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J10

Ground Temps

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Over the past few days I think we are seeing the real difference that frozen ground has on snow sticking and snow retention.

Over the past few winters, there have been occasions that despite heavy snow, it has struggled to stick and the ground has become wet and any further snow has found it even harder to stick, in effect negative feedback.

Whereas on this occasion, the ground is frozen and snow is sticking and this helps further snow to stick, causing positive feedback. Overnight the ground freezes under the snow making it even colder.

Of course it does help that for once we are having a real cold spell in the heart of winter, rather than mid to late February.

Does anyone have any thoughts on this, as perhaps at lest for Scotland and other Northern parts of the UK, this feedback loop may continue for some time.

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The ground here has been frozen since Saturday night. I think the rain helped the ground to freeze and stay for so long after it rained, then a -3c occured.

Pond is really frozen here, people been walking to the middle of it - This has also stayed frozen despite temperatures maxing out at almost 3c.

Not had a higher maximum than 3c since Saturday now, although it looks like my record will be spoiled on Boxing Day - Followed by another cold spell? :whistling:

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If snow falls heavily enough, at low enough temperatures, it will settle. However, ground temperatures are certainly an important factor, because if the ground temperatures are warm it has to snow more heavily for it to settle.

The classic example in recent years was on the 12th March 2006, when most western areas got very heavy snow so it settled despite the high ground temperatures, but in most eastern areas the snow was light and so struggled to settle, especially on concrete surfaces, despite air temperatures below freezing. In contrast, snow settled here in Cleadon last Saturday with the temperature standing at 1.5C, and the same happened at 1.6C on last Thursday night. During the Arctic outbreak of Easter 2008, snow actually settled during some of the showers with the temperature standing at 2 or 3C.

There are quite a few other factors that affect the likelihood of snow accumulating on the ground as well, air temperatures and dewpoints in particular being of paramount importance.

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Quite an interesting sight today with snow having settled and stuck to both the Forth Bridge (think big red columns of iron plastered in white) and the sea wall in Dundee/Broughty Ferry. The decks of both the Tay and Forth bridges were also covered in white. Those type of surfaces rarely get cold enough for snow to settle, being so close to (or should I say actually in) the sea, so my point is it really has been that cold for that long that it's a very positive feedback indeed Jackone! :crazy:

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Interesting topic.. looks like being one of the coldest nights here in many a year. I got a taxi home about 40 mins ago and the temps were already down to minus 7 at 2300. The 18z GFS is forecasting a minimum of minus 2 tonight, so a long way below that already.. check out Middle Wallop or Benson for a local reading.

We have about 6cm of lying snow here and it has been clear since midday hence the ingredients are all there for an exceptionally cold night to the point where the antifreeze may not work and we have a lot of frozen engines on cars it is that cold.

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We have some heavy sleet here, but this has had little impact on the lying snow.

Normally the snow would have simply been washed away by now.

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Rained all day today and although i only had 2 inches of snow id say most of it still on ground except it is like an inch and half of solid ice now and froze very quickly because we have had several nights of frost down to -7/8 and even one -9

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Certainly I don't think the snow we've just had would have settled were it not for all the frozen ground/icy pavements; even heavy snow almost never settles here if it starts off as rain and fell onto wet ground. However I also think that the weather in the days, even weeks leading up to an event are important. Even 3-4 frosty nights preceding a snowfall can often help in my experience; we did get that in the days before the Feb 2007 event, and it had dropped below freezing most nights for a fortnight or more before 12th March 2006. On the other hand the lack of settling snow in Feb 2005 came after there had barely been a frost since Christmas.

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Interesting thread.

Sorry to go off at a tangent but I think sea temps, particularly the North Sea temps, also matter.

During the cold spell so far North Sea temps have fallen at least 1 degree Celsius. The small patch of 7.5C water must reflect a 2.5C drop! But let's not get carried away - the North Sea is still warmer than the running average.

picture2zu.jpg

Even so, the slightly colder SSTs means any subsequent blasts of cold air from the East won't be modified mild as much as this last one was. This could be the difference between snow and rain for some.

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I've noticed snow doesn't settle on wet concrete initially, until it gets heavy enough. The ground builds up loads of slush which the snow then settles on. This was the case in some places Today where snow had melted.

