Thursday, 5 January 2012

Colder than air - beating the second law of thermodynamics

The temperature of the roof seems to go lower than air temperature at night. I noticed this before with a simulation from OM Solar on the temperature of their roof. OM Solar runs on the principal of heating air under the roof, which it uses for space heating in the winter or  heating water, via a heat exchanger in the summer.

It starts to get cold at this time of year, and if you go out on a clear night, it really feels like the heat is being sucked from your face by the starry sky. This is because the heat IS being sucked from your face by the starry sky. Heat transfer by conduction depends on the temperature of the air, and convection keeps making sure that as soon as the air next to your face wams up, it  moves away and will be replaced by some cold air. Wearing a fur lined hood reduces the air flow around your face. Something more is happening on a clear night. Beyond the clouds, that are not there, it's very very cold, around fifty five degrees below freezing.  

But in fact the heat is not really being sucked from you at all. Your body is constantly radiating heat, depending on its absolute temperature and regardless of whether you stand in front of a starry cold night or a burning open fire. Usually, something is radiating the heat back again though. If you're inside, the walls and ceilings. If they're low e, the windows are going to be reflecting your heat back to you, because low emissivity means high reflectivity. If you're outside, the clouds are radiating heat back. Even though they are a long way away, below freezing and have a high emissivity, they're still radiating a lot more heat than the vacuum of space beyond. If it snows, the snow is radiating much more heat than any clouds. It may be below freezing, but that's still a few hundred degrees above absolute zero, where nothing is going on at all. That's why it feels warmer when it snows.