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A Cold and Frosty Morning (Posted on 2022-08-15) Difficulty: 2 of 5
Neil has just come in from a walk around his garden. The grass is covered with a heavy frost everywhere except under the trees.
Was the sky clear or overcast? How do you know?

No Solution Yet Submitted by K Sengupta    
Rating: 4.0000 (1 votes)

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soln | Comment 1 of 12
Likely it is clear. Clear (night) sky thermal temperatures are colder than cloudy sky temps and the thermal radiation from a tree can be stronger than either. 

Sky temps are given by the Swinbank Formula which shows a significant contribution from any cloud cover.  As air temperatures drop overnight, dew may freeze to make frost. This suggests that Neil is on a morning walk before the Sun has reached an elevation high enough to warm the air.  

Note: low trees may also outshine a cloudy sky, say, if the trees are low enough, and the clouds are high enough, but it is less likely. 

Edited on August 15, 2022, 9:12 am
  Posted by Steven Lord on 2022-08-15 09:01:31

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