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?
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
soln
