492 Dam Air

[londonsnow]

probably one for mr data, but i just wondered if 492 air has ever reached our shores. i think almost certainly not, as lets face it 510 air is a rarity these days. just a passing query. LS

[SP1986]

I recall that it once descended the Shetlands/Northern Scotland.. this was a topic I vaguely remember being brought up in 2003! I cant be sure but I would suggest yes it has in the past not sure when though

[Methuselah]

Maybe Xmas 1995, Ste?

[Thundery wintry showers]

I don't think Xmas 1995 managed it- the severity of cold then was due mainly to snow cover, clear nights and hardly any wind, and I doubt it got below around 510 dam.

Shetland might have managed it a few times, and in the January 1987 easterly parts of south-east England might have sneaked below 500 dam.

[Methuselah]

I don't think Xmas 1995 managed it- the severity of cold then was due mainly to snow cover, clear nights and hardly any wind, and I doubt it got below around 510 dam.

Shetland might have managed it a few times, and in the January 1987 easterly parts of south-east England might have sneaked below 500 dam.

81/82???

[Thundery wintry showers]

I doubt it. December 1981 had a similar type of pattern to late December 1995- an Arctic airmass trapped over Britain with frequent cold days and nights. January 1982 had some very deep cold air towards the NE but again unlikely to have approached 492 dam.

[Summer of 95]

1795? 1683-84? (clutching at straws....)

What about January 1940 (IIRC a northerly rather than easterly month, with -20 recorded in western Britain)?

[Paranoid]

Jan 11th 1993 according to Wetterzentrale http://www.wetterzentrale.de/topkarten/fsreaeur.html

EDIT: Link doesn't go to the actual chart for the day, you'll have to put that in yourselves i'm afraid, i don't know how to link it to the proper chart. Sorry.

[Methuselah]

I doubt it. December 1981 had a similar type of pattern to late December 1995- an Arctic airmass trapped over Britain with frequent cold days and nights. January 1982 had some very deep cold air towards the NE but again unlikely to have approached 492 dam.

Oh, sod it, Ian!

How's about Jan-Feb 16,000 BC???

[I remember Atlantic 252]

Jan 11th 1993 according to Wetterzentrale http://www.wetterzentrale.de/topkarten/fsreaeur.html

im just learning this, does it mean the numbers down the right side of WZ charts? because that day is mild, and I do know the early 500's is cold

[shuggee]

im just learning this, does it mean the numbers down the right side of WZ charts? because that day is mild, and I do know the early 500's is cold

Yes - that's the wrong chart.

You need the 500-1000 HGT chart not the 500 Hpa chart. To illustrate, here's the difference between the two for right now:

500 Hpa

HGT 500-1000

You can see the difference.

[Thundery wintry showers]

I recall that a rough method of calculating the DAM figures from the hPa charts is as follows:

Height (dam) = hPA + (0.8*(sea-level pressure-1000))

I think it was Steve Murr who originally suggested that formula.

The 11th January 1993 was actually quite a cold day over Scotland with heavy blizzards, but a warm one in the south- the low hPa values were mainly a consequence of the very low pressure.

[I remember Atlantic 252]

Yes - that's the wrong chart.

You need the 500-1000 HGT chart not the 500 Hpa chart. To illustrate, here's the difference between the two for right now:

500 Hpa

HGT 500-1000

You can see the difference.

Thanks Shuggee yeah that's what I thought

[Stu_London]

If you are talking HGT 500-1000, then I think 1987 is the closest we came with something like 496 in east anglia

500HPA is not that uncommon - a deep depression say sub 960mb, in mid winter might bring sub 492 heights though not necessarily any cold weather.

[Optimus Prime]

I don't think Xmas 1995 managed it- the severity of cold then was due mainly to snow cover, clear nights and hardly any wind, and I doubt it got below around 510 dam.

Not sure that's true. There was very little snow snow outside the Northern Isles. There was very little snowcover if any. It was mostly due to a very deep low level cold draw of air from very far north and little wind.

[Weather-history]

I've always wondered what the DAM and thickness levels were like for the start of April 1917 over the UK with sub-zero maxima being recorded in the north. Spitsbergen recorded a minimum of -49.2C on the March 28th of 1917 and the airmass over the UK came from that region.

Not sure that's true. There was very little snow snow outside the Northern Isles. There was very little snowcover if any. It was mostly due to a very deep low level cold draw of air from very far north and little wind.