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Certainly I don't think the snow we've just had would have settled were it not for all the frozen ground/icy pavements; even heavy snow almost never settles here if it starts off as rain and fell onto wet ground. However I also think that the weather in the days, even weeks leading up to an event are important. Even 3-4 frosty nights preceding a snowfall can often help in my experience; we did get that in the days before the Feb 2007 event, and it had dropped below freezing most nights for a fortnight or more before 12th March 2006. On the other hand the lack of settling snow in Feb 2005 came after there had barely been a frost since Christmas.

Feb 2005 is often quoted as an example of snow failing to settle to any meaningful depth because of warm ground temperatures, but on that occasion, the air temperatures and dewpoints were both very marginal, featuring maxima of 4 or 5C for most and minima and dewpoints close to 0C. In some inland parts of SE Scotland and NE England, where temperatures were a couple of degrees lower, snow accumulated to aggregate depths in excess of 10cm.

If ground temperatures had been lower during the February 2005 easterly, the snow would have settled more readily but it would still have thawed in the sunshine in between the snow showers except for the areas that saw maxima more like 2 or 3C.

I mentioned in my earlier post that March 12th 2006 was, in eastern areas, a classic example of snow struggling to settle due to high ground temperatures despite temperatures and dewpoints both below freezing. I got 5cm in Leeds out of that, but only after the snow had become heavy. In contrast, the snow was heavier in the west and that's why it settled readily there.

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On Thursday, the air temperature was just around 1C for the entire day, but due to the ground being frozen overnight, the first shower lay with ease. Even Boxing day looks like it won't ruin our snowfall, as the ground is so cold and the snow will be so frozen (-6C here now with a possible ice day and another frost to come) that even if it falls as rain, I reckon it will freeze on contact with the ground. The lack of heat from the sun has also been a major factor - daytime temperatures yesterday were about 8C colder than overnight temperatures, with -16C and sunshine recorded at Tulloch Bridge at 4pm!

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I noticed that in Cleadon last Thursday, snow settled on the ground (albeit only temporarily) with a temperature of 2.0C at one point, which came as a big surprise. Dewpoints were very low, but it was also clear that the ground had frozen over- thus ideal for settling snow despite a temperature of 2C.

Sometimes in marginal conditions like those, a hail shower can help matters by providing a cover, on which snow can settle more readily- this particularly applies when the surface underneath is warm and wet.

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I had an average snow depth of 16cm at 9am, 24th February 2005, in fact I have a picture of my ruler up to 18cm in snow from about midday the same day (although the 16cm at 9am was recorded for record keeping purposes and is still my record)

There was widely 25-30cm in areas such as Consett and Copley at c900-1,000ft and Boltshope Park c1,500ft had 50cm.

The snow didn't melt too quickly because of the sheer amount of the stuff, but certainly sunshine damaged it quickly...but there was a lot to burn through and it kept getting topped up - I had in excess of 30cm snowfall over a period of days to achieve 16cm. To have 8cm snow the other day, I had recieved in total...8cm snow! A bit of a difference between mid December and late February for accumulating and keeping snow.

I know I keep banging on about it, and the learned ones amongst you talk about a combination of factors, but ....SUNSHINE. Sun has next to no effect on snowcover in mid December, this truly is the absolute optimal time for snowfall "stickability". Get to about the 3rd week of January and the sun becomes noticeably stronger.

People, enjoy this spell while you can, its not going to be very often when you see such little snow last so long.

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Feb 2005 is often quoted as an example of snow failing to settle to any meaningful depth because of warm ground temperatures, but on that occasion, the air temperatures and dewpoints were both very marginal, featuring maxima of 4 or 5C for most and minima and dewpoints close to 0C. In some inland parts of SE Scotland and NE England, where temperatures were a couple of degrees lower, snow accumulated to aggregate depths in excess of 10cm.

If ground temperatures had been lower during the February 2005 easterly, the snow would have settled more readily but it would still have thawed in the sunshine in between the snow showers except for the areas that saw maxima more like 2 or 3C.

I mentioned in my earlier post that March 12th 2006 was, in eastern areas, a classic example of snow struggling to settle due to high ground temperatures despite temperatures and dewpoints both below freezing. I got 5cm in Leeds out of that, but only after the snow had become heavy. In contrast, the snow was heavier in the west and that's why it settled readily there.

