# Why does barometric pressure differ from measured pressure?

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1. Nov 26, 2015

### mfig

I live in Denver CO, and I have come across a mystery having to do with barometric pressure. If you go look at any weather site, like:

http://www.wunderground.com/cgi-bin/findweather/getForecast?query=denver+co

You will see the local pressure (at the time of this writing) is listed as 30.33 inches of Hg, which is 14.89 psi. However, I have a brand new certified absolute pressure transducer that shows the local pressure is 11.85 psi. Also, if you look at how pressure is supposed to vary with altitude, such as at this site:

http://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html

You can verify that the predicted pressure at Denver (5300 ft elevation) is around 11.9 psi. This is much closer to the pressure I measured with the pressure transducer. The difference is small and can probably be explained away due to local variations, temperature and humidity effects, etc.

But if you think this is a single anomaly, think again. I first noticed this in July when the weather was hot. I have observed this over and over the past few months in different conditions. The weather pages consistently give values around 29-31 inHg, which is far higher than I have ever measured with my PT.

So my question is, why the difference?

2. Nov 26, 2015

### Staff: Mentor

The only explanation that comes to my mind is that those general weather sides may calculate their numbers by large scale weather pattern and don't take the special height situation in Denver into account. Would be interesting to know whether you can find similar discrepancies for let's say Santa Fe or Salt Lake.

3. Nov 26, 2015

### rootone

For purposes of drawing weather map charts local elevation is ignored.
The pressure shown is the equivalent sea level pressure.
The reason for that is because we can then draw isobars on the map which indicate weather systems.
If actual pressure at elevation were used, mountainous areas would appear to have a permanent low pressure systems with the associated poor weather conditions.
If actual pressure were used instead of sea level equivalent then a major storm at sea level would look like a trivial weather feature when compared to a mountain range.

4. Nov 26, 2015

I believe for the purposes of weather reports they usually scale the values to their sea level "equivalents" so the same barometric pressure means high or low independent of altitude. A quick Google search ought to clear that up.

5. Nov 26, 2015

### Staff: Mentor

This makes sense. However, if you display local weather conditions as on the side spamanon has mentioned, in this case for Denver including the height given, it's a poor service. And they didn't even explained their data - at least at first sight. Just the number and unit. Funny is that I get the pressure in hPa, snow in cm and temperature in °C. If they can transform the units it would be no big deal to adjust the pressure to the heigth (also given in m!).

6. Nov 26, 2015

### mfig

Thanks, everyone.