Arctic Air Masses

By: 24hourprof , 5:14 PM GMT on December 12, 2012

Share this Blog
25
+

At 65, I'm one of those old timers who still enjoys cold and snowy weather. In the early 1980s while I was in graduate school at McGill University (Montreal, Quebec), I would chase lake-effect snow squalls downwind of Lakes Ontario and Erie, inserting myself into convective bands with snowfall rates of six inches per hour (or higher) with thunder and lightning. Those were the days, my friends.

Here in central Pennsylvania, cold weather has been hard to come by this autumn and this winter, so I looked enviously at the Arctic air mass that has dipped into the Upper Middle West this morning (09Z surface analysis).

Take a closer look at the 09Z temperatures and dew points on the station models over western and central Canada (09Z is 4 A.M. EST). With temperatures and dew points as low as minus 44 degrees and minus 47 degrees Fahrenheit respectively, there's no question that this is a genuine Arctic air mass. What is an air mass? Are polar air masses colder than Arctic air masses?


The source regions for air masses that affect North America. Credit: Courtesy of A World of Weather: Fundamentals of Meteorology.

For the record, an air mass is a large chunk of air with horizontal dimensions on the order of several hundred to a couple of thousand miles. Within any air mass, temperatures and dew points near the earth's surface (or at any other arbitrary altitude) vary only gradually with increasing distance away from the center of the air mass (a center of high pressure).

Winter's most frigid air masses, like the air mass shown on the 09Z surface analysis, are tagged continental-Arctic air masses (cA) to herald their extreme cold and very low dew points. There are a couple of tests for Arctic air masses. For starters, I avoid looking at nighttime temperatures because sometimes, nocturnal cooling can mislead you into thinking the air mass is colder than it really is (this happens sometimes in Nevada and other parts of the Intermountain West on clear, calm nights during the cold season).

No, I like to look at daytime temperatures in order to determine the presence of Arctic air. When Arctic air masses invade the United States, daytime high temperatures are typically in the single digits or they're below zero degrees Fahrenheit. Yesterday's high temperatures over central and western Canada (right panel) seal the deal on the Arctic nature of this air mass.

I also look at 850-mb temperatures (standard altitude is 1500 meters) to verify the presence of Arctic air. As a general rule, 850-mb temperatures equal to or lower than minus 20 degrees Celsius are consistent with Arctic air. Check out the 06Z GFS 3-hour forecast for 850-mb temperatures (valid at 09Z this morning) and note the large area over Canada where 850-mb temperatures were predicted to be minus 20 degrees Celsius or lower.

For your benefit, here's a close-up of the Arctic front on the 09Z surface analysis this morning, which marks the leading edge of Arctic air. In case you're wondering, the cold front shows where Arctic air was advancing, and the stationary front indicates where Arctic air stalled.

Despite its name, continental-Polar air masses (designated cP) are not as cold as continental-Arctic air masses during winter. Yes, I agree, the convention of using "polar" to describe these air masses is somewhat confusing, but revisit the source-region map above to drive home my point that continental-Polar air masses form farther equatorward than continental-Arctic air masses.

A Pet Peeve

Earlier, I made a reference to "nocturnal cooling." For the record, I always try to avoid "radiational cooling" whenever I'm discussing low nighttime temperatures. That's because I often hear people describe the reason for very low nighttime temperatures as "great radiational cooling." Aaaarrrgggghhhh...

If the truth be told, the greatest radiational cooling occurs around the time of the daytime maximum temperature. Indeed, radiational cooling is proportional to the fourth power of absolute temperature (in Kelvins), according to Stefan-Boltzmann's Law. So that's why I prefer "nocturnal cooling," a term that gets to the heart of the real reason for very low temperatures on a clear, calm night...yes, there is radiational cooling, but the amount of radiation from the atmosphere is relatively small. So the ground runs an energy deficit, and temperatures continue to fall until solar energy arrests the decline after sunrise. I'll have more to say about nocturnal cooling in future blogs.

So, yes, I'm sort of a stickler for the accuracy of science. There will be an underlying theme of science in all of my blogs, and I hope readers learn more about meteorology and atmospheric science as I delve into deeper topics.

I am very grateful for this opportunity.

Lee Grenci

Reader Comments

Comments will take a few seconds to appear.

Post Your Comments

Please sign in to post comments.

or Join

Not only will you be able to leave comments on this blog, but you'll also have the ability to upload and share your photos in our Wunder Photos section.

Display: 0, 50, 100, 200 Sort: Newest First - Order Posted

Viewing: 40 - 1

Page: 1 — Blog Index

40. Bannyok
7:17 AM GMT on February 24, 2014
Hi there. I really appreciate the points you made. I don't think I've actually thought about it in that way. I can really appreciate how you approached the subject matter and what you said really gave me a new perspective. Thanks for taking the time to write this all out.
Your opinion this blog:
Harga Jual Kenari
Hewan Langka
Memelihara Anjing Chow chow
Anjing Kintamani
Anjing Labrador Retriever
Anjing Pitbull
Anjing Samoyed
Anjing Siberian Husky
Memelihara Ternak kambing
Cara merawat lovebird Untuk Lomba
Member Since: February 24, 2014 Posts: 0 Comments: 6
39. 24hourprof
6:57 PM GMT on December 24, 2012
Quoting bappit:
Radiational cooling occurs when the temperature drops. If there is incoming radiation to offset the outgoing radiation, then there is less cooling. So we get more radiational cooling at night when the sun goes to sleep.

