Al Stahler: Understanding fire |

Al Stahler: Understanding fire

A rock does not have a brain. When I hold a rock up off the ground, and let go, we could say that the rock “wants” to fall down. But the rock is just following the laws of nature – in this case, the law of gravity – as it plummets downward.

Atoms are tiny things – it takes gazillions of atoms to build the tiniest grain of sand – no room for a brain in an atom.

Glue two, three, a hundred atoms together, and you’ve got a molecule. In joining up to make molecules, an atom must make choices: Would it rather glue itself to this atom, or to that one? Again, no brain necessary: In “deciding” where to attach, atoms – like rocks – simply follow laws of nature.

Carbon and oxygen atoms – following the laws of nature – really “like” to stick to each other … they “love” to make carbon dioxide. Likewise, hydrogen and oxygen atoms “love” to stick together to make molecules of water.

If small fuels – kindling – are very moist, fires are less likely to spread; if such fuels are super-dry, spot fires – outside the main fire – are inevitable.

Plants do not need to eat – plants can make food, by busting up molecules of carbon dioxide and water, then re-using the atoms to make sugar.

But the atoms in carbon dioxide and water – following the laws of nature – are “happy” where they are – they don’t “want” to be pulled apart. The plant needs energy to pry atoms out of carbon dioxide and water … which is why plants need sunlight. Plants use solar energy to bust up molecules of carbon dioxide and water, then re-arrange the atoms to make sugar.

All the time the atoms are sitting in that sugar molecule, they would “rather” be back in their carbon dioxide and water molecules. The plant “knows” this … this is why it went to all the effort of busting up CO2 and H2O in the first place.

Sugar is high-energy food, and not just for us animals – sugar is high-energy food for plants, too. We – animals and plants – harvest sugar’s energy by allowing its atoms to follow their “druthers” – to do what they’d “rather” do: We encourage the atoms to break off the sugar, and to re-combine, once again, into carbon dioxide and water.

Busting up carbon dioxide and water requires energy – energy from the sun. Putting carbon dioxide and water back together is the same trick backwards. When atoms are allowed to follow their “druthers” – to do what they’d “rather” do – they give back that energy.

And give us energy to live.

Billions of years after plants’ early ancestors learned to make sugar to store energy, plants discovered another use for sugar: Gluing hundreds of sugar molecules together into chains, they could make wood. Building their bodies with wood, plants could grow upward, toward the light. Plants evolved into trees.

But wood is, still, basically sugar. And those atoms in sugar would still “rather” be glued-together as carbon dioxide.

When we – animals and plants – encourage the atoms in sugar to re-assemble themselves into carbon dioxide and water, we make sure they do so slowly. Allowed to go from sugar to carbon dioxide and water too quickly, the atoms would go out of control.

Such an out-of-control re-arrangement is what happens in fire.

We fight fire with water. Putting water on fire forces the flames to waste energy: Instead of putting energy into busting up sugar molecules (sending their atoms on their way toward becoming CO2 and H2O) , the fire puts its energy into boiling off the water. Putting water on fire slows the fire.

Not all the fire-slowing water comes from hoses.

Even in a living forest, we are surrounded by deadwood: Twigs, tree limbs, tree trunks, once-living, now dead … the wildfire sees these as fuel.

In camp with a fire crew, awhile back, I watched a crewman weigh some dowels – thin sticks, a quarter-inch across. He knew what the dowels had weighed in the lab, bone-dry. Here in the woods, the dowels had sucked up moisture from the air around. Weighing the dowels, he could learn how much moisture they’d absorbed … and from that, how much moisture – fire-slowing moisture – was in the deadwood around us.

If small fuels – kindling – are very moist, fires are less likely to spread; if such fuels are super-dry, spot fires – outside the main fire – are inevitable.

Firefighters rate wood fuels by how quickly their moisture comes into balance with the moisture in the air. One-hour fuel comes into balance with the air in one hour … this is kindling, a quarter-inch thick, or less. Ten-hour fuels – up to an inch thick – come into balance in ten hours. Hundred-hour fuels – up to three-inch diameter – take four days. Thousand-hour fuels – small logs, up to eight inches across – take a month-and-a-half – a fairly long time … but California’s dry summers last more than a month-and-a-half.

I was up before dawn, Monday morning, to check out the crescent moon and planet Venus, suspended together in the eastern sky. Normally, the moon – and, especially, Venus – shine a brilliant white. But not this past Monday … moon and planet both shone orange.

Fiery atoms follow their “druthers,” attempting to re-form into carbon dioxide and water. But fires burn fast, and there’s a lot going on … many atoms do not have time to go all the way to CO2 and H2O … they blow away just partway through the process. The smoke coming off a wildfire thus contains a LOT of different molecules … some of which are very “unhappy” – they’d “rather” be carbon dioxide or water … or ANYTHING … other than the half-way molecules they find themselves in.

If the particles and gases in smoke – following the laws of nature – can get closer to where they “want” to be, they’ll snatch atoms off, or latch onto, anything they can to get there … atoms in our lungs, for instance.

As I write, on Tuesday morning, the winds have shifted, and the air is cleaner. Please join me in visualizing fall rains coming early this year … and in expressing gratitude to the folks fighting the fires.

Between virus and fire … masks and finances … smoke … evacuations … blame and trust and distrust … folks … are … on … edge. Let’s remember to be kind and patient with our neighbors … and with ourselves.

Al Stahler enjoys sharing science and nature with friends and neighbors in The Union and on KVMR-FM; teaches classes for both kids and grown-ups; and can be reached at

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