Come to think of it the snow in early March 1995 did come after a month with virtually no frost as well, but that had no trouble settling. As for 12th March 2006, I remember from reports/pics that it settled right on the Lancashire coast (Liverpool, Blackpool) but not in Manchester (remember a Sky TV report from Old Trafford that morning, it was chucking it down with snow but not sticking at all)- that it settled further east in Leeds makes that even stranger.

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People, enjoy this spell while you can, its not going to be very often when you see such little snow last so long.

Good Point, under 1 inch of snow has lasted 5 days, and is only now thawing due to heavy rain/sleet today.

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Sunshine certainly becomes a more important factor regarding snow retention as we head outwards from the coldest time of year. In December or January, it's a relatively minor factor, but in February and more especially March it becomes a major factor.

The main issue with a strong sun is the larger increase in air temperature that it brings. The widespread maxima of 5C, in sunshine, during the February 2005 spell were a case in point. Once a hole has been punched in the snow cover, exposing bare ground, the sun can then heat up this bare ground and accelerate the thaw in surrounding areas. Regarding the aforementioned example of March 1995, in Tyneside that was a snowy month, especially near the beginning and end, but while the snow often settled readily during snow showers, it invariably thawed during the subsequent sunny intervals.

But even in March, a solid 8cm snow cover will usually survive longer under unbroken sunshine, highs of 4C and sharp night frosts than under cloud, highs of 2C and lows of 0C- in the latter case higher dewpoints and higher ground temperatures come into play.

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Mate if 1 inch of snow had fallen in early March, it'd be gone by 11am the same day even at 2ºc...because the sun shining on it would be 25ºc. Like you said 1 inch in december in cold temperatures = 5 days.

There is hardly any difference between the shade temperature and sun temperature in December, by February the difference is massive.

Snow doesn't know that your shaded thermometer says freezing, snow only knows the temperature it is exposed to.

I personally feel that ground temperatures have little bearing on snow laying unless it is only light wet snowfall.

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The "the sun shining on it would be 25C" isn't entirely accurate, because as long as there is a continuous snow cover, the snow cover will help to reflect heat from solar radiation. The problem arises when the snow thaws enough to allow some ground to show through- then the strong sun really does heat it up considerably and accelerate thawing.

I've seen 1 inch coverings in early March last in sunshine and 2C up to 11am (Leeds on 4 March 2006 for example)- but by 5pm there would usually be just a smattering left because of the above process.

I think ground temperatures can be an important factor even with light powdery snow, but certainly if it's heavy enough at a low enough temperature, it will settle no matter how warm (and even wet) the ground is.

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^^ I agree yes :).

I've not had much snow, around an inch but it's stuck around longer than 6/7inches of snow we sometimes get in Feb/March.

As soon as the Sun gets on it, it melts very quickly.

This time of year the daylight hours are limited and there's not much thawing occuring even when there has been some sunshine.

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A couple of years ago we had two inches of snow in the evening. Temperatures below zero all night I went to bed expecting a good photo opertunity. Next morning hardly any left.

This year we had a light dusting at the start of this cold spell and it stuck.

Go and figure. Pollution in the snow ????

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But even in March, a solid 8cm snow cover will usually survive longer under unbroken sunshine, highs of 4C and sharp night frosts than under cloud, highs of 2C and lows of 0C- in the latter case higher dewpoints and higher ground temperatures come into play.

Indeed in pages 51 to 52 of Philip Eden's book a "Change in The Weather" he said of the March 1970 cold wave in Britain that the snowman he made called Snowman Ted persisted till the 22nd March 1970 despite unbroken sunshine.

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In response for Jackone's request for snow day information:

Cupar, Fife

Since 17/12/09

11 consecutive days of snow lying on the ground

10 days of snow falling

5 days where more than 2 cm of snow fell

Total snow depth is hard to measure since there is a layer of ice under almost everything, in some places the ice is 5 cm thick with around 15-20 cm of level snow on top.

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As per JackOne's request

9 days snow (more than 30 minutes

11 Days Snow on Ground

Lowest Temp -12

Snow depth currently 10"

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