Drifts 07: theoretical influences of air pressure

[DavorJ]

Modeling air-pressure is mainly done using the ideal gas law.

The density depends on 1) humidity, 2) altitude and 3) flow. The temperature depends on altitude, cf. laps rate. Ignoring humidity, assuming the air static (i.e. ignoring the "flow"-part of the Bernoulli equation) and assuming a linear laps rate up to 11km, one ends up with the barometric formula:

Altitude

Barometric formula can be used to determine the effect of altitude on air pressure.

Using this formula it can be easily computed that the expected drop in air pressure per 100 m increase from sea level is 12 cmH2O.

Temperature

The effect of drops in temperature in function of height is taken into consideration in the above-mentioned barometric formula. Here I consider only the local temperature change (ignoring height). Subsequently, I am left with 2 unknowns (density and temperature), and one equation (ideal gas law) which thus can not be solved. Cf. my question here.

Preliminary conclusion is that local temperature should have no significant effect on air pressure from a theoretical perspective. (More research needed.)

Humidity

The effect of humidity on air pressure is described here. In BE/NL region we have the following seasonal humidity pattern:

To check...

Flow

To check...

[DavorJ]

The KNMI Handbook for the Meteorological Observation (2000) is referring to Guide to Instruments and Methods of Observation (WMO-No. 8), Volume I – Measurement of Meteorological Variables and these are some relevant excerpts:

Temperature

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Seems all barometer types need temperature compensation. But their compensations vary based on type. Just wondering whether these differences in compensation could explain the seasonality effects, cf. #51? Also, not all compensate automatically.

For aneroid barometers this suggests that the temperature effect is significant if not compensated:

Wind

KNMI suggests more than 1hPa effects.

Hysteresis

Quartz-based barometers do not have this effect.

Some other excerpts

[DavorJ]

The WMO formulae from Guide to Instruments and Methods of Observation (WMO-No. 8), Volume I – Measurement of Meteorological Variables for pressure reduction to MSL are based on Technical Note No. 61 which in turn refers to the hypsometric equation in Technical Note No. 7, pp. 20-21

See also the wiki.

The assumptions (i.e. ideal gas law and hydrostatic equation) and derivation are the same as for the barometric formula. The hypsometric equation above is extended with a vapor pressure component, but for us this is not important as stated in Technical Note No. 61:

Since this equation is very sensitive to temperature, the following is also stated:

Note that in the barometric formula the assumption is that MSL is at 1013.25 hPa and 15 °C, In Belgium we use TAW as the MSL. So although we all use the same pressure reduction techniques, the "0-calibration point" is not necessarily the same. That is why I believe (based on what I have read) the altitude calculated with the barometric formula (A-BF) is related to TAW in the following way:

A-BF = TAW + constant

Analysis 16 confirms this for as much as it is worth. This constant seems to be around -32.6m. The slope should be 1, but the estimate is 0.86. Note though that 1 is still within the 99% confidence interval, so we either have bad luck or temperature or some other entity also have a strong confounding effect.