From the Lee Side |
|
| Posted by: Lee Grenci, 5:14 PM GMT on December 12, 2012 | +22 |
| Permalink | A A A |
|
Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.
|
Tropical Blogs
Tropical Weather Stickers®
 |
 |
Get Free Weather Stickers® |
 |
Upload Your Photos |
 |
Weather™ API for Developers NEW! |
 |
Become a Site Member |
 |
Find us on Facebook |
 |
Follow us on Twitter |
| Copyright © 2013 Weather Underground, Inc. | About Us - Contact - Support - Employment - Press Releases - Terms of Service - Privacy Statement |
Page: 1 — Blog Index
More than every-once-in-a-while I find that I have the science wrong. I look forward to the lessons provided by your posts.
Generally, at what altitude does cloud cover/type no longer play a measurable role in actual radiational cooling (not "ground deficit") with respect to surface temps?
To re-word the question, I frequently read NWS forecast discussions that attribute mid and upper level cloud cover to placing a limit on actual radiational cooling.
Is this good science?
Thanks again.
8:31 PM GMT on December 12, 2012
That's a great question. Actually, neither has complete control of the radiational cooling at the ground, which is governed strictly by the temperature of the ground. See this link.
But you're on the right track. Think of clouds as space heaters, which radiate infrared energy in all directions. The higher the altitude of the space heater, the cooler the cloud, the less energy emitted by the cloud (space heater). Thus, low clouds emit the greatest amount of energy and therefore have the greatest impact on the energy budget at the ground (infrared energy emitted by low clouds can dramatically offset the ground's radiational cooling, so the ground runs a smaller deficit or even a surplus of energy at times). Indeed, I've seen nighttime temperatures plummet under clear skies, only to rise later as a low overcast arrives.
The impact of high, colder clouds is to offset the radiational cooling at the ground to a lesser degree. Thus, high clouds have an impact on the energy budget at the ground, but because they emit a relatively small amount of energy, the ground runs only a slightly smaller energy deficit.
Make sense? Thanks for your question.
P.S. You may call me Lee.
8:41 PM GMT on December 12, 2012
Understood and thanks.
My attitude is that we're all here to learn. So I check any ego I have at the door. The only reason I'm here is to to teach folks interested in learning...and to stop driving my wife crazy by looking to do things around the house! :-)
P.S. You may call me Lee.
8:42 PM GMT on December 12, 2012
Great! Many thanks!
8:44 PM GMT on December 12, 2012
Way too kind, but thanks!!!!
8:46 PM GMT on December 12, 2012
You made me laugh! Thanks.
P.S. I used to enjoy shoveling snow at 4 A.M. after a snowstorm (so peaceful and beautiful), but I have to admit that the older I get, the less appealing winter weather becomes.
8:49 PM GMT on December 12, 2012
Glad you liked it. I really enjoy writing and teaching. I hope it comes across that way. Communicating online has its drawbacks compared to standing in front of a class...you can't see my body language and I can't see yours. That makes communication a greater challenge, but, in the end, communicating online can be just as rewarding.
8:49 PM GMT on December 12, 2012
Many thanks.
Your informative, thought-provoking articles (and writing style combining wit and humor), through the years in Weatherwise magazine always had me looking forward to each new issue... LOL, of course I still have 'em, and re-read from time to time. Learned quite a bit from you, sir, thanks!
PS: and thanks for answering Beell's question above.
G'day to you!
9:09 PM GMT on December 12, 2012
A Weatherwise fan! I miss writing for the magazine. Many thanks for the warm welcome.
By the way, er, did your parents ever forgive you for flooding their downstairs living room after bringing your "bathtub to resonance"? (Weatherwise, Jan / Feb 2001)
Nah, probably no more than mine did after my early venture into alchemy - dropping a block of paraffin into a pot of molten lead. Ya know, in the name of science. Seems parents often fail to behold young genius in their midst, my reward for such an astounding accomplishment matched yours!
;)
11:28 PM GMT on December 12, 2012
Thanks for your generosity!
11:28 PM GMT on December 12, 2012
Many thanks!
