Take a swirling dust devil, pick up some tumbleweeds, add fire, and the result looks like an elemental monster straight out of a video game. But it isn't a computer-generated effect; it's 100% real science.
Thomas Rogers was working on a controlled burn at Arsenal Rocky Mountain Wildlife Refuge in Colorado when a dust devil spawned and quickly evolved into a swirling mass of hot air and fire. The building of whirling tumble weeds into a dark tower over a base of swirling flame echoes a crazy final-level video game boss, an elemental ominously pulling together from individual pieces.
Tumbleweeds and a dust devil add excitement to a controlled burn near Denver.
We already covered the what/when/where of this event, but what's the science behind this incredible, bizarre video of earth, wind and fire?
It's all about the Eagles
Starting with a bit of context, the video is shot in the Rocky Mountain Arsenal Wildlife Refuge. Arsenal is a national wildlife refuge northeast of Denver, Colorado. The refuge is a mix of prairie, lakes, wetland, and woodlands. It has a complicated history for a conservation area — it has previously served as farmland, and even housed war-time manufacturing.
During 1980s cleanup operations to meet state and federal regulatory requirements, the Army and Shell Oil Company found a roost of bald eagles. This triggered the intervention of the U.S. Fish and Wildlife Service; by 1992 Congress had declared the land a wildlife refuge. Cleanup and expansion of the refuge continued until by 2010, the cleanup was complete. Now, at just over 60 square kilometers (15,000 acres) the Arsenal is one of the largest urban refuges in the country.
Deer in front of Denver. Photograph credit: Oborseth
Birds, mammals, reptiles, and amphibians all loiter in the refuge. Birds included everything from the bald eagles that triggered all the fussing, raptors, songbirds, and more common ducks and geese. Mammals are everything from photogenic bison, sneaky raccoons, charismatic coyotes, cotton-tail rabbits and black-tailed prairie dogs, and a whole lot of deer. Bullfrogs, bull snakes (which are frequently confused for rattlesnakes), and snapping turtles also find homes within the refuge.
The soil ranges from sands to clay, with different types of plants in each region. The grasslands are carpeted in blue grama, sideoats grama, and sand bluestem grasses, with pops of wildflowers like prickly poppy and the Rocky Mountain bee plant. Cottonwood trees fringe the lakes and rivers.
Okay, cool, it's a patch of wilds next to a city with some cute but not particularly noteworthy flora and fauna. What's with the guys in uniforms setting fires?
Burn, baby, burn!
Land management is tricky business. If you set aside wilderness for preservation from human activities, the our quashing fires to "protect it" actually creates a totally unnatural environment. Undergrowth gets thicker and denser, and plants that depend on heat for germination don't sprout. Insect populations boom, and the heaps of dead plants on the forest floor can reduce air quality and enhance the spread of disease. On top of all that, the risk of a larger, higher intensity fire increases. Yet, if we let every fire rage out of control, someone is bound to get hurt.
Historically, low-intensity fires crept through most forests every 5 to 15 years, burning spindly trees and bushy undergrowth. This cleared out fuels, and recycled those nutrients back into the soil to feed the growth of new plants. These frequent cool fires actually reduced the risk of huge, high-intensity devastating fires, but we've gotten so good at suppressing every little flicker of flame that they don't happen anymore.
Fires clear out dry and dead litter on the forest floor. Photography credit: Takeaway
Enter the prescribed burn. While that sounds like a rocketry maneuver for sliding into the right orbit, it's the formal name for a controlled burn. Controlled burns are any fire that is intentionally ignited for land management — intentionally set fires by pyromaniacs or accidental fires from careless campers don't count.
The purpose of a controlled burn can be reducing flammable fuels, restoring ecosystem health, recycling nutrients back into the soil, or preparing an area for the growth of new plants. The fires clear out overcrowding and encroachment by species that would normally be wiped out once a decade by small fires, and ensure that strong, fire-resistant species reproduce.
Setting fires is a science. The moisture level of dead and live fuels and atmospheric conditions like air temperature, wind speed, and humidity all impact if a burn will be successful and not rage out of control. Land management agencies like the U.S. Forest Service keep an eye out for the right combination of factors — the burn window — to start a targeted burn.
