Burning Bushes: The Science Behind Water Droplets and Fire Ignition

[jollytime]

It is said that bush fires in Africa can be started by water droplets in the grass. How could this work? What actually causes the burning?

 

[jtbell]

Have you ever used a magnifying glass in the sunlight to light fires?

 

[jollytime]

yes, but i never knew how that worked..

 

[jtbell]

The lens (or water drop) concentrates the light that enters it, at its focal point.

 

[Andy Resnick]

This is how some old weather-recording devices worked- a large glass ball sat in the center of a ring of paper. If the sun was out, the paper would have a char line, and if it was cloudy, there would be no line.

 

[pallidin]

This is how some old weather-recording devices worked- a large glass ball sat in the center of a ring of paper. If the sun was out, the paper would have a char line, and if it was cloudy, there would be no line.

OK, and I'm not trying to be silly here at all, but I question the water droplet scenario.

A water droplet tends to be quite small, and I would imagine also grossly distorts while clinging to, say, a blade of grass or some such thing.

It seems to me that the droplets size itself, even if it could be shaped into an ideal lens for starting a fire, would simply be inadequate. After all, with a true lens the size of a water droplet, I tend to doubt there is enough surface area to concentrate sufficient sun-light to start a fire. Indeed, could a fire be started with a lens which has a diameter less than that of a pencil eraser?

But I could be wrong on that aspect.
Even if I were, a water droplet in nature is so grossly deformed apart from an ideal lens that the potential is even much less, it seems to me.

Just my thoughts...