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View Full Version : So how often does atmospheric pressure drop below 26 anyway?

YogSosoth
04-10-2012, 11:28 AM
This has bugged me for like a decade but I hardly ever see barometers measure pressure outside of the range of 27-30. I even looked up how low it dropped during Hurricane Katrina because I knew that low pressures come with hurricanes. The lowest it dropped was something like 26.50.

It seems to me that there's a whole huge part of the barometer just gathering dust. What kind of condition would it be if pressure was 0, for instance? Is that outer space or something? What about 15? Maybe the bottom of the ocean? I have no idea. So my question is, why bother having 0-25 on a standard barometer? It makes more sense to create a new scale from 25-30, and renumber it 0-100 or something like that. As it stands, 29.48 pressure vs 29.37 tells me absolutely nothing, and I feel nothing. But a difference of 18 (on my new scale) vs 84 would definitely make me aware of atmospheric pressure as a thing

And if someone can help me understand pressure, please tell me just exactly what kind of conditions will result in a pressure of 0, 10, and 20

kanicbird
04-10-2012, 11:45 AM
Higher altitude will give you lower numbers, as to how much I'm not sure but would suspect readings of 24 is not out of the question.

Zeldar
04-10-2012, 11:51 AM
What is the record low and high barometric pressure ever recorded ? (http://findanswers.noaa.gov/noaa.answers/consumer/kbdetail.asp?kbid=544)

The lowest sea-level pressure ever recorded 870 millibars (or 25.69 inches of mercury) in the storm Typhoon Tip. It was recorded 300 miles west of the island of Guam in the Pacific Ocean at latitude 16 deg 44 min north, longitude 137 deg 46 min east, on Oct. 12, 1979. A hurricane hunter plane flying through the Category 5 storm's eye of Wilma, on October 19th of 2005 found a minimum central pressure of 882 millibars, this would be an all-time record for an Atlantic storm surpassing the record set in 1988 by Gilbert which was 888 millibars (mb).

Anne Neville
04-10-2012, 11:54 AM
This has bugged me for like a decade but I hardly ever see barometers measure pressure outside of the range of 27-30. I even looked up how low it dropped during Hurricane Katrina because I knew that low pressures come with hurricanes. The lowest it dropped was something like 26.50.

It seems to me that there's a whole huge part of the barometer just gathering dust. What kind of condition would it be if pressure was 0, for instance? Is that outer space or something? What about 15? Maybe the bottom of the ocean? I have no idea. So my question is, why bother having 0-25 on a standard barometer? It makes more sense to create a new scale from 25-30, and renumber it 0-100 or something like that. As it stands, 29.48 pressure vs 29.37 tells me absolutely nothing, and I feel nothing. But a difference of 18 (on my new scale) vs 84 would definitely make me aware of atmospheric pressure as a thing

And if someone can help me understand pressure, please tell me just exactly what kind of conditions will result in a pressure of 0, 10, and 20

30 is approximately standard atmospheric pressure at sea level, measured in inches of mercury. (http://en.wikipedia.org/wiki/InHg) Blood pressure is measured in similar units, but those are millimeters of mercury, not inches.

0 inches of mercury would be space, with no air. The air pressure at the summit of Mt. Everest would be roughly 10 inches of mercury.

Enginerd
04-10-2012, 11:59 AM
One inch of mercury (the units you're talking about) is about 34 kPa. Normal atmospheric pressure at sea level is around 100 kilopascals, or 29.9 inches of mercury. A pressure of 26" Hg would be equivalent to 88.4 kPa, which is the normal air pressure at an elevation of ~3730 ft.

