My time in Australia’s dry tropics amounts to three words: Fires. Fires everywhere. I’m sure the region has a certain amount of paranoia about this year’s growing El Nino, which brings droughts to the region, and the increased threat of wildfires. But, drought or no drought, controlled burns have been a traditional part of Australia’s ecological management for 40,000 years.
The Aborigines used fire to thin forests to create grasslands for grazing, and to limit the buildup of kindling that could fuel catastrophic wildfires. They could control the length and temperature of these fires to target trees with certain fire resilience, and shape the ecology of the remaining forest to suit their needs. Those practices dwindled after the Europeans arrived, until an uptick in nasty wildfires made them reconsider incorporating controlled fires into their arsenal.
Sometime after hearing this, I was watching the forest drift past the car window, and realized that this wilderness wasn’t wild at all. It was cultivated.
This was a photo I took on a hike on Castle Hill, Townsville. There’s a lot going on here: most relevant, there’s a big controlled burn visible at the center left. The streak on the right is an ascending airplane. As a bonus, there’s the conjunction of Venus and Jupiter overhead.
Sometime during my stay, fires stopped being a spectacle, and became routine. No car trip was complete without a glimpse of a smoke plume crossing the horizon, or a whiff of the mouthwatering barbecue smell of burning hardwood.
On the trip back to the U.S., I got a window seat (TV? bah…the real in-flight entertainment is watching the landscape go by!), and I lost track of the number of fires I saw. Fifteen? Seventeen? I snapped a picture of a couple of them.
This fire, just outside Sydney, caught my eye. You can see the brownish smoke plume rising upward, but that flat, white cloud at the top isn’t smoke, but water vapor. The smoke is promoting cloud formation. This can happen in a couple of ways:
1. The “Speed Bump”
The rising warm, smoky air is creating a “speed bump” in the overlying air. That air is pushed upwards, where it cools. Cool air can hold less moisture, so the water vapor condenses, and voila! A cloud is born. This is the same principle behind lenticular clouds, those flying saucers that form over mountains.
2. Seeding the clouds
Moisture in the air can’t spontaneously turn to liquid. That gas needs some sort of particle to begin condensation. These particles are called Cloud Condensation Nuclei (CCN’s), and can be specks of clay, cosmic dust, sea salt, or, in this case, soot. Water vapor condenses to liquid on these particles and form clouds. If these water droplets grow large enough, they overcome gravity and rain out of the cloud. In principle, this means that every raindrop, hailstone, or snowflake contains a microscopic speck of Saharan dust, micrometeorite, or ocean salt. But don’t think you’ll amass a collection of cosmic dust anytime soon–individual particles are tiny.
Cloud condensation nuclei are the principle behind cloud seeding, an old technique that strays into the realm of science fiction. We’ve been doing it accidentally for centuries. Smoke and smog can act like CCN’s, creating clouds–so our pollution can actually alter weather patterns. If those droplets grow enough, they rain. Some governments have been seeding clouds with dry ice or silver iodide for fifty years in the hopes of creating rain in drought-stricken areas. However, precipitation in one place creates drought someplace else, which leads to accusations of “stealing rain”. Weirder still, cloud seeding was deliberately used in the Vietnam War. Operation Popeye used cloud seeding to lengthen the monsoon season, washing out roads to strand opponents and their supplies. After this program was declassified, Congress moved to ban weather modification techniques in warfare. Today, using cloud seeding for military purposes is illegal under the Environmental Modification Convention. Let that sink in: there’s an international law, proposed by the U.S., that prevents governments from playing Zeus with the weather.
This photo might imply, then, that controlled burning affects more than the region’s ecology – it can change its weather. Intentionally or otherwise, the Aboriginal masters of fire may have shaped the sky of Australia as much as they shaped the land beneath it.