I'm confused by QNH compared to MSLP. Can anyone explain?

Hi,
I’m trying to check the calibration of my new HB barometer. I’m lucky enough to live about 10 kilometres from an Australian Bureau of Meteorology automatic weather station which updates its readings to the web every 30 minutes. They give two readings for barometric pressure one being QNH and the other MSLP. They usually are 2 or three hPa different. So I went to the BoM glossary and looked up their definitions and the following is what I found:-

“QNH is obtained by taking the station level pressure and reducing it to mean sea level by assuming the International Standard Atmosphere (ISA) conditions between the station and sea level.”

“MSL Stands for Mean Sea Level. It is necessary to convert the pressure readings to equivalent mean sea level pressures, otherwise the important horizontal changes in pressure would be overwhelmed by vertical variations simply due to differences in height between observing stations.
In this way, a Mean Sea Level Pressure (MSLP) map will then show pressures affected by changing weather conditions, not because of changing altitude.”

My question is which pressure should I be comparing my readings to? The definitions seem to be pretty similar to me but they obviously wouldn’t bother showing both if they were’nt different and as I said earlier the readings are usually 2 or 3 hPa different.

Can anyone enlighten me?? Which figure should I use and what’s the difference?
Regards to all,
w0mbat

In my opinion they are pretty much the same. I’m used to the QNH term (aviation environement), understanding it is the actual site pressure reduced to sea level.

There are two ways to reduce that pressure: by simple ISA pressure gradient (1 HPa per 9 meters, or 1 inch per 1000 feet), or applying more complex formula taking into account site temperature and humidity. Maybe this is the difference for BoM (just guessing).

In aviation QNH is not an absolutely calculated value because it varies slightly throughout the region. As you know, an aircraft’s altimeter uses the pressure to determine the altitude - the pilot sets in the QNH and the altimeter reads out the altitude. So here’s the trick: if it varies slightly throughout the region then that means the altimeter doesn’t always show the pilot the correct altitude. So QNH is adjusted so that the altimeter reads low when it’s not giving the correct altitude. That is done, by example, so that if there’s a mountain ahead at 5000ft high and your altimiter reads 5500ft then you know for sure that you have at least 500ft clearance because if anything the altimeter will be reading low. If that were not done then your plane might be reading 5500ft but you’re actually flying at 4900ft and might hit the mountain.

In the UK we use altitudes in ft and pressure in HPa and we adjust 30ft per 5 HPa. HPa and millbars work out the same.

So for example, Tollerton airport in Nottingham (EGBN) is 142ft above mean sea level so to adjust QFE (the barometric pressure that reads 0ft off the ground) to QNH we add 5 Hpa which is roughly 150ft.

Hope that helps.

Kind Regards
Steve

I agree with you (I know you know it all as a pilot :slight_smile: ), but QNH is a calculated value, valid only for the site it was calculated. To prevent ground collisions due to QNH changes throughout your flight path, you have to fly at least MSA, MEA, etc. flight levels, setting your altimeter with local QNH values or flying above transition level with a safe QNE altimeter setting.

For a static device such a weather station QNH value is calculated roughly by simple press gradient or applying more complex formulas as Davis does: http://www.davisnet.com/product_documents/weather/app_notes/apnote_28.pdf . Of course it is only valid for the weather station location.

This may explain even more :wink:

Go with QNH. Highs and Lows and lines of equal pressure etc. are all calculated based on the reported QNH of weather reporting stations. These are normally based at airports.

Rick

Do you mean that? Being near sea level, as a rule of thumb I estimate 1hPa/10m, so approx 1hPa/30ft.

You are correct… 1 mb = 30ft in normal parlance

Thanks for the input guys. I have to say I’m still unclear as to the significance of reporting both values at a fixed location such as an automatic weather station (which, by the way, is nowhere near an airport and the only flight activity we see around here is very distant vapour trails of high flying jets).
Perhaps I might try emailing the Bureau of Met. to see if they will explain.
Thank you all,
w0mbat

The high flyers you see are all flying in the standard pressure region so they all set their altimeters to 29.92. By the way I misspoke the other day, the maps are all drawn from the MSLP.

rick

I guess that’s an American fetish for palindromic numbers - here in the UK we use 1013.2 HPa :smiley:

Sorry about this little off-topic. Are you using metric units in the UK? This summer I want to spend 15-20 days flying France and the UK, so I started gathering all possible info.

