Thickness Charts

[mike Meehan]

A long time ago I was taught that the 528 metre dam on the 500 mb thickness chart was the "snow line" i.e. below 528 the liklehood was for snow and above more likely for rain and that all this was due to colder air being more dense than warmer air.

On looking at the thickness charts on your site, it occurs to me that the sea level pressure could have a lot to do with this as well, for instance in an area of high pressure the 528 mb dam is likely to be higher because it is starting from a higher level of sea level pressure and vice versa for the low pressure.

This being the case it would appear that the 528 mb dam is not quite the dividing line I thought it was and leads me to think that for a more definitive and accurate analysis the sea level pressure should be brought into account.

If this were to be the case is there a formula to be applied involving sea level pressure and the height of the 500 mb dam which would give a more accurate picture?

[johnholmes]

here is Mike I'm pushed for time and am away until Tue evening but will come back to you if you have not found it or someone else has not given it

briefly though 8mb is taken as being equivalent to 6dm

[shuggee]

A long time ago I was taught that the 528 metre dam on the 500 mb thickness chart was the "snow line" i.e. below 528 the liklehood was for snow and above more likely for rain and that all this was due to colder air being more dense than warmer air.

Mike it's not the 528 dam line on the 500Hpas that indicates the possibility of snow - that is the wrong chart. You need to look at the 500-1000 HGT charts for the correct dam line that's fits your understanding

[Guest]

Mike it's not the 528 dam line on the 500Hpas that indicates the possibility of snow - that is the wrong chart. You need to look at the 500-1000 HGT charts for the correct dam line that's fits your understanding

Thanks Shuggee and where do I find the 500-1000 HGT charts?

[Paul]

Hi Mike, you can find them here:

http://www.netweather.tv/index.cgi?action=nwdc;sess=

4th one down on the list - hgt500-1000

[Guest]

Paul, Shuggee,

Thanks I have found it but what what is the difference, apart from sea level pressures etc between this and the SLP 500 mb chart and how can I interpret them - I've got a basic idea but need more explanation in the way of specifics.

[johnholmes]

Mike I will try and find time to try ad explain it to you-just keep reminding me please?

in f this link has oodles about it Mike

well worth a read- I suspect it was written initially by Martin Rowley and ex colleague-if it is then it will be very good.

hope it helps anyway

I might end up putting it in our Guides area

http://weatherfaqs.org.uk/node/152

this is the bit that usually causes head scratching for those new to the idea

An example of the former would be

500 hPa height = 5407 m

1000 hPa height = 23 m

Thickness = 5407-23 = 5384 m (or 538 dam)

Careful note must be made when the height of the 1000 hPa surface is below msl thus: 500 hPa height = 5524 m

100An example of the former would be

500 hPa height = 5407 m

1000 hPa height = 23 m

Thickness = 5407-23 = 5384 m (or 538 dam)

Careful note must be made when the height of the 1000 hPa surface is below msl thus: 500 hPa height = 5524 m

1000 hPa height = - 13 m

Thickness = 5524 -(-13) = 5537 m (or 554 dam)

0 hPa height = - 13 m

Thickness = 5524 -(-13) = 5537 m (or 554 dam)

Edited December 15, 2009 by johnholmes

[Guest]

Thanks John,

I appreciate you are busy in what is very difficult set up at at the moment.

I'll just say that I have now looked at both, plus the Méteo Ciel and they all seem to be indicating to me the potential for snow until just after Christmas, though there is a lot which can change in the meantime, depending on how firmly established the cold air gets.

In the meantime I will try to excercise patience.

Edited December 15, 2009 by mike Meehan

[johnholmes]

okay Mike

I hope that link helped get your head round what thickness is

its similar for the 1000-850mb there the 'magic' figure is at or below 1290DM.

enjoy the cold, frost and snow while its here-what is round the corner is hard to be sure about-the models have certainly shown less wish to end it quickly than they did at first.

before computers and I suspect its much the same now it was always difficult to work out when the cold block would give way and allow the Atlantic back in-now of course, rather than wondering about tomorrow, we look at T+144 or even further and wonder!

[Guest]

Interesting - when I was a humble scientific assistant at Heathrow during the winter of 1963 some of the forecasters kept trying to pull in warm fronts to end it but having just plotted a chart with lots of low minus figures I was always a bit sceptical. I always understood that it is very difficult to move cold air once it was firmly established. Unfortunately although eastern Europe and Scandinavia are quite cold at the moment I think it needs a little longer to get really well established, so we cannot rely on this cold snap staying - yet!

Incidentally do you still use tephigrams?

I'm not sure whether my memory is playing tricks but I seem to remember one observation from New Orleans coming in with a temperature of -5C and freezing rain. I suppose it is possible with the wind in the right direction.