Скачать или смотреть Obstructing microconvection in a cup of water

Why does an entire cup of water become cooler than room temperature after a long time? We know there is evaporative cooling occurring at the surface of water, but that cools down only the surface layer.

The cooled surface water then sinks because of natural convection. This convection is so weak that we cannot see it at all, which is why I call it "microconvection." But this microconvection still transfers heat faster than conduction. This IR video shows it.

In the two cups of water, one had a plastic disc that stuck in the middle to prevent water from sinking all the way down. Water convection could cross this barrier. But heat could conduct through because the disk was very thin. The IR view shows that the water below the disc was slightly warmer than the water above it, indicating that it was the microconvection, not conduction, that was responsible for the uniform temperature in a cup.

Visit http://energy.concord.org/ir for more.