We use a mix I’m afraid…

  • Visibility (horizontal) is measured in kilometers or metres depending on how poor it is
  • Altitude and Height are measured in feet
  • Barometric pressure is measured in millibars. In training we are taught that HPa and mb are equivalent and that we should use HPa but on the radio you will only hear millibars - the convention is that no units are mentioned on the radio if the pressure is 4 digits but if it is only 3 digits then the word “millibars” should be used to avoid confusion with other 3 digit items
  • Distance is measure in nautical miles (on the radio we says “miles” but we mean nautical miles)
  • Speed is measured in knots
  • Temperature is measured in Celcius

I think that’s the lot? :slight_smile:

If you want to ask any flying questions privately you can get me on steve at sandcmills dot co dot uk
Kind Regards
Steve

Ah, Ok. Pretty much the same here. Sure I’ll email you, thanks Steve :slight_smile:

Ok, I emailed the BoM. Here’s what I wrote and the reply I got:–

From: Ian Delaney [mailto:[email protected]]
Sent: Wednesday, 28 January 2009 14:10
To: Web Climate requests
Subject: Technical enquiry re BoM AWS barometric readings

Hi,
I am not sure I have directed this to the best address so if not would you be kind enough to forward my email to an appropriate person.
I am an amateur weather observer who runs my own weather station. I use the nearby Gelantipy AWS web page to check the calibration of my instruments, particularly my barometer.
On the site two pressure readings are given,QNH and MSLP which are about 3 hPa apart. I have checked the definitions in the BoM glossary and I’m afraid the difference is not clear to me.
Can you tell me where I will find an full explanation of the difference and/or why they are both given?
I would like to know which reading I should use to compare with my barometer.
Thanyou for your time,
Regards
Ian Delaney

From: John Cornall-Reilly On Behalf Of Climate VIC
Sent: Thursday, 29 January 2009 11:34
To: Web Climate requests
Subject: RE: Technical enquiry re BoM AWS barometric readings [SEC=UNCLASSIFIED]

Hello Ian
Our National Climate Centre has forwarded you inquiry to us in the Victorian Climate Services Centre. In future please use the following email address for requests of this nature : [email protected]
You really only have to concern yurself with the MSL readings.
The QNH is a reading used by pilots to set equipment on their aircraft.
The MSL pressure is the one we recommend people to set their home barometers to. Weather maps in the paper and on TV etc are prepared according to Mean Sea Level, so you can then check you barometer against those charts.
Definition from our glossary.
MSL MSL stands for Mean Sea Level. It is necessary to convert the pressure readings to equivalent mean sea level pressures, otherwise the important horizontal changes in pressure would be overwhelmed by vertical variations simply due to differences in height between observing stations.
In this way, a Mean Sea Level Pressure (MSLP) map will then show pressures affected by changing weather conditions, not because of changing altitude.
Just why both MSL and QNH pressures appear on the web page I am not sure but it is the result of the format of the incoming observation message from the station concerned.
Hope this helps.
Yours sincerely,
John Cornall-Reilly
Technical Officer 3
Victorian Climate Services Centre
Victoria Regional Office
Bureau of Meteorology, 6th Floor, 1010 Latrobe St, Docklands, 3008
Postal: GPO Box 1636, Melbourne, 3001
Phone: +61 3 9669 4943
Fax: +61 3 9669 4977

So it seems MSLP (as I originally believed) is the one to use. I would have liked a fuller explanation but at least I am confident of which figure to use now.
Thanks to you all for your input
w0mbat

Here is a guess and I’m not sure it is a good one. QNH is based on the elevation above sea level. For instance the runway is 100 feet ASL. In the tower we used to do a correction based on cistern elevation, the actual location of the barometer, for mean sea level. Most towers I’ve worked in are at least 50 feet above ground level. That could be your difference.

Rick