# Difference between Rho, Theta and Pressure Levels

**URL:** <https://cms-helpdesk.ncas.ac.uk/t/difference-between-rho-theta-and-pressure-levels/1376>\
**Category:** General\
**Created:** [22 March 2024 10:01 UTC](https://cms-helpdesk.ncas.ac.uk/t/difference-between-rho-theta-and-pressure-levels/1376 "2024-03-22T10:01:16Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![ajw1g19](https://avatars.discourse-cdn.com/v4/letter/a/35a633/32.png) [@ajw1g19](https://cms-helpdesk.ncas.ac.uk/u/ajw1g19)\
**Post date:** [22 March 2024 10:01 UTC](https://cms-helpdesk.ncas.ac.uk/t/difference-between-rho-theta-and-pressure-levels/1376/1 "2024-03-22T10:01:16Z")

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Hi CMS,

Could somebody please explain the difference between rho, theta and pressure levels within UM diagnostics?

On the one hand, MetOffice documentation seems to suggest that rho = upper boundary of the gridcell and theta = lower boundary of the gridcell. On the other hand, the UM documentation and the vertlevs file ($UMDIR/vn13.2/ctldata/vert/vertlevs\_L85\_50t\_35s\_85km) suggest that rho = centre of gridcell and theta = bounds of the gridcell.

And I cannot find any definition of pressure levels.

Any advice would be much appreciated, thank you!

Alfred

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**Author:** ![RosalynHatcher](https://dub1.discourse-cdn.com/flex013/user_avatar/cms-helpdesk.ncas.ac.uk/rosalynhatcher/32/11_2.png) [@RosalynHatcher](https://cms-helpdesk.ncas.ac.uk/u/RosalynHatcher)\
**Post date:** [27 March 2024 12:12 UTC](https://cms-helpdesk.ncas.ac.uk/t/difference-between-rho-theta-and-pressure-levels/1376/2 "2024-03-27T12:12:46Z")

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Hi Alfred,

I’ve asked around the group and we think:

`"I believe Rho is the middle and Theta is the edges of the cell, but some variables are only on particular grids, i.e. density is on the rho grid and potential temperature on the theta grid. Pressure level fields are interpolated at runtime and need to have the relevant Heaviside function considered to properly process them."`

If you want further information or have further scientific questions that you can’t find answers to in the UM documentation, I suggest posting on the UM github discussions board: [MetOffice/simulation-systems Um · Discussions · GitHub](https://github.com/MetOffice/simulation-systems/discussions/categories/um)

Cheers,  
Ros.

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**Author:** ![ajw1g19](https://avatars.discourse-cdn.com/v4/letter/a/35a633/32.png) [@ajw1g19](https://cms-helpdesk.ncas.ac.uk/u/ajw1g19)\
**Post date:** [27 March 2024 12:28 UTC](https://cms-helpdesk.ncas.ac.uk/t/difference-between-rho-theta-and-pressure-levels/1376/3 "2024-03-27T12:28:30Z")

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Hi Ros and the rest of the group,

Thank you for the help!

Alf

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**Author:** ![system](https://europe1.discourse-cdn.com/flex013/uploads/cms_support/original/1X/1fd2411499ffcbc299fe756cd5cdf26e44956558.png) [@system](https://cms-helpdesk.ncas.ac.uk/u/system)\
**Post date:** [26 April 2024 12:28 UTC](https://cms-helpdesk.ncas.ac.uk/t/difference-between-rho-theta-and-pressure-levels/1376/4 "2024-04-26T12:28:45Z")

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