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dylfish

Geopotential Heights

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Hi Guys, 

 

I was wondering if somebody could explain to me why in Geopotential heights.

 

Lower heights - Have low pressure or a cold atmosphere below.

High heights  - Have a high pressure below or a warmer atmosphere below.

 

I was under the impression a cold atmosphere would decease heights due to the higher density of cold air and place more pressure on the surface, while warm air would do the opposite. 

 

geopotential_height_1.gif

 

In the above image heights seems to follow the temperature, with the poles having lower heights that closer to the tropics.  That makes sense to me,but don't the poles have higher pressures compared to the equator? The fact that lower heights also represent low pressure seems to contradict this.

 

Is there any way to tell if the change in height is due to thermal or pressure changes?

 

Sorry for the incoherent rambling. This is just getting me really stuck as of late!

 

Thanks. 

 

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For what it’s worth I’ll have a bash at answering this.

 

The geopotential heights for all pressure levels are a function of temperature but they are also affected by the surface pressure.

 

In simplistic terms they are calculated thus. You take the temperature upper air profile for a particular location and measure the ‘height’ of each layer between pressure levels using the mean temperature of the layer. This is of course now done by computers using thermodynamic equations but in the old days it could be done quite easily using thermodynamic diagrams such as a tephigram (what you probably know as a skew-t).

 

For example if you want the height for the 500mb level you measure the height between 1000mb and 900mb using the mean temperature between that layer. Move on and measure the height for the next layer 900mb -850mb and so on for each subsequent layer until you reach 500mb. As the heights are added together as you go along your final height will be the 500mb geopotential height. This will vary according to the temperature between layers as the warmer the mean temperature the greater the height increments and vice versa for cold.

 

Well it would be except for one important omission. The height between the surface pressure and the 1000mb level has to calculated and included in the addition. For example if the surface pressure was 1030mb then the height between that and 1000mb, say 250m, has to be the starting point which will obviously raise the 500mb height by the same. Conversely if your surface pressure is 970mb your height will be negative -250m which will lower the 500mb height. This is one reason why the 500mb charts can indicate low or high pressure but they are still essentially functions of temperature.

 

One reason why thickness charts are often more useful than contour charts because they just use the temperature profile and exclude the initial surface to 1000mb calculation.

 

I hope this ramble clarifies it a little and don't forget cold and warm is relative.

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Thanks Knocker!

 

That seriously answers a few questions that were lingering. Just to follow up and make sure I was on the right track I'll try re explain it?

 

oBhy55A.png

 

What I gathered (Numbers and letters correspond above)

1) The geopotential height is the height of all the pressure surfaces below it. (all figures are made up!)

2)

A) Colder air below will have a lower height than relatively warmer air since it is more dense / compact and takes up less space. 

B) Higher pressure will have a higher height since the 1000mb level that the first measurement is taken from is usually higher than the surface pressure and with low pressure the surface pressure will be lower than the 1000mb mark.

 

A few more questions

  1. What is the easiest way to determine if the heights are caused by thermal changes or pressure? Is it as easy as consulting a MLSP to check for pressure. 
  2. If there is warming mid level (eg. 500mb Inversion) will the pressure surface below it lower due to the air below being relativity colder with the pressure surface above (Say 400mb) rise due the warmer layer below?

 

Sorry if Q2 is a bit ambiguous. I'm just having problems trying to articulate my thoughts.  Oh and sorry if I take a while to answer. I'm in Australia =)

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Dylfish

 

  1. Yes that’s correct or put another way. The geopotential height of all pressure levels is their distance above the Earth's surface if it was a perfect and flat sphere.
  2.  

A)  More or less.

B)  Not quite. I think it got confused because you changed them around.

 

Higher pressure will have a higher height since the 1000mb level that the first measurement is taken from is usually lower than the surface pressure and with low pressure the surface pressure will be lower than the 1000mb mark.

 

A few more questions

 

  1. Use thickness charts as pressure isn’t involved.
  2. The pressure levels below will not be affected. A mid level inversion on it’s own will not make any significant difference to the subsequent contour levels (geopotential height levels) but if it marks a change in airmass properties such as above a subsidence inversion then the subsequent pressure levels will be higher and may well be noticeable on the higher contour charts.

 

Hope this helps but feel free to ask if it doesn’t and I’ll attempt to clarify if I can :)

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B)  Not quite. I think it got confused because you changed them around.

 

Higher pressure will have a higher height since the 1000mb level that the first measurement is taken from is usually lower than the surface pressure and with low pressure the surface pressure will be lower than the 1000mb mark.

 

 

Sorry mate, I'm sure it was due to my wording. I had just finished work so my brain was a little cooked. 

 

I'll do a little more reading for now and see what else pops into my head. Thanks for you help thus far =)

 

Cheers

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