The air pressure is an expression of the weight of the atmosphere above you, so if the air pressure is reading zero, you're somewhere with no atmosphere, and you've got bigger problems than an inefficient barometer. An atmospheric pressure of 20 inches of mercury is equivalent to around 10500 feet, which (IIRC) is what airplanes are pressurized to. The air pressure at the top of Mt. Everest (29029 ft) is a little under 10 inches of mercury

Gray Ghost
04-10-2012, 12:03 PM
A inch of mercury is equal to 0.491 pounds per square inch, per Google (https://www.google.com/search?aq=1&oq=inches+of+mercury&sourceid=chrome&ie=UTF-8&q=inches+of+mercury+to+psi). 0 inches of mercury would be a vacuum. 10 would be 4.91 PSI, or the air pressure at around 27,500 feet of elevation (http://www.sensorsone.co.uk/altitude-pressure-units-conversion.html). 20 would be 9.82 PSI, or the air pressure at 10,500 feet, by the same chart.

The barometer, if anything doesn't properly measure high pressure, given the highest barometric pressure recorded is 32.06 inches. (http://www.ncdc.noaa.gov/oa/climate/extremes/2001/december/extremes1201.html) In very cold, very dry places like Verkojansk (http://www.wunderground.com/history/station/24266/2011/2/11/DailyHistory.html?req_city=NA&req_state=NA&req_statename=NA), Russia, or parts of Antarctica, it's not uncommon for the air pressure to remain over 30 inches for quite some time.

Edit: and partially ninja'd by Enginerd

Gary T
04-10-2012, 12:24 PM
So my question is, why bother having 0-25 on a standard barometer? It makes more sense to create a new scale from 25-30, and renumber it 0-100 or something like that.The thing is, those numbers refer to standard units (inches of mercury) so they do make sense. It's like having a yardstick to measure sticks when every stick you can get your hands on ranges from 25-30" long. What you're proposing is equivalent to erasing the marks on the yardstick, then putting a scale of 0-100 starting from what used to be 25" and ending at what used to 30" on it. I suspect you'll find precious few folks who see that as much of an improvement. You can get a reasonable sense of higher vs. lower pressure situations from just seeing where the needle sits on a standard weather barometer which just shows the range from 28-31 (http://image.shutterstock.com/display_pic_with_logo/82773/82773,1223093007,2/stock-photo-barometer-18426463.jpg).

I feel compelled to point out that your numerical examples are not equivalent. Note that your proposed 18 vs. 84 comparison corresponds to 28½ vs. 30½, which is very noticeable on either scale, while 29.48 vs. 29.37 corresponds to 46 vs. 49 which is quite small on either scale.

Anne Neville
04-10-2012, 12:50 PM
I wonder if you can buy a barometer that shows a different range if you live somewhere at high altitude, say, Colorado. From the table Gray Ghost posted, it looks like the average barometric pressure in Denver should be around 25.

Mr. Neville has been to the telescopes on Mauna Kea at about 13,600 feet. That should have a pressure around 18.3. Mr. Neville said it was hard to think straight at that altitude (of course, the fact that he was totally thrown off his normal schedule by going on an observing run probably didn't help with that).

The average pressure at the Dead Sea, the lowest elevation on Earth, is around 31.5 inches of mercury.

Colophon
04-10-2012, 02:04 PM
I wonder if you can buy a barometer that shows a different range if you live somewhere at high altitude, say, Colorado. From the table Gray Ghost posted, it looks like the average barometric pressure in Denver should be around 25.
You don't need to. A properly set up barometer is adjusted to show the sea-level pressure, wherever it is situated.

All weather reports and so on are based on sea-level pressure. (Unless you're flying, in which case you'll want to know the actual pressure at the air field.)

On days with very settled weather (typically a large and fairly stationary high-pressure system), TV weather forecasts sometimes mention that "it's a good day to set your barometer". If the sea-level pressure is, say, 1030mb (or 30.42 inches) over a wide area, you can adjust the dial on your barometer to that figure, regardless of how high you live. (IIRC the pressure falls by approximately 1mb for every 30ft of altitude, close to sea level.)

Chessic Sense
04-10-2012, 02:10 PM
In very cold, very dry places like Verkojansk (http://www.wunderground.com/history/station/24266/2011/2/11/DailyHistory.html?req_city=NA&req_state=NA&req_statename=NA), Russia, or parts of Antarctica, it's not uncommon for the air pressure to remain over 30 inches for quite some time.