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.


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.
Member Since: October 24, 2012 Posts: 98 Comments: 839
38. georgevandenberghe
1:30 PM GMT on December 17, 2012
One point about that airmass diagram. The source region for Arctic air can be a lot larger than that. If the atmosphere is very dry (general in winter under subsidence) and the ground is snow covered, esp with new snow, incoming absorbed radiation will be exceeded by outgoing and the air mass will cool at the surface until it is bitter cold (-20s for lows 0s for highs at 40N) and it may maintain deceptively high thicknesses while doing so since the cooling is mostly at the surface. I suspect this doesn't last long enough to be noticed very often because the cooling air will either move away somewhere else or come under lifting which increases moisture depth and downward IR radiation.

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)
Member Since: February 1, 2012 Posts: 19 Comments: 2236
37. Balwanz
9:01 PM GMT on December 15, 2012
I apologize for not reading all replies, this is probably already mentioned. I don't dismiss radiation cooling at any time. Instead I consider it an aspect of radiation equilibrium. That others would consider one without the other makes no sense to me.
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.
Member Since: November 23, 2011 Posts: 0 Comments: 9
36. bappit
8:04 PM GMT on December 15, 2012
Radiational cooling occurs when the temperature drops. If there is incoming radiation to offset the outgoing radiation, then there is less cooling. So we get more radiational cooling at night when the sun goes to sleep.

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.
Member Since: May 18, 2006 Posts: 10 Comments: 6158
35. georgevandenberghe
8:44 PM GMT on December 14, 2012
Professor Grenci. I also am very pleased to read and comment on your blog. It stirs old memories of when I was a student at PSU (and later FSU) and of areas in my education as a dynamic meteorologist that I am no longer exercising in my year to year work as an HPC specialist for NCEP or things that I perhaps learned wrong.


George Vandenberghe
Member Since: February 1, 2012 Posts: 19 Comments: 2236
34. georgevandenberghe
5:08 PM GMT on December 14, 2012
Oh by the way cold air is even harder to come by in the DC area where I'm still picking lettuce from my garden on December 14 with NO protection. (Last night's 25F may have damaged the plants though)
Member Since: February 1, 2012 Posts: 19 Comments: 2236
33. georgevandenberghe
5:06 PM GMT on December 14, 2012
One of the rules of thumb for positioning the Arctic Front (abbreviated by John Dutton as ROT [ for good reason ] ) is the position of the 528DM 100-50hpa thickness contour in winter It is somewhat correlated and that's the strongest support I'll make about its validity. But there are many many cases where there is no arctic air in an air mass that exhibits much lower thicknesses and when arctic air gets deep into the southeast the arctic front is found along a higher thickness contour.
Member Since: February 1, 2012 Posts: 19 Comments: 2236
32. georgevandenberghe
4:18 PM GMT on December 14, 2012
I should say "almost always extends to 850mb" "Always" is a dangerous word easily disproven with a single contradictory example.
That said, I saw colder 850mb temperatures with other airmasses
in Tallahasse where surface temperatures did not get nearly as cold.
Member Since: February 1, 2012 Posts: 19 Comments: 2236
31. georgevandenberghe
4:15 PM GMT on December 14, 2012
Alistair Fraser maintains a bad science page (C 1995)

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)


Member Since: February 1, 2012 Posts: 19 Comments: 2236
30. bappit
4:12 PM GMT on December 13, 2012
Quoting beell:
Thanks for another great entry, Professor.

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.

Maybe you should have asked about "placing a limit on actual nocturnal cooling."

Member Since: May 18, 2006 Posts: 10 Comments: 6158
29. WunderAlertBot (Admin)
3:58 PM GMT on December 13, 2012
24hourprof has created a new entry.
28. STLweatherjunkie
3:56 PM GMT on December 13, 2012
Welcome to the community Lee, 6 in/hr snowfall rates sound fantastic indeed!

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.
Member Since: September 9, 2001 Posts: 0 Comments: 1040
26. FormerAussie
12:26 PM GMT on December 13, 2012
Good morning from the far side of the pond where, judging from the longer range forecasts, spring will be arriving just in time for Christmas. The first spring, that is, we often have several false starts.
Member Since: October 10, 2006 Posts: 3 Comments: 58
25. beell
12:09 PM GMT on December 13, 2012
Lee,
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!


