Understanding 500 hpa charts

[stewfox]

I like to follow the model out put discussions but im having a problem understanding what the 500 hpa show

Clearly I can look at the colours and assume the blues and mauves are cold , green milder and yellows etc warmer

Looking at this site it would suggest these are temps at 18,000 ft ?

http://www.franksingleton.clara.net/500_hpa.html

However some further google searches and I cant find much about 500hpa

qu are there any good sites to read up on what the numbers mean on the right that relate to the colours ?

so for example

http://www.wetterzentrale.de/pics/Rtavn961.png

I assume 1005 is the pressure and -45 the temp ??

What does the 600-476 mean on the right?

I assume after fully understanding them you could equate -45c at 18,000ft given below average readings at ground level

Would average readings year round be Green ?

[johnholmes]

Ill come back with hopefully some guidance in a bit

[stewfox]

Ill come back with hopefully some guidance in a bit

Thanks i did see some breifs comments going back to 2003 on this site but didnt seem to explain all of it

[johnholmes]

The site you quote above is a good one, Frank Singleton was a sernior forecast in the UK Met.

this is another good site, again he was a senior forecaster in the Met O

http://booty.org.uk/booty.weather/metindex.htm

there is enough to keep you going for days not hours! Its all explained in simple non technical language in most cases.

to your questions

The solid white lines you see are the sea level pressure lines.

The colours you see are the temperatures at 500mb, around 18,000ft.

The values on the right with the colours refer to those temperatures, and the dashed/dot grey lines refer to those temperatures and are given in the white boxes. Thus the dark blue/purple is cold and the yellow to orange/red are warm.

These temperatures, like those at the surface will change through the seasons.

The green ones are about 'average' over much of the year IF there is such a term.

The black line you see with a number 552 by it is something called the ;'thickness' or temperature thickness between the surface(sea level) and 500mb(18,000ft). It is something forecasters use, perhaps not quite so much now with computers, but pre computers, it was a good guide to showing where surface highs and lows might form and their likely trajectory. Computers still have that basic data fed into them along with many other factors.

Hope that helps a bit and do ask again if you are still not sure about anything. If I cannot answer it there will be someone who can.

Also try the Met Office site, there are various links in there and explanations about most things to do with the weather and charts along with further links to help.

You may also find some help in the NW Guides section.

Edited March 13, 2008 by johnholmes

[Nick F]

Yes, the site John has given a link for is good for trying to explain the 500mb charts.

Basically, the colours you see on the WZ and N-W 500mb charts represent the geopotential height of the 500mb level, i.e. the lower the geopotential 500mb height - the colder the air is likely to be at that height and the greens and blues also indicate the presence of low pressure at that level or an uppper low. Geopotential height is measured in gpdm - short for geopotential decameters (528 gpdm = 5800m asl).

Geopotential height shouldn't be confused with 500-1000mb thicknesses (although they confusingly have similar figures e.g 528 DAM line you hear about is the 500-1000mb thickness not 500mb height!) - thicknesses are a measure of the temperature of the air between those two pressure levels, usually between 500-1000mb or 850-1000mb. Some examples of 500-1000mb thickness chart are below:

The black line you see with a number 552 by it is something called the ;'thickness' or temperature thickness between the surface(sea level) and 500mb(18,000ft). It is something forecasters use, perhaps not quite so much now with computers, but pre computers, it was a good guide to showing where surface highs and lows might form and their likely trajectory.

John, isn't the black 552 line geopotential height rather than the 500-1000mb thickness line?

The 500-1000mb thicknesses are shown on different charts on WZ, have posted the WZ slp/geopotential heights and slp/500-1000mb thickness charts side by side for t+96:

[johnholmes]

oh dear I was afraid my attempt at trying to keep things simple woud come unstuck!

I think we need definitions of the terms we have seen used already.

Give me time and I'll be back with my version of them.

[Nick F]

Sorry John! Perhaps I'm overcomplicating things. Maybe a suggestion that the looking at the slp/500 hPa charts - the colours basically represent the temp anyway at the 500mb level, despite the technical jargon I may have impose wrt heights. The yellows/oranges represent warmer temps at 500mb and greens/blues/purples cool-cold temps at 500mb. Low thickness numbers bascially the same =cold, high thickness numbers = warmer/milder.

[johnholmes]

this is an attempt at a simple but correct explanation of the two terms Nick has mentioned, geopotential and thickness and the one I mentioned thickness or as I loosely termed it ‘temperature thickness’

Geopotential heights measured in decametres is the correct way of quoting what we see on 500mb charts.

The 500mb charts are often called ‘contour’ charts

So what do we mean by contour?

it’s a line of constant height on an isobaric chart,

another term already isobaric, what is that?

=a surface of constant atmospheric pressure; iso means equal)

What is geopotential?

1 geopotential metre=0.98 dynamic metres

geopotential height is a better measure of height in the atmosphere as energy, is in general, lost or gained when air moves along a geometrically level surface. This is because geopotential depends on geometric height and gravity with mean sea level being selected as the zero potential height.

The unit of geopotential is the potential energy acquired by unit mass on being raised through unit distance in a field of gravitational force of unit strength.

(no I do not remember that without reference to one of my basic met books!)

I did say it is very complex which is why I tried to answer the question as simply if ever so slightly less than totally accurately.

To get back to an attempt to explain things as simply but as accurately as possible.

500mb contour charts are not the same as 500mb thickness charts, which are not the same as surface charts.

500mb and surface charts are contour charts, one at 500mb(about 18,000ft) and one at sea level(whatever the pressure may be) A thickness chart, in this case surface(1000mb-500mb) is constructed to show what the thickness, sometimes loosely called the temperature is between the two levels.

In days of yore forecasters use to draw the surface chart (1000mb) then the 500mb chart. We then ‘gridded’ the two together!

6dm=approx 8mb (if my memory is correct) and thus we were able to construct a third chart called the 1000-500mb ‘thickness’ chart. This was, and still is, useful in, as I posted, discovering where lows and highs are or may form and also how they may move. There is a fantastically complicated and very long equation beloved by all trained meteorologists called ‘the thickness change equation’ Before the advent of computers then all that forecasters had was this thickness change equation which helped to try and place lows and highs in the right place and with the correct depth. factors such as rate of change of temperature at the surface and through the 1000-500mb were possible to calculate taking into account sea temperatures or land temperatures at varying times of the day and seasons and much much more.

So to return to geopotential and thickness.

The charts you see from GFS etc are 500mb geopotential charts with heights (the 552) I quoted, whilst the temperatures are those at 500mb. Strictly not the thickness temperature as Nick correctly pointed out.

I hope that has not totally muddied the plot!

Sorry John! Perhaps I'm overcomplicating things. Maybe a suggestion that the looking at the slp/500 hPa charts - the colours basically represent the temp anyway at the 500mb level, despite the technical jargon I may have impose wrt heights. The yellows/oranges represent warmer temps at 500mb and greens/blues/purples cool-cold temps at 500mb. Low thickness numbers bascially the same =cold, high thickness numbers = warmer/milder.

too late you bugger(!) I've just posted again and not seen this until too late, never mind

Edited March 13, 2008 by johnholmes

[Nick F]

Good explanation there John of the differences between heights and thicknesses. Puts both our minds at rest. Is a confusing subject - took me a while to figure out when I first started model watching and read-up on all the meanings wrt heights and thicknesses for different levels of the troposphere.

[johnholmes]

cheers Nick

one day I'll do a screen print of that thickness change equation.

almost drove some of us to drink during the first forecast course of about 6 months!