Edit: and partially ninja'd by Enginerd

Huh? How's that work? Warm air has a higher pressure than cold air, on account of the kinetic energy. High pressure systems bring warm weather, and when the day heats up, my airplane's altimeter indicates lower alititude (i.e. reads higher pressure). A cold place would have very low pressure.

Chessic Sense
04-10-2012, 02:15 PM
All weather reports and so on are based on sea-level pressure. (Unless you're flying, in which case you'll want to know the actual pressure at the air field.)

Actually, aviation weather reports also give the pressure at sea level. Sure, they read it at the field, but as you said, the barometer then reads the sea level pressure, and that's the setting that goes into the METAR. If the field elevation is 240 ft, then the altimeter will read 240 when you land, not 0.

Colophon
04-10-2012, 02:35 PM
Huh? How's that work? Warm air has a higher pressure than cold air, on account of the kinetic energy. High pressure systems bring warm weather, and when the day heats up, my airplane's altimeter indicates lower alititude (i.e. reads higher pressure). A cold place would have very low pressure.
Cold dense air sinking creates high pressure, such as the archetypical example, the Siberian High (http://en.wikipedia.org/wiki/Siberian_high). Here in the UK, while we don't usually feel the effects of the Siberian high, pressure tends to be higher in winter than in summer. We regularly get 1040mb+ highs in winter, which are usually associated with cold, clear, frosty weather. The cold Scandinavian High is also fairly common, which brings cold easterly winds to Britain.

Machine Elf
04-10-2012, 03:54 PM
I wonder if you can buy a barometer that shows a different range if you live somewhere at high altitude, say, Colorado. From the table Gray Ghost posted, it looks like the average barometric pressure in Denver should be around 25.

Here's where it starts to get weird. Barometric pressure reported by weather services is ordinarily compensated for altitude (http://en.wikipedia.org/wiki/Barometer#Altitude). So for example, whereas the actual air pressure in Denver, Colorado is somewhere around 24.7 inches of mercury, Weather Underground (http://classic.wunderground.com/cgi-bin/findweather/hdfForecast?query=denver%2C+co&searchType=WEATHER) is currently reporting it as 29.9 inches.

If you have a cheap barometer, it won't allow for altitude compensation (though it will show temporal trends, which is what matters for weather forecasting).

In any event, the range isn't that much lower; in Denver for example, the pressure will fluctuate around 24.7 inHg.

Nava
04-10-2012, 04:15 PM
Huh? How's that work? Warm air has a higher pressure than cold air, on account of the kinetic energy.

Uh, no. By that reasoning, the gas/plasma (I've heard it called both) inside a fluorescent tube would be at high pressures. Those are vacuum tubes... low pressure.

The pressure being measured is linked to density, not to kinetic energy. As Colophon explained, cold air is denser than hot air - this is why hot air rises, such as in balloons.

Kevbo
04-10-2012, 04:23 PM
All weather reports and so on are based on sea-level pressure. (Unless you're flying, in which case you'll want to know the actual pressure at the air field.)

This is incorrect. The barometric setting in a standard Kollsman altimeter is also a sea-level compensated barometric pressure value, not the ground level static pressure.

There IS a difference in that someone at the airport will make the measurement by adjusting an altimeter to read the field's known elevation, while the weather service will use a mercury manometer to directly measure the pressure and apply an altitude based correction. The altimeter applies a simple offset to the pressure, where the weather service uses a more sophisticated model, so you can end up with significantly different numbers especially at high altitude airports.

Chronos
04-10-2012, 04:24 PM
Quoth Chessic Sense:

Huh? How's that work? Warm air has a higher pressure than cold air, on account of the kinetic energy.Are you assuming a fixed number of gas molecules in a sealed container of constant volume? That's not a reasonable description of the atmosphere. Atmospheric pressure is literally the total weight of all of the air directly above a given area, so lower-density air will result in lower pressure.

