Al Stahler: Ecology of fire

Nature piles sand in dunes, snow in drifts. Nature piles air into huge domes, covering huge regions. Such a dome has parked itself over us, bringing with it a monstrous heat wave.

The air in our dome weighs quite a bit, and presses down hard on the ground. In the northern hemisphere, such high pressure systems rotate slowly clockwise.

In mid-August, Hurricane Elida, off Baja, lifted huge amounts of water vapor into the air. Hurricanes in the northern hemisphere rotate counterclockwise.

Put your hands together, palm-to-palm. Slowly spin your hands in opposite directions, moving your thumbs apart, but leaving your little fingers touching.

As your hands move apart, you right hand spins clockwise, like our high-pressure dome of hot air. Your left hand spins counter-clockwise, like Hurricane Elida — or, rather, the remnants of Hurricane Elida.

Where your little fingers touch, Elida’s remnants feed sub-tropical moisture into our spinning dome of hot air.

There’s one more player in the game: A kink in the winds blowing into North America, from over the Pacific. This kink sends air in the upper levels of our dome – air moistened by input from Elida – sends that moist air spinning upwards. Rising upward, the air cools; water vapor condenses into clouds; clouds grow into thunderheads; lightning flashes, mostly cloud-to-cloud — but sometimes, cloud-to-ground.

A lightning strike pumps a huge jolt of electricity into a tree. Tree sap boils, flashes to steam. The tree – feeling tremendous steam pressure – explodes. Flaming shrapnel flies in all directions. The Jones Fire ignites.

Chris Mertens, of Cal Fire, is investigating the fire’s origin. A charred, shattered tree would be good evidence. Merten’s report should be out around now.

But it doesn’t require lightning to make tree sap boil. Standing anywhere near a clump of burning trees, the infrared – heat radiation – is unbearable. Nearby tress, not yet alight, feel that heat. Their sap boils off into the air above – the rising, vaporous air is visible to the eye, resembling the shimmering plumes of hot air rising off sun-baked asphalt.

Light a candle, let it burn a few minutes, then blow it out. Fume rises off the still-hot wick. Bring a match near this fume, some inches from the candle, and the fume ignites … and carries the flame back to the candle.

As fume rises up from that pine, baking in the heat of nearby flames, something – perhaps a stray ember, perhaps intense infrared – something ignites the plume, which carries the flame back to the tree, engulfing the tree in fire. The fire spreads.

The purpose of a fire line is to keep a wildfire from spreading beyond. I visited the eastern fire line of the Jones Fire – SR 49 – with sheriff’s Captain Jeff Petitt. One of the fire-fighters’ major goals was to prevent the fire from crossing SR 49 – to keep the fire west of 49. To the east was Nevada City.

The fuel that drips from a drip torch is a mix of gasoline and diesel. Falling from the torch, already burning, the fuel ignites whatever it falls on. Firefighters use the drip torch to create a backfire, a fire burning back toward the advancing wildfire. When the wildfire does arrive, it finds no further fuel. A backfire tells the fire, “This far, and no farther.”

The line drawn by the drip-torch on the SR 49 fire line ran some inches from the western side of the blacktop. With firefighters hosing down the smoking vegetation, with pink fire retardant coating leaves and twigs, with fuel removed by the backfire, the SR 49 fire line held.

Most of the fire line around the perimeter of the Jones fire had to be constructed from scratch, by bulldozers … except for the part of the line closest to the South Fork. Too steep for dozers, the northern fire line could only be built by hand crews … using hand tools. Wearing thick pants, shirt of yellow, fire-resistant NOMEX fabric … it’s hard, hot work.

Sunlight is composed of every color of the rainbow: red-orange-yellow-green-blue-indigo-violet (memorized by making up a name: ROY G. BIV). We can enjoy rainbows because, when sunlight is bent by droplets of water, blue is bent most, red least. Thus, water droplets in the air separate the colors, creating the rainbow.

Dry air, too, can separate colors, by scattering them every which way. Blue is scattered most, all over the sky, which is why the sky is blue; red scatters least. Light from the setting sun comes to us sideways, through many more miles of air, allowing the air to scatter not just blue, but also green and yellow, turning the setting sun orangey-red.

Smoky air scatters even the orange from sunlight, painting the setting sun a deep red.

There’s another color, below red, which our eyes cannot see, that penetrates smoky air even better than red: Infrared (IR). Cal Fire rep Mary Eldridge explained that aircraft fly over the fire at night, sensing infrared to map hotspots – hotspots for fire-fighters to jump on, next day.

In addition to keeping the Jones Fire west of 49, anther objective of the fire-fighters is to keep the fire north of Newtown Road. Yet there’s a stand of trees that straddles Newtown, both north and south of the road, not far west of Hwy 49. If trees could hope and wish and pray … these trees would be hopin’, wishin’, and prayin’ for the fire to reach Newtown Road, then work its way east, toward 49.

McNab Cypress trees are conifers – cone-baring evergreens, like pines – but their cones are small and round, an inch or so across. Unlike pine cones, McNab cones don’t open when they’re ripe, don’t drop their seeds – they’re held tightly closed by a glue that does not wash off. McNab cones open – and release their seeds – only when they feel the heat of a fire. McNab cypress is one of many, many species, native to the foothills, that require fire.

I pulled a book off the shelf last week: “The Wildfire Reader” – a collection of essays by scientists, naturalists, land managers, and turned to an essay by Gary Snyder in which he sums up our situation: We live in an ecosystem that has evolved over hundreds of millions of years to burn.

Looking at how fire burns over the landscape … it’s not monotonous … fire burns hot here, cooler there. It burns some areas completely, others it merely singes … or avoids entirely. And much of what does burn will be better for the burn.

I know folks who have lost their homes to the Jones Fire. Can we somehow return much-needed fire to the landscape, without losing more homes, and lives? We’ve got to talk.

Last week, another hurricane – Genevieve – stormed off Baja. Remnants of Genevieve fed moist sub-tropical air into the upper levels of our dome. Sunday night, Monday morning, another kink in the upper atmosphere tried to use this moist air to fuel another round of electrical storms. This attempt, fortunately, was abortive. We … lucked … out.

Al Stahler enjoys sharing science and nature with friends and neighbors on KVMR-FM, teaches science classes for both kids and grown-ups, and can be reached at [email protected]

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