The Naked Scientists

The Naked Scientists Forum

Author Topic: How does frost form when the air temperature is above zero?  (Read 4590 times)

Offline thedoc

  • Forum Admin
  • Administrator
  • Sr. Member
  • *****
  • Posts: 511
  • Thanked: 12 times
    • View Profile
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.

 

« Last Edit: 25/09/2013 23:01:30 by _system »


 

Offline thedoc

  • Forum Admin
  • Administrator
  • Sr. Member
  • *****
  • Posts: 511
  • Thanked: 12 times
    • View Profile
How does frost form when the air temperature is above zero?
« Reply #1 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 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...
« Last Edit: 25/09/2013 23:01:30 by _system »
 

The Naked Scientists Forum

How does frost form when the air temperature is above zero?
« Reply #1 on: 25/09/2013 23:01:30 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums
 
Login
Login with username, password and session length