A firefighter uses a drip torch during prescribed burn at Coconino National Forest. Photography credit: Ian Horvath
To control the area that burns, firegaurds are used to contain the fire. A firegaurd is anything that fire won't cross — streams, bluffs, roads, bare soils, burned areas, thinly vegetated areas, or even vegetation that is soaked to keep it from burning. But nature is complicated, and fire is tricky. Some days even a well-laid plan doesn't turn out as expected and fire jumps the line and burns beyond the target area.
The Arsenal was inside a burn window when the federal U.S. Fish and Wildlife Service and U.S. Forestry Service, and the local South Metro Fire, West Metro Fire, Denver Fire, and Fairmount Fire services started the controlled burn to clear out 600,000 square meters (150 acres). The fire was contained within firegaurds, until a dust devil formed and drew the fire into its winds. Wind blows where it pleases, so the dust devil drifted, crossed the line, and burned an additional 4,000 square meters (1 acre).
Summoning a Dust Devil
A big chunk of atmospheric science is the transfer of heat. If a column of hot air rapidly rises inside a pool of cool air and picks up a bit of rotation, the column pull in and up, lengthening as it spins faster and faster through the conservation of angular momentum. More hot air is pulled horizontally along the ground, drawing in at the base and feeding the dust devil. The hot air cools as it rises, loosing buoyancy until the now-cool air descends along the outside of the tower. The combination of a steady supply of hot air rushing in at the base and the descending cool air stabilizing the tower creates a self-sustaining system.
On Earth, most dust devils stretch a few meters wide. On Mars, the same phenomena takes it up a notch, commonly spawning monstrous dust devils spanning tens of meters wide. While the really big ones can be a hazard to our Martian explorers, smaller ones have helpfully cleaned off dusty solar panels, giving the rovers a new burst of energy.
The Mars Reconnaissance Orbiter spotted a towering dust devil from orbit, so the JPL team animated what it would look like as seen from the surface.
Friction between the dust devil and the ground paired with the whirling air drives the dust devil in a staggering dance forward to tap new sources of hot air. On hot, barren, flat surfaces, the hot air supply can be enough to sustain the dust devil for minutes. When cold air is sucked in, the temperature differential is broken, and the dust devil dissipates. Dust devils are fragile structures that form in calm conditions. A bit of wind will disrupt the stabilizing rotation, blowing the dust devil apart.
Variations on the Dust Devil: A Zoo of Elemental Creatures
By default, dust devils are Wind elementals, but in the right conditions they can incorporate Earth, Water, and Fire.
Over barren, dusty terrain, an active dust devil is a hungry beast, scooping up a gram of dust every second for every square meter of ground it crosses. With all the dust moving quickly, the particles scrape against each other, building up a charge. Charged particles moving in circles produce electromagnetic fields. Even small dust devils produce radio static.
In different terrain, a dust devil can swirl up powdery snow, the steam from geysers and power plants, or the low layer fog blanketing warming oceans or fields on a wet morning.
But most spectacular are the fire whirls. These badass cousins to dust devils form over the hot air of a burning fire, pulling the flames into the rotating air. Temperatures can soar over 1,000°C (2,000°F), reigniting ash pulled into the tower. They move slowly, but last longer than pedestrian dust devils and cannot be directly extinguished.
Fire + Tumbleweed + Dust Devil = Arsenal's Elemental
The controlled burn in Arsenal spawned what was originally a tame dust devil, pulling up dust but ignoring the fire. Then dry tumbleweeds — part of the dead fuel the prescribed burn was set to clear out — got picked up along with the dust.
Within seconds of the video start, the dry tumbleweed catches fire, swirling flames into the devil. That's about when the truck and fire crews back up to a safer distance, then turn and run to a safe distance.
When the fire whirl returns to the field of view, it's a tower of dark dust with flames flickering up the column and a fringing swarm of flying tumbleweeds. While the event itself cannot be directly extinguished, crews killed the ground fires, cutting off burning tumbleweeds enhancing this whirl, or worse, spawning another. The devil crossed over the control line, carrying the fire outside the target area to burn an additional 4,000 meters.
Thank you to Observation Deck contributor Tomb: R.O.A.C.H. ᶘ ᵒᴥᵒᶅ for first sharing this video. Images are screen captures from the video unless otherwise credited. Want more strange phenomena? How about lightning that generates gamma ray bursts!