Member Since: September 11, 2007 Posts: 145 Comments: 16926
24. NasBahMan
3:01 AM GMT on December 13, 2012
Welcome Prof. Grenci...A great addition to WU, very informative, I know this isn't a Tropical Weather Blog but would love for Dr. Masters to add a quick link on his blog
Member Since: August 3, 2001 Posts: 0 Comments: 152
23. Barefootontherocks
1:40 AM GMT on December 13, 2012
Very interesting. Thank you.
Member Since: April 29, 2006 Posts: 159 Comments: 19397
22. Hoynieva
11:51 PM GMT on December 12, 2012
Good read and looking forward to more.
Member Since: January 20, 2009 Posts: 0 Comments: 1521
21. 24hourprof
11:28 PM GMT on December 12, 2012
Quoting stormchaser43:
welcome aboard & i enjoy your perspective,,


Many thanks!
Member Since: October 24, 2012 Posts: 98 Comments: 839
20. 24hourprof
11:28 PM GMT on December 12, 2012
Quoting whitewabit:
Welcome to WU .. you stated "I am very grateful for this opportunity.", But we are the one's who should be grateful for the opportunity to read what I am sure will be a very interesting blog !!


Thanks for your generosity!
Member Since: October 24, 2012 Posts: 98 Comments: 839
18. whitewabit (Mod)
10:32 PM GMT on December 12, 2012
Welcome to WU .. you stated "I am very grateful for this opportunity.", But we are the one's who should be grateful for the opportunity to read what I am sure will be a very interesting blog !!
Member Since: August 17, 2005 Posts: 368 Comments: 32536
17. DocNDswamp
9:56 PM GMT on December 12, 2012
Quoting 24hourprof:


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!
;)
Member Since: September 21, 2005 Posts: 94 Comments: 4803
16. KoritheMan
9:14 PM GMT on December 12, 2012
Easily one of the best bloggers I've ever read. Very informative and straight to the point. Keep up the good work, Lee.
Member Since: March 7, 2007 Posts: 602 Comments: 21333
15. 24hourprof
9:09 PM GMT on December 12, 2012
Quoting DocNDswamp:
Great to have you onboard Wunderground, Prof Lee!

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!


A Weatherwise fan! I miss writing for the magazine. Many thanks for the warm welcome.
Member Since: October 24, 2012 Posts: 98 Comments: 839
14. DocNDswamp
8:50 PM GMT on December 12, 2012
Great to have you onboard Wunderground, Prof Lee!

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!
Member Since: September 21, 2005 Posts: 94 Comments: 4803
13. 24hourprof
8:49 PM GMT on December 12, 2012
Quoting auburn:
Welcome !!!


Many thanks.
Member Since: October 24, 2012 Posts: 98 Comments: 839
12. 24hourprof
8:49 PM GMT on December 12, 2012
Quoting calpoppy:
I'm hooked!!!!! This is great!


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.
Member Since: October 24, 2012 Posts: 98 Comments: 839
11. 24hourprof
8:46 PM GMT on December 12, 2012
Quoting TomballTXPride:



Keep your Arctic air North of the U.S. / Canadian Border.

Thanks!

:)




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.
Member Since: October 24, 2012 Posts: 98 Comments: 839
10. 24hourprof
8:44 PM GMT on December 12, 2012
Quoting calpoppy:
I'm hooked!!!!! This is great!


Way too kind, but thanks!!!!
Member Since: October 24, 2012 Posts: 98 Comments: 839
9. 24hourprof
8:42 PM GMT on December 12, 2012
Quoting barbamz:
Thanks a lot and welcome. New stuff learned.


Great! Many thanks!
Member Since: October 24, 2012 Posts: 98 Comments: 839
8. 24hourprof
8:41 PM GMT on December 12, 2012
Quoting JNCali:
Welcome Prof. Grenci, looks like you will be wonderful addition to the WU blog family! Looking forward to many informational articles! I'm certain that you will find the thick skin you most certainly have developed from the years at Penn coming in most handy on this blog. The WU clan is quite varied on so many levels....


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.
Member Since: October 24, 2012 Posts: 98 Comments: 839
7. 24hourprof
8:31 PM GMT on December 12, 2012
Quoting beell:
Thanks for another great entry, Professor.

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.


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.
Member Since: October 24, 2012 Posts: 98 Comments: 839
6. JNCali
8:23 PM GMT on December 12, 2012
Welcome Prof. Grenci, looks like you will be wonderful addition to the WU blog family! Looking forward to many informational articles! I'm certain that you will find the thick skin you most certainly have developed from the years at Penn coming in most handy on this blog. The WU clan is quite varied on so many levels....
Member Since: September 9, 2010 Posts: 5 Comments: 1034
5. barbamz
7:41 PM GMT on December 12, 2012
Thanks a lot and welcome. New stuff learned.
Member Since: October 25, 2008 Posts: 64 Comments: 6724
4. calpoppy
6:53 PM GMT on December 12, 2012
I'm hooked!!!!! This is great!
Member Since: February 18, 2008 Posts: 57 Comments: 4034
3. beell
6:29 PM GMT on December 12, 2012
Thanks for another great entry, Professor.

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.
Member Since: September 11, 2007 Posts: 145 Comments: 16926
2. auburn (Mod)
6:29 PM GMT on December 12, 2012
Welcome !!!
Member Since: August 27, 2006 Posts: 547 Comments: 50882

Viewing: 40 - 1

Page: 1 — Blog Index

Top of Page

About 24hourprof

Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.