Naked Science Forum

General Science => General Science => Topic started by: thedoc on 25/09/2013 23:01:30

Title: How does frost form when the air temperature is above zero?
Post by: thedoc on 25/09/2013 23:01:30

If water freezes at zero, we get frost at three degrees C?
Asked by Alan in Lowestoft


                                        Visit the webpage for the podcast in which this question is answered. (http://www.thenakedscientists.com/HTML/podcasts/naked-scientists/show/2007.02.18/)

 

Title: How does frost form when the air temperature is above zero?
Post by: thedoc on 25/09/2013 23:01:30

We answered this question on the show...

The ground is loses heat by radation. It can see the clear cold sky above it on          a clear night and the ground radiates away its energy, getting down to zero or [img float=right]/forum/copies/RTEmagicC_FROST.jpg.jpg[/img]below          zero very effectively, because the ground is good at giving          up energy.
But the air, and the atmosphere sitting above          the ground, is effectively transparent to the radiation, it doesn't absorb or emit radiation well and so it doesn't cool down nearly as fast or insulate the ground very much.
So the air loses its energy much more          slowly than the ground does. This means that the air can          be several degrees above zero but the ground is now lower that, say          zero or even minus three degrees. And when the warmer, damp air meets the freezing ground, it cools down, the moisture condenses out and forms frost on          the ground, even though the air temperature is still not quite          freezing...