon, May 3, 2016. Dean Twardzik, Global News
Fort McMurray wildfire from the Prairie Creek Trailer Park Tuesday, May 3, 2016.
Fort McMurray wildfire from the Prairie Creek Trailer Park Tuesday, May 3, 2016. Cam Cook, Global News
Fort McMurray wildfire from the Prairie Creek Trailer Park Tuesday, May 3, 2016.
Fort McMurray wildfire from the Prairie Creek Trailer Park Tuesday, May 3, 2016. Cam Cook, Global News

The hot weather is expected to stick around in Alberta.

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Global News meteorologist Anthony Farnell says the conditions in Fort McMurray have contributed to the raging fire. He says humidity dropped to just 15 per cent by late afternoon as temperatures soared to a record-breaking highs.

READ MORE: Fort McMurray wildfire: Residents take to social media as situation intensifies

“Firefighters use this 30-30-30 rule. It involves temperature, humidity and wind speed. When they’re all around 30, it’s bad news,” Farnell says. “That is what we have seen throughout the day today.”

The forecast may only worsen Wednesday, with no rain in sight, even stronger winds and temperatures predicted to remain above 30 C.

“This is going to be a rough 48 hours coming up,” Farnell says.

-With files from Nick Logan