850Hpa

[geoffw]

what is the lowest 850hpa the uk has ever seen? also what is the lowest 850hpa the world has ever seen?? if you can, post charts to show this

thanks

[Polar Continental]

what is the lowest 850hpa the uk has ever seen? also what is the lowest 850hpa the world has ever seen?? if you can, post charts to show this

thanks

hi Geoff

Probably not when you would think actually, in recent times anyway.

I suppose most of us would think the lowest 850 hpa temperatures would have occurred during our long and severe winters, of 1947, or 62, 63, or 78, 79. Actually during most of the 1962 winter and 78-79 winters the 850 hpa didn’t drop much below –10c around the UK, many days the 850’s were around just –5c hpa, it was the incredibly prolonged periods that sub zero 850’s persisted.

Possibly the most appalling outburst on record was the very severe winter of 1739-40.

Below is a post from myself a couple of years ago..

'The winter of 1739 started early during October when easterly winds set in giving widespread frosts. But the real cold set-in on Christmas Eve with severe frosts persisting day and night with a great freeze lasting until February 17th 1740.

By far the worst outbreak of the incredible bitter cold came at the end of December in London, when temperatures below – 9c / 15f arrived on an easterly gale. Near by on the Continent in Holland the gale was accompanied by temperatures of –19c / -2f. for hours on end.'

I would think from the above, the 850’s reaching our shores must have been below –20c.

The most recent very low 850hpa, as low as –20c was during January 1987, see chat below, many places in southern England saw maximums of just –6c accompanied by heavy powder snow.

Regarding the lowest in the world, that would almost certainly be in Antarctica, i have no idea of the lowest 850hpa, at a guess i would say around -40c to -45c or lower. Surface temperatures there though can be much lower than that.

Paul

Edited October 21, 2009 by Polar Continental

[Nights King]

Marginallyoff topic but what kind of 850's do you need to have a chance of getting snow at sea level and how much does it change with height?

what 850temperature would give 300m a chance of snow?

[Thundery wintry showers]

At sea level the broad rule of thumb is that below -5C 850hPa gives a reasonable chance of snow, and below -10C makes it almost certain, but it depends on the airmass. A sunshine-and-showers regime will require lower values than a frontal battleground for example, as the lapse rates mean that the difference between the surface and upper air temperatures is larger in a showery regime.

The 850hPa temperature required for snow at 300m probably isn't much different actually, just a couple of degrees higher perhaps.

[Polar Continental]

Was going to post this morning, but had to go out. I see TWS has explained as well, but since I started, will post this also.

There are many factors involved, under the right conditions with the help of evaporation cooling, snow can fall at low levels even when the 850 hpa temperatures are around 0c or just below.

Then there is the source of the cold airflow to take in to account.

If the cold air supply is of a continental origin, -5c hpa is usually enough for snow to fall at low levels, as the surface air will be much less modified over a short sea track.

Maritime surface air though will become considerably more modified at the surface than at the 850 hpa level because of the long sea track, so -8c to -10c 850hpa is better for snow reach the ground at low levels.

Paul

Edited October 22, 2009 by Polar Continental

[Thundery wintry showers]

If the cold air supply is of a continental origin, -5c hpa is usually enough for snow to fall at low levels, as the surface air will be much less modified over a short sea track.

Maritime surface air though will become considerably more modified at the surface than at the 850 hpa level because of the long sea track, so –-8c to –-10c 850hpa is better for snow reach the ground at low levels.

Paul

I may also add that this only works for southern and central England. For northern England and Scotland, easterly outbreaks usually have a long track over the North Sea as well and so 850hPa values of -8C or below are desirable in those cases as well.

[Polar Continental]

I may also add that this only works for southern and central England. For northern England and Scotland, easterly outbreaks usually have a long track over the North Sea as well and so 850hPa values of -8C or below are desirable in those cases as well.

Thanks TWS, I should have made that clear.

[Backtrack]

Is 850HPA the upper air temperature? If it is, I think i'm starting to understand the charts!

[Polar Continental]

Is 850HPA the upper air temperature? If it is, I think i'm starting to understand the charts!

Yes mate, roughly at 1.5km

[Backtrack]

Yes mate, roughly at 1.5km

Thanks PC. Whats the rough difference from upper air temps and temperatures we see at ground level? Is it 5c difference ish?

[Summer of 95]

At sea level the broad rule of thumb is that below -5C 850hPa gives a reasonable chance of snow, and below -10C makes it almost certain, but it depends on the airmass. A sunshine-and-showers regime will require lower values than a frontal battleground for example, as the lapse rates mean that the difference between the surface and upper air temperatures is larger in a showery regime.

The 850hPa temperature required for snow at 300m probably isn't much different actually, just a couple of degrees higher perhaps.

It's an interesting puzzle, looking through the charts at Wetterzentrale, it's surprising how many of the biggest snowfalls here have come with relatively high 850hpa values.

For instance on 12 Mar 2006 we got 12cm of snow despite being right on the -5C isotherm (frontal battleground); 8 Dec 1990 and 3 Mar 1995 it was around -7, at Christmas 2004 (a Sunshine and showers setup) it also only reached about -7.

Yet in late Feb/early Mar 2005 the -10 regularly reached here, it seldom went above -5 for two weeks, yet we never got more than 2cm which always disappeared in a couple of hours. That spell had similar 850s to other easterly spells which produced plenty of snow, which stuck for longer (Feb 1994, Dec-Jan 96/7, Feb 1991)- why did it not produce comparable wintry weather?

[Terminal Moraine]

Thanks PC. Whats the rough difference from upper air temps and temperatures we see at ground level? Is it 5c difference ish?

It's roughly 8c as, on average, temperature falls about 1c for every 180m in height ( 600ft ); there is some variability around this figure though depending on the air mass.

It can be very misleading during spells of high pressure is winter when the air stagnates and inversions develop; in this case the 850hp temp can be, for example, around 5c but the surface temperature can be around 0c or below.

[Backtrack]

It's roughly 8c as, on average, temperature falls about 1c for every 180m in height ( 600ft ); there is some variability around this figure though depending on the air mass.

It can be very misleading during spells of high pressure is winter when the air stagnates and inversions develop; in this case the 850hp temp can be, for example, around 5c but the surface temperature can be around 0c or below.

So in the case of a Northerly there would be upper air temps of say... -10c it would mean there would be roughly a -2 temperature at sea level?

[Thundery wintry showers]

It can be, but in a northerly there is usually some modification at lower levels due to the north Atlantic and North Sea, so the temperature differential is often a lot higher- the same reasons as why polar westerly flows are heavily modified and also why easterlies are more modified in the north than in the south.

[Backtrack]

It can be, but in a northerly there is usually some modification at lower levels due to the north Atlantic and North Sea, so the temperature differential is often a lot higher- the same reasons as why polar westerly flows are heavily modified and also why easterlies are more modified in the north than in the south.

Oh I see... Thanks TWS.