By concoures on Sunday, May 17, 2009 - 10:13 pm: Edit Post
I live in the Metro Detroit area and the NWS has issued a frost advisory for tonight. They also state that the low will be 37. How can we have a frost when the temperature is above 32 or freezing?
By frnash on Monday, May 18, 2009 - 01:15 am: Edit Post
Of course that frost advisory also said:
Maybe the key word is as mentioned in the heading of the Detroit/Pontiac NWS FO home page:
".SURFACE HIGH PRESSURE WILL REMAIN IN PLACE ACROSS THE GREAT
LAKES REGION OVERNIGHT. LIGHT WINDS AND CLEAR SKIES WILL ALLOW FOR TEMPERATURES TO DIP INTO THE LOWER TO MIDDLE 30S BY SUNRISE MONDAY. THESE CONDITIONS WILL FAVOR WIDESPREAD FROST FORMATION...WITH A FREEZE EXPECTED FROM THE I-69 CORRIDOR NORTHWARD.
* * *
INCLUDING THE CITIES OF...HOWELL...PONTIAC...WARREN...ANN ARBOR...
320 PM EDT SUN MAY 17 2009
...FROST ADVISORY IN EFFECT FROM 2 AM TO 8 AM EDT MONDAY...
THE NATIONAL WEATHER SERVICE IN DETROIT/PONTIAC HAS ISSUED A FROST ADVISORY...WHICH IS IN EFFECT FROM 2 AM TO 8 AM EDT MONDAY.
TEMPERATURES WILL DROP INTO THE LOWER TO MIDDLE 30S OVERNIGHT. THESE CONDITIONS WILL SUPPORT FROST FORMATION."
"One more chilly night - freeze likely in spots." Mainly as a cautionary note for agricultural interests?
Although I will agree that the warnings of "WIDESPREAD FROST FORMATION" don't seem consistent with either that, or their graphical temperature forecasts. Wierd, eh?
By admin on Monday, May 18, 2009 - 12:19 pm: Edit Post
It has to do with the way some solid objects react to changes in solar radiation vs. how air reacts. Typically solid objects both heat up and cool faster to changes in sunlight than air. Plus, the darker the object, the faster it reacts.
For example, put your hand on a dark colored car that is in the sun and that cars surface will be warmer than the air and it will also be warmer than a car painted white. That same car surface will also cool faster once the sun sets than the air around it.
The same things also happens with other objects and surfaces like vegetative surfaces. They will heat up and cool faster than the surrounding air.
So even though the air temperature (officially measured 6 feet off the ground) may not cool to freezing, these darker surfaces do and thus the microscopic layer of air molecules that bounce against those surfaces also cool to freezing and if there is moisture in the air, it can sublimate (go from a vapor to a solid) onto that surface.
By booondocker on Monday, June 01, 2009 - 12:30 am: Edit Post
Clear nights also allow a temperature inversion, where the warmer air will rise rapidly and the colder overhead temps will sink. This is one of the reasons that valleys often receive frost when the lands adjacent may not. This sinking of cold air develops quickly and if you recall the last time you experienced this "night air" chilling rapidly, this was exactly why. Colder dense air aloft will fall and cause cooling rapidly in addition to the "solid surface" principles which John has explained.
When there is cloud covering very little of this process can occur because the clouds form a blanket interfering with this process and preventing the heat loss and/or cold air displacement.
Next time you are out for a walk at twilight, with full stars and no wind, watch how quickly you can feel the temps drop.
Being a biker, I can tell you that in late spring or late summer this process can be felt extremely well as you travel along after dark and hit a low spot where the cold has accumulated. These same cold accumulations can quickly drop temperatures to below freezing and cause frost.