from the Environmental Sciences department...
Relation between Ambient Temp and Barometric Pressure
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Posted by Grishma on 23 August, 2012 - 7:31 am
Hi all,

I am looking after installation and commissioning of Meteorological Sensors. We commissioned Wind Speed, Direction, Relative Humidity, Barometric Pressure, Ambient Temperature and Pyranometer at a select location. The installation is nearing completion.

But I just happened to plot a graph of Ambient Temperature Vs. Barometric Pressure (Y Axis Pressure and X Axis Temp). In general, I observed that as the temperature rose, the barometric pressure either remained constant OR rose. The Regression Line goes linearly upwards from 25 Deg C to 29 Deg C.

My question is that isn't it contrary to the idea that as temp rises, the air gets hot and rises up and hence the pressure should fall.

I asked the guy doing the commissioning and he said that all equipments are directly imported from MetOne, US and they come with certificate of calibration, which he showed to me. Further, there's nothing the I&C guy can do at site.

Should I be worried about some calibration drifts? Or there can be other factors too, which can affect the Barometric Pressure apart from Ambient Temperature.

Some other information about the site:

The location of our site is at sea coast. The sensors are mounted on a small hill-top adjacent to the sea coast. The process plant is on the slope of the hill. The location of Meteorological sensors is the highest location.

Thanks in advance

1 out of 1 members thought this post was helpful...
Posted by Rob on 23 August, 2012 - 9:22 am
This isn't really an automation question - you need to grab some weather textbooks. Some of the easiest to read weather information will come from books aimed at pilots. 3668318 is quite readable

The relationship between temperature, pressure and volume is described by Boyles law. However in practical terms the temperature of the air on any given day is controlled by 2 things: 1. Where the air came from, and 2. How much energy has been added by the sun. The surface pressure of the air on any given day is governed by the movement of upper level winds known as jet streams.

Yes, as you heat the air in a small local area a parcel of air will begin to rise - but ONLY if that air is hotter than the surrounding air. Colder air will be pulled in from the surrounding area. There is only a small pressure difference in this case. The important point is that just because air is warm does NOT mean that it will rise - it has to be warmer (less dense) than the surrounding air.

If this happens over a large enough area that it can occasionally lead to the formation of a "heat low". ie a localised low pressure weather system.

Atmospherics is a complex subject - too complex for a small automation related post :) Yes, there are LOTS of influencing factors on the temperature of the air around you - not least of which is the location of the station near a very large body of water.


Posted by Grishma on 23 August, 2012 - 1:18 pm
Thanks Rob,

So its not JUST the Temp vs Barometric Pressure relation, as I understood in the most simplistic way.

Yes, it was a wrong question on the automation forum. But I thought that since it involves sensors and calibration issues, it would be somewhat close. :). I don't even know how to calibrate a Barometric Pressure sensor. Temperature sensor is fine.

So my initial worry was that, should the Barometric Pressure sensor need re-Calibration / Validation then what all things I'll have to be prepared with.

Anyways, thanks again.


Posted by Rob on 23 August, 2012 - 6:21 pm
> So its not JUST the Temp vs Barometric Pressure relation, as I understood in the most simplistic way.

Correct. The reason that the air temperature is usually higher when the pressure is higher is that high pressure days have less cloud and hence more sun. More sun means a hotter day. (But clear skies often also cause a colder night)

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