Hi all, I've been looking at this sort of chart in here for quite a while now.... I am no expert, so could someone give a quick but concise synopsis on what I am looking at weather wise at ground level? I.E what the colours, and numbers actually represent please? As I think I have been misinterpreting some of the data on this type of chart.........
Thanks
VSC
Originally Posted by: VIRTUAL STORMCHASER
That chart gives you two measures, the surface level pressure (SLP) and the 500hPa temperatures. 500hPa is approximately 18,000 feet up.
As you will know, I think, air expands as it warms, so warmer air equates to higher pressure. When you see orange colours on the 500hPa chart it tells you there is, in effect, an 'upper high'. On that chart at the surface (SLP) there is also a high pressure cell. When you read people commenting on 'false highs' over Greenland it is because the SLP indicates a high pressure cell but there is no support for it at 500hPa, with only cold air. For an effective block you want both an upper and a surface high - the upper one acts as the block, "the boulder in the stream" that blocks and diverts the jet and LP systems.
That chart doesn't tell you much about what's going on at the surface, other than some clues from wind direction. Ideally you want to look at the 850hPa chart for temperatures (that's about 5,000 feet) and thickness values. If you want to check whether there is a chance of snow you look at the 500-1,000hPa thickness and want values generally below 525dam. For better guidance look also at how cold the lowest layer of the atmosphere is, 850-1,000hPa (effectively 5,000 feet down to near the surface). In that lowest layer you need values below 130dam.
On that chart for 19th January:
500-1,000hPa thickness: http://modeles.meteociel.fr/modeles/reana/1963/archives-1963-1-19-0-2.png
850hPa temp: http://modeles.meteociel.fr/modeles/reana/1963/archives-1963-1-19-0-1.png
That's sub-zero at the surface but the relationship between the airmass above and surface temperature depends on lots of factors.
Hope this helps
Originally Posted by: colin46