Why weather apps show absolute pressure?

[pines-demon]

When displaying atmospheric pressure, weather apps show absolute pressure. I understand that it is the definition, but usually the atmosphere is at 1 atm $\pm$ corrections of the order of 1% or 0.1%. Such changes are what matters. Would it not be better to use differential pressure put 1 atm at zero and show variations around that?

Here is an example for Paris:

Clearly the the scale is trimmed to avoid this issue but it shows that we do not need such a ridiculous scale on the left.

 

[.Scott]

On the other hand, sometimes a precise absolute pressure reading is required.
Airports will report an adjusted (sea-level) altimeter setting - in the US, it is reported in inches. This is the altimeter setting that pilots in the US and Canada will use when flying below 18,000 feet (other countries have other "transition altitudes"). Above that, the "International Standard Atmosphere" altimeter setting (29.92 inches) is used and altitude is reported as a "Flight Level".
When you set your altimeter to that local adjusted (sea-level) pressure reported by the airport, your altimeter will report correct MSL altitude readings. This is particularly important when approaching an airport for landing and in instrument conditions (fog/clouds) when flying below the transition altitude.

On another point, even the difference between the absolute barometric pressure and the average pressure (for example, 29.92) is not the best indicator of an approaching storm. Rather, it is the how the pressure is changing. I have an old mechanical barometer hanging on my wall. And, although it is mostly an ornament, I do occasionally use it for a quick prediction. When I do that, I first tap the barometer to see if it changes. If the needle jumps up, the pressure is increasing. If it jumps down, that's an indication on impending inclement weather.

 

[Frabjous]

One atm is average pressure at sea level at 15 degrees Celsius so things are not going to fluctuating around zero at many places on the Earth.
https://en.wikipedia.org/wiki/Standard_temperature_and_pressure#International_Standard_Atmosphere

 

[Baluncore]

With a relative air pressure system, reported pressure would go negative in the eye of a cyclone, when flying, or when climbing in the mountains.

The formation of cloud is dependent on absolute pressure, temperature, and RH.

 

[pines-demon]

On the other hand, sometimes a precise absolute pressure reading is required.
Airports will report an adjusted (sea-level) altimeter setting - in the US, it is reported in inches. This is the altimeter setting that pilots in the US and Canada will use when flying below 18,000 feet (other countries have other "transition altitudes"). Above that, the "International Standard Atmosphere" altimeter setting (29.92 inches) is used and altitude is reported as a "Flight Level".
When you set your altimeter to that local adjusted (sea-level) pressure reported by the airport, your altimeter will report correct MSL altitude readings. This is particularly important when approaching an airport for landing and in instrument conditions (fog/clouds) when flying below the transition altitude.

I understand its use in technical occassions but I am talking about apps for everyday life.

On another point, even the difference between the absolute barometric pressure and the average pressure (for example, 29.92) is not the best indicator of an approaching storm. Rather, it is the how the pressure is changing. I have an old mechanical barometer hanging on my wall. And, although it is mostly an ornament, I do occasionally use it for a quick prediction. When I do that, I first tap the barometer to see if it changes. If the needle jumps up, the pressure is increasing. If it jumps down, that's an indication on impending inclement weather.

But that's my issue here, you need to see the change, but in my phone app I can barely see it dropping because a bad choice of scale.

 

[pines-demon]

With a relative air pressure system, reported pressure would go negative in the eye of a cyclone, when flying, or when climbing in the mountains.

A negative value is not a problem, we have that with temperatures already.

 

[Filip Larsen]

Clearly the the scale is trimmed to avoid this issue but it shows that we do not need such a ridiculous scale on the left.

This graph seems to provide what you ask for, namely a plot of the variation over time where people uninterested in the absolute pressure values just look at the curve to get a sense of how pressure varies over time and people who somehow benefit from knowing the corresponding absolute pressure can then read specific values off the scale.

Are you complaining that the graph include the extra numbers on the left? I mean, if the plot was made insisting that the scale should include zero pressure, then I would agree it would be ridiculous.

 

[Baluncore]

A negative value is not a problem, we have that with temperatures already.

That is another problem. Monotonic scales are easier than zero crossings. What would you recommend for the relative zero pressure ?

Zero absolute pressure is never shown on weather maps, only isobars that exist on the weather map are shown. The weather map pattern is the same, independent of the magnitude attached to the lines.

 

[pines-demon]

That is another problem. Monotonic scales are easier than zero crossings. What would you recommend for the relative zero pressure ?

Zero absolute pressure is never shown on weather maps, only isobars that exist on the weather map are shown. The weather map pattern is the same, independent of the magnitude attached to the lines.

I was thinking 0 should be at 1 atm or at least some average atmospheric pressure.

 

[Frabjous]

the atmosphere is at 1 atm ± corrections of the order of 1% or 0.1%.

I was thinking 0 should be at 1 atm or at least some average atmospheric pressure.

The average atm pressure of Denver is .83 atm, so here is the plot for Denver

Those poor airless bastards

 

[Baluncore]

I was thinking 0 should be at 1 atm or at least some average atmospheric pressure.

We are not arguing about the plotted line or the isobars here, they will remain the same, only the numeric labels on the vertical axis or the isobars will change, leading to confusion over the sign, and which way is up.

So what do you have against simply writing the absolute pressure? Why must it be offset?

If it was offset, I would argue that it should be labelled with absolute pressure, so we would not need to record the critical offset that was used for each plot.

 

[.Scott]

But that's my issue here, you need to see the change, but in my phone app I can barely see it dropping because a bad choice of scale.

It sounds as though you want a dP/dt plot. Something showing the current mm/hour rate of barometric pressure change.

 

[pines-demon]

The average atm pressure of Denver is .83 atm, so here is the plot for Denver

View attachment 361667

Those poor airless bastards

The thing is that this is what my app looks like:

The drop indicated a sudden thunderstorm, barely noticeable with this scale.

 

[pines-demon]

It sounds as though you want a dP/dt plot. Something showing the current mm/hour rate of barometric pressure change.

That would be great, do you know of websites that do that?

 

[russ_watters]

The thing is that this is what my app looks like:
View attachment 361703
The drop indicated a sudden thunderstorm, barely noticeable that scale.

I'll give you that's a terrible scale (sometimes 1kpa, sometimes 2, all the same spacing?) but creating a brand new way of expressing pressure (and having to teach it to people) just as a workaround for one bad app seems like overkill to me. Better to just find/write a better app.

I have a couple of apps that I use and both say the trend in words but neither show it graphically. And I feel like pressure has somewhat been superseded by radar for short term forecasting.

 

[renormalize]

...creating a brand new way of expressing pressure (and having to teach it to people) just as a workaround for one bad app seems like overkill to me.

That's some bad app, Harry. (50th anniversary joke)

 

[Frabjous]

The thing is that this is what my app looks like:
View attachment 361703
The drop indicated a sudden thunderstorm, barely noticeable with this scale.

Since I previously brought up Denver, here’s a page with it’s pressure.
https://barometricpressure.app/denver
I believe that you have an app problem, not a units problem.

 

[Baluncore]

If you want a long-range forecast, you can get it from a long way away. Scroll down for pressure.
https://www.yr.no/en/details/graph/2-5419384/United States/Colorado/Denver/Denver

 

[pines-demon]

If you want a long-range forecast, you can get it from a long way away. Scroll down for pressure.
https://www.yr.no/en/details/graph/2-5419384/United States/Colorado/Denver/Denver

Those pressure graphs are also pretty bad due to the scale.