Colophon
04-10-2012, 04:55 PM
This is incorrect. The barometric setting in a standard Kollsman altimeter is also a sea-level compensated barometric pressure value, not the ground level static pressure.
Yeah, I was getting confused with, um, QFE? Is that the code for the uncorrected pressure at the airfield? IANAPilot, and you should be thankful for that. ;)

DHMO
04-10-2012, 05:08 PM
A related bit of data:

It is the change in the reading which is used to forecast approaching weather, more than the absolute number on the device. A falling barometer foretells stormy weather, while a rising barometer indicates clearing, sunny days. Usually. YMMV.

Richard Pearse
04-11-2012, 07:13 AM
Yeah, I was getting confused with, um, QFE? Is that the code for the uncorrected pressure at the airfield? IANAPilot, and you should be thankful for that. ;)
Yes, setting QFE will make your altimeter read 0 at the airfield. This normally isn't done though.

Chessic Sense
04-11-2012, 08:29 AM
Are you assuming a fixed number of gas molecules in a sealed container of constant volume? That's not a reasonable description of the atmosphere. Atmospheric pressure is literally the total weight of all of the air directly above a given area, so lower-density air will result in lower pressure.

I asked about this in my piloting thread from last year. According to the manuals, pressure goes up on hot days. As for a sealed container, here's one response...

Another question: Weather - On a hot day, the pressure rises. This seems counter-intuitive to me, as I'd expect the air to expand in volume, not pressure. But when you've got gas in a container and heat it up, pressure (not volume) increases. Is the atmosphere really just one big container?

The lower atmosphere essentially is, yes - air pressure is just the total weight of all the air above you. If lower layers heat up but upper ones don't, the weight of the upper layers, resting on the lower ones, will contain them and cause their pressure to rise.

So apparently, it's a container.

Chessic Sense
04-11-2012, 08:45 AM
Uh, no. By that reasoning, the gas/plasma (I've heard it called both) inside a fluorescent tube would be at high pressures. Those are vacuum tubes... low pressure.

The pressure being measured is linked to density, not to kinetic energy. As Colophon explained, cold air is denser than hot air - this is why hot air rises, such as in balloons.

I know that it's denser, but I'm saying there's not more of it. You have to think of it as a constant volume of air, not a constant volume of space. PV=nRT, where V, n, and R are constant yields lower pressure at lower temperatures.

I'm not sure what that has to do with vacuum fluorescent tubes, though. That has to do with getting the mercury (or whatever) to evaporate in the tube.

ftg
04-11-2012, 08:50 AM
Why rescaling is a bad idea:

A change from 26 to 27 is a about a 4% increase. If you changed the scale so that 25 becomes 0, etc., then a change from 1 to 2 sounds like a 100% increase in pressure. Even worse, going from 0 (25) to 1 (26) is an infinite percent increase. This would confuse things.

It also would create problems for the ideal gas law: PV = nRT. So Scientists would stick with a suitable "0 = a vacuum" scale and regular folk would have a different scale. And there's already too much of that nonsense.

It's bad enough that you meet people who think 100 degrees is "twice as warm" as 50 degrees (C or F but not K). We don't need to add to the confusion.

Chronos
04-11-2012, 03:40 PM
It's bad enough that you meet people who think 100 degrees is "twice as warm" as 50 degrees (C or F but not K). We don't need to add to the confusion. Personally, in regards to weather, I think of "twice as hot/cold" as meaning "twice as far from optimal comfortable temperature". You wouldn't say "It was perfect out yesterday, and twice as cold today", but you might say "It was uncomfortably cold yesterday, and twice as cold today".

Enginerd
04-11-2012, 04:58 PM
Why rescaling is a bad idea:

[snip]

It also would create problems for the ideal gas law: PV = nRT. So Scientists would stick with a suitable "0 = a vacuum" scale and regular folk would have a different scale. And there's already too much of that nonsense.

[snip]
Meh, it happens all the time. No scientist I know works in inches of mercury (I had to look up the conversion before my first post). Scientific work is mostly in hPa or millibars.

Personally, in regards to weather, I think of "twice as hot/cold" as meaning "twice as far