Why does smoke from cigarettes, smokestacks, and volcanoes rise in a straight line before spreading out into a billowing cloud? And how does this help Forest Service workers spot wildfires early?
Although most people have seen the way smoke moves, it's mostly one of those visible-but-invisible physics phenomena. Once people have seen it a certain number of times, they develop a sense of it ‘just being the way it is' – like the popularity of lower back tattoos, and the storyline of every romantic comedy ever made. But consider how smoke usually behaves. It rises up in a narrow band, and then suddenly starts to billow out in all directions. On the surface, this doesn't make sense. Hot air expands, so smoke should billow out right away. And if the smoke is rising in one thick column, it shouldn't suddenly start scattering around.
The key to understanding why smoke rises in a column is understanding why it rises in the first place. Smoke is usually accompanied by fire, and fire makes air very hot. Hot air expands, and because it has expanded, it is less dense than the colder air around it. When something is less dense than its surrounding material, and when that material is free to jostle around, the less dense material rises. A good example of this is a helium balloon. Helium is lighter than air, so packing it into a balloon makes the balloon less dense than the air surrounding it. The air molecules push it upward.
The same thing happens with hot air. It takes off for the sky, while denser air surrounds it, keeping it in line. But as it gets away from the source of heat, it cools. Once it cools down it loses its drive skyward, and it can scatter around.
This can present a problem in the case of forest fires, since burning debris can be carried up along with the hot air, scatter sideways when it gets high above the forest, and come down in new places, igniting smaller blazes that grow and burn on their own. Smoke can also be carried back down to earth, and can settle into valleys, displacing air and injuring or smothering inhabitants.
So far this entry may sound like it's listing the many different ways that innocent people can get killed by some jerk who throws a cigarette out of a car window (how many people remember that public service announcement), but smoke can be used to fight fires instead of just exacerbating the harm from them.
By studying the plumes, scattering and movement of the smoke from forest fires, fire fighters are able to spot fires early, get information on them from afar, and anticipate the direction of a blaze. Remember – only math can prevent forest fires.
Top image via Hawaii Volcano Eruption Updates