Avain

Very low temps here last night, -9.9! Where I live is a bit of a frost hollow and I have two thermometers, one outside which is a Glass an Alcohol type bolted to a north facing shed wall about 2 feet up from GL, the other an internal digital with an outside sensor at about 3 meters up and both are giving the same reading right now. With the 850 temps at -8 I am a bit surprised as I would have thought it would not go much lower, how much below the 850's is possible then?

Hi John, well, as I said, I had read your explanation on the page referred to by BF, but it still left the questions made in my second post regarding the hieghts and temps and the apparent reversal of logic, i.e. this bit: I am assuming that the colour coded numbers refer to the height of the 500mb line, and are abbreviated meters (i.e. 534 = 5340 meters) this being the case I note that all the action on the colour section of these charts takes place in side 4000ft of upper air from 15700 ft to 19700ft (approx) But what is puzzling is the way the colours seem to be the reverse of what \i would expect, in the instance I have seen, the lower temp are associated with the 500mb line being at a lower altitude and vis versa, also they seem to be more associated with low pressure systems, which by there nature are warm wet air rising, with High pressure being cold descending air columns, on top of that I would expect an increase in alt to give a lower temp reading, but the opposite seems the case with this, as the lowest alt of 480 is the coldest temp and 600 the warmest. Obviously I don't have an accurate grasp of what the chart is intended to convey, so would still like further clarification if possible please. So yes, I am still wondering about that aspect.

Hi BF, indeed yes, thanks for posting it. It is though the very page linked to in my second post, you must have missed it, but I do appreciate you taking time to help.

I should have titled this "trying to understand" Anyway, I also read another thread in which this question was addressed by John Holmes Here It has helped some but also raised further questions. I am assuming that the colour coded numbers refer to the height of the 500mb line, and are abbreviated meters (i.e. 534 = 5340 meters) this being the case I note that all the action on the colour section of these charts takes place in side 4000ft of upper air from 15700 ft to 19700ft (approx) But what is puzzling is the way the colours seem to be the reverse of what \i would expect, in the instance I have seen, the lower temp are associated with the 500mb line being at a lower altitude and vis versa, also they seem to be more associated with low pressure systems, which by there nature are warm wet air rising, with High pressure being cold descending air columns, on top of that I would expect an increase in alt to give a lower temp reading, but the opposite seems the case with this, as the lowest alt of 480 is the coldest temp and 600 the warmest. Obviously I don't have an accurate grasp of what the chart is intended to convey, so would still like further clarification if possible please.

I see these charts used frequently in the model discussion thread and although I have read the thread in this forum about the 500hpa charts I still was not able to grasp their significance, particularly in relation to the colour codes and what they mean, also in relation to the numbers on the key. Also the value of the charts, what is is you can learn from them? So if anyone has time to give a more detailed explanation I would be grateful. Thanks in advance!

About 3.5cm of snow here now and the current stream is about to pass by my location, the temp rise was significant too, from -3 before the showers to +0.1 now, snow is biggish flakes of wet snow but it's all settling.

Another streamer coming in above Aberporth and the temps on the radar are showing -9 for Brecon and -11 for Dublin already! Here it is -2.4 and the recent snow has frozen to the road.

The 162hr GFS 12z shows the 510 dam over Kent! Albeit a small pool and very F1 but striking nonetheless.

Heavy snow falling, OAT 0.6 degrees! Everything now becoming covered. And I thought this might be a cold dry week! You never can tell what you're going to get with these conditions.

Snow falling from Canaston bridge to Haverfordwest now, big flakes from the first clouds,a heavier looking set of ppn to the north on NW Radar.

The current view to the NNW of Mid Pembs

Woke up to a low of -6.4, outside buckets of water frozen solid! With the charts showing it to get much colder still I am going to up the concentration of antifreeze in the car!

In the 81/82 (I think) snowfall, we had rain at first that was supercooled and turned instantly to ice on everything it touched, blades of grass, twigs, the entire side of my house even had a good inch of solid ice on it in a single sheet, then it snowed and built up huge drifts, in one part a drift was topped out above a telegraph pole, the night of the snowfall though was the most spectacular with a moon close to full I walked across a snow filled lane with an easterly wind whistling across the fields to go to the pub, more for the walk than a drink, and in doing so passed a row of beech trees that were covered in ice from trunk to twig, the whole lot sparkling in the moonlight, it was a sight I'll never forget, I wonder if it could happen again?

I cant get an image from the Radar, the rest of the page is OK, i.e. the sliders and various controls, but no image. Is this the server or something else?

http://earthobservatory.nasa.gov/Newsroom/...E-27426_lrg.jpg The story: http://earthobservatory.nasa.gov/Newsroom/...ges/images.php3

Well according to Metchecks forecast for Milford Haven ( http://www.metcheck.com/V40/UK/FREE/7days.asp ) we are going to be at one with the universe come Sunday night with a predicted windchill of -209 !!! Now, where's that Damart catalogue!! No doubt it will be corrected by the time anyone clicks it so here is a screenshot.

Thanks FFO, that does help a good deal, I had to read it several times to picture the actual processes, and admit that although I understand it a good deal better, I could not explain it (I think being able to explain something is the true test of understanding) So, despite two excellent presentations, anyone with a slightly different angle is equally welcome to add their own explanation to build as full a picture as possible (Graphics welcome!!) Cheers!

Thanks for that input Steve, and I am sure it is smack on, but still, the question remains, why? Why can the air from one airmass not mix with the other? For sure the answer lies with the physical nature of the airmasses, but I can't picture just what it is that forms the "membrane" so to speak.

In several posts here in the learning centre and on the forum in general I have read a description of an event, the event in question being that a high blocks a low, in other words, an easterly moving low pressure system will tend to drift north or south of a high, skirting round it's boundary, so the high effectively seems to "Block" the easterly progress of the Low from it's original course at any rate. But is this what is actually happening? Why does the low do this, the high consisting as it does of a mass of cold descending air, albeit less cold probably on the borders than at centre, it seems that the warm aired Low (at the frontal zone at least) air should ride over the cooler air, or mix in with it and disperse if the gradient is close, how is it that the low maintains it's structure and exists almost as an entity in it's own right rather than an amophous mass of different temp air? I am learning such a stack in this learning centre, thanks to all who have contributed the variuos articles etc!

This is a good one too. http://www.wxmaps.org/pix/hemi.jet.html

Cheers Paul, works fine now! (Learning centre links etc)

I am having trouble accessing any links to images and files in the learning centre, so far I have been trying to follow John Holmes variuos instructional posts but each time I click a link it says I do not have permission to view the file and need to log in, needless to say, I am logged in! So, have I missed something or is there broken links, or some other problem? Cheers!

Thunderstorms to the North East and West (esp West) of Pembs active now, heavy rain.