Thanks for the response. Your use and explanation of the term "energy deficit" made my fuzzy understanding clear. It's the ground, not the clouds!
I have a problem with your pet peeve though. Nocturnal cooling occurs regardless of clear skies and light winds, which is why I frequently reference good radiational cooling conditions in my forecast discussions. (I am a student agricultural forecaster for AgEBB at the University of Missouri) "If the truth be told, the greatest radiational cooling occurs around the time of the daytime maximum temperature." If you can be a stickler so can I, any cooling or warming refers to the direction and magnitude of the net radiation flux present at that time. You are referring to outbound long wave radiation as radiational cooling, but this is only half of the problem. If there is still incoming solar radiation at peak temperature then the net radiative flux is not minimized. Radiational cooling will actually be maximized at sunset, when the only radiative component is outbound and the surface temperature is maximized. I use the phrase ideal radiational cooling conditions to describe a particular set of atmospheric conditions that "nocturnal cooling" neglects. In some cases strong warm air advection will cause nocturnal temperatures to actually rise, which would create a hole in your "nocturnal cooling" term. I understand that every object with a temperature emits radiation and this is what makes you cringe about the term. However, when used properly I believe it does a better job at describing the atmospheric conditions present than the "nocturnal cooling" term, which does not imply anything about the atmospheric conditions (except the sun has set).
"Yes, there is radiational cooling, but the amount of radiation from the atmosphere is relatively small." I assume this is a typo since the atmosphere is merely the medium through which radiation passes. Additionally, this statement does not consider the presence of clouds, which can most certainly create a significant amount of radiation.
Maybe you should have asked about "placing a limit on actual nocturnal cooling."
http://www.ems.psu.edu/~fraser/BadScience.html
I did undergrad work at PSU 1979-81
For what it's worth I was not taught (in middle or high school) that skylight is blue, only that
it is more blue than the direct beam of the sun because of scattering and it is perceived as blue for this reason even though there are a lot of
other frequencies. I was never taught that it was either monochromatic or had a narrow frequency band in the short visible range.
Arctic air always extends up to 850MB at the Pennsylvania latitudes. However in the southeast U.S. it can be MUCH shallower and -20C at 850mb
(or even exceptionally cold for the latitude)
is not an indicator that far south. (Florida State where I did graduate
work comes to mind; in the coldest of the 20'th century outbreak in January 1985, surface temperatures were around 5-8F with strong
winds (and good mixing) at 30mph) but 850MB temps only got to about -10C. (14F)
That said, I saw colder 850mb temperatures with other airmasses
in Tallahasse where surface temperatures did not get nearly as cold.
George Vandenberghe
That's surely what people mean so I'm comfortable with calling it radiational cooling. Nocturnal cooling could be caused by cold air advection, so it is less specific.
This is similar to focusing upon birth without recognizing its necessary companion, death. Doing this just doesn't work. Society is the balance between these two. Without the balance of death, birth cannot happen.
I was struck by this phenomenon while at my grandfather's funeral in Minneapolis in mid December 1983 (if you bury your grandfather as a young adult you're lucky. to have known him so long and I count that as a blessing). THere was fresh snow (over a foot of it) on the ground and an arctic air mass had oozed in with lows around -14 and highs around -5. The day was forecast to be and verified fully sunny and I expected a stiff breeze at the grave site since there was a pressure gradient and an airmass that cold would of course mix some during the day.
It didn't. The wind remained a slow ooze out of the north and there were no gusts. I realized, standing there in the full noontime sunshine (in the blessedly light breeze) that this airmass was not only cold, it was getting COLDER as it moved over us.
(The next day I flew to FMY and went from -20 at the MSP airport to +83F at FMY with dewpoints near 70. A week later all of FLorida expericened the Christmas freeze of the century)
6:57 PM GMT on December 24, 2012
That's just not true. Around the time of maximum temperature, the ground is undergoing its largest radiational cooling (sigma T to the fourth power). But it is receiving much more energy than it's losing.
Viewing: 1 - 39
Page: 1 — Blog Index