No there was a snowcover further south outside the Northern Isles. It may not have been as deep but there was certainly a snowcover further south. Even here in Manchester there was a dusting Christmas morning. I think Glasgow may have got a couple of inches.

[trickydicky]

Not sure that's true. There was very little snow snow outside the Northern Isles. There was very little snowcover if any. It was mostly due to a very deep low level cold draw of air from very far north and little wind.

I think there was widespread snow as far south as about Manchester. There was some in Cumbria, not deep as such, 3 or 4 inches and no drifts to speak of, but very dry powder. That would suggest it came fom cold upper air.

[Thundery wintry showers]

Not sure that's true. There was very little snow snow outside the Northern Isles. There was very little snowcover if any. It was mostly due to a very deep low level cold draw of air from very far north and little wind.

Not so- there was 4cm at Cleadon (Tyne & Wear) on Christmas morning, most of it from Christmas Eve's snowfall, which increased to 8cm by the end of the day due to heavy east-coast snow showers. Lancaster had 7cm. Dyce (Aberdeen), Aviemore, Abbotsinch (Glasgow Airport), Leuchars, Aldegrove (N Ireland), and Tynemouth all had snow cover on Christmas Day, judging by Weather Log. Much of it resulted from a polar low that brought a dumping of snow to much of Scotland on Christmas Eve but fizzled as it headed south across northern England. Surprisingly it appears that nearby Durham contrived to get less than 50% cover from the system despite Cleadon getting a few centimetres.

Some parts of Norfolk also had a white Christmas due to east-coast snow showers, and Scarborough was cut off due to frequent heavy blowing snow. Snow showers penetrated a little further inland across Yorkshire, Lincolnshire and East Anglia on Boxing Day giving a more widespread snow cover in those areas.

The spell was generally nothing like as cold from the Midlands southwards as it was from northern England northwards and I think the fact that the former areas were largely free of snow cover, and the latter largely snow covered, had a lot to do with it.

Edit: charts for Boxing Day and the 27th reveal that the upper air was not exceptionally cold (and this is as cold as it got):

http://www.wetterzentrale.de/archive/ra/1995/Rrea00119951226.gif

http://www.wetterzentrale.de/archive/ra/1995/Rrea00119951227.gif

http://www.wetterzentrale.de/archive/ra/1995/Rrea00219951226.gif

http://www.wetterzentrale.de/archive/ra/1995/Rrea00219951227.gif

[Methuselah]

Not sure that's true. There was very little snow snow outside the Northern Isles. There was very little snowcover if any. It was mostly due to a very deep low level cold draw of air from very far north and little wind.

No OP, there was lots of snow, about 6" if I recall...

[johnholmes]

interesting to read about snowfall but its hardly what the thread it supposed to be about chaps!

[Iceberg]

I recall that a rough method of calculating the DAM figures from the hPa charts is as follows:

Height (dam) = hPA + (0.8*(sea-level pressure-1000))

I think it was Steve Murr who originally suggested that formula.

The 11th January 1993 was actually quite a cold day over Scotland with heavy blizzards, but a warm one in the south- the low hPa values were mainly a consequence of the very low pressure.

I think that formula has been in text books for many years, but it should be

496.6=520-(0.8*(1035-1000))

If you take the above which can be figured out using the below chart, then apply a bit of local variation and also that this likely isn't the coldest point of that day it's possible to get down to 495 maybe during the 13th of Jan 1987.

No doubt a few other very cold years have got down this, so the 492 figure has likely been reached. certainly 495 to 496 has been reached in the last 25 years.

Also notice that the Yellows never touched Greenland or Svalbard so this isn't a definate pre req for a severe cold spell.

reading through this means that I pretty much concur with Stu.

[londonsnow]

thanks for the feedback guys. i guess the best place near the uk to experience 492 air is greenland! problem is getting there takes quite a while

[johnholmes]

to try and clear up what appear to be some misunderstandings on what is meant by DM and how its arrived at I hope the link below will help.

I've also dropped it into the Guides area for ready reference.

DAM heights or total thickness between two levels.doc

[Snowyowl9]

A deep cold severe atlantic storm like this would bring low dam 504 for this,only found this as I remember it.

And it did give snow.

http://www.wetterzentrale.de/archive/ra/1988/Rrea00119880210.gif

[bluearmy]

A deep cold severe atlantic storm like this would bring low dam 504 for this,only found this as I remember it.

And it did give snow.

http://www.wetterzentrale.de/archive/ra/1988/Rrea00119880210.gif

sorry snowyowl - they're not actual thicknesses as you need to take sea level pressure into account - see JH's post above.