May 31st, my morning walk.
It was 43°F, dew point 41°F, and humidity 94%. I was very surprised to see night sky frost on my neighbor’s house roof. I have been keeping track of earliest and latest night-sky-frost for several years. My data for latest frost has been stuck on May 20th for several years.
As the nights get shorter it becomes more and more difficult for there to be enough time for daytime heat build-up in an attic to cool enough for frost to form. One can easily see this effect in the way roofs with attics as opposed to vaulted roofs (or unheated roofs) support frost development.
Take the following picture for example.
Notice how there is frost build-up on the left side (vaulted side) and no build-up on the right side (attic area). The overhang also shows frost. So these are the colder areas where the night sky could suppress the roof surface temperature enough for frost to form. The attic side has so much more volume to cool down and the short nights do not provide enough time.
Here is some more frost from this morning’s walk, This is a little detached storage shed that is more or less open on one side and obviously not heated. It is a place I can always count on seeing frost when conditions are “marginal.”
There is also something else that has to happen for night sky frost to form as late as May 31st. The day prior has to have been cloudy. So a very cloudy day followed by a very clear night with high enough humidity, temperatures that reach the dew point, and low enough ambient temperature, will allow some areas to form frost.
Here is a link to learn more about Night Sky Radiational Cooling.
Charles Buell, Charles Buell Consulting LLC