Height/pressure conversion?

[Agile]

The standard pressure at sea level is 1013.25 mB.

A few questions!

How does that figure relate to the UK ODN reference for Newyln?

How does one calculate the pressure at different heights above ODN ?

Say I am at 100m ODM and the pressure today is exactly 1013.25 at sea level then what pressure will I be experiencing? Or whats the conversion ?

All these questions are in readyness for my compass/altimeter watch!

Tony

 

[empip]

The old Newlyn datum
Search Newlyn datum here

Given the atmosphere is fluid affected by weather, temp, density etc, you'll only get an average idea from a calculator...

Pick a few stations here and note the barometric differences... Some graph the changes ... Pressure varies like the weather
http://www.weatherstations.co.uk/aws_map.htm

Here is a calculation from Nat' Physics Lab:-

1013.25 - ( 1.224 x 9.81 x 100 / 100 ) = 1001.24mb for 100 m above your N datum having 1013.325 Millibars
http://www.npl.co.uk/server.php?show=ConWebDoc.1377

NASA atmos' modeller gives 100.099 kPa or 1000.99 Mb at 100 m but uses 101.292 Kpa or 1012.92 Mb as it's base at 0 m... the mean level of the sea at Newlyn in Cornwall may be lower than NASA's datum hence the slight difference...
http://www.grc.nasa.gov/WWW/K-12/airplane/atmosi.htm

Do your conversions if necessary here :-
http://www.cleavebooks.co.uk/scol/ccpress.htm

http://www.shodor.org/metweb/interact.html

Mind you this is mostly guess work on my part, but it is interesting stuff...

I guess you need a datum offset for the watch???

 

[Agile]

Thanks for that!

I have a "super weather station" which showed the absolute pressure today as 1043 but after selecting relative and calibrating it in accordance with the nearest weather station its now 1022. It seems rather out of calibration on the absolute but I dont know if there is any way to recall it.

When I get the watch I will have to see how its easyest/best to calibrate it when I leave home.

Tony

 

[megawatt]

As Empip has said it varies day by day based on environmental conditions.

For calibration of altimeters pilots are given both QFE during approach to give height above ground level (useful during an approach ) and QNH to give regional height above sea level as the heights of features on aeronautical charts are above sea level.

The QFE and QNH settings can vary quite a bit in certain conditions.

MW

 

[JohnD]

I set using the nearest Weather Stations (and check it is not changing fast as they are not up-to-the-minute) and adjust for height above sea level <5m for me

 

[empip]

The Use of Altimeters in Height Measurement
http://www.biber.fsnet.co.uk/altim.html
-

 

[Agile]

I set using the nearest Weather Stations (and check it is not changing fast as they are not up-to-the-minute) and adjust for height above sea level <5m for me

So your QFE and QHN will be virtually the same then!

Just make sure the sticky tape have been removed from the altimeter sensing ports after the plane has been washed down!

Tony

 

[JohnD]

no plane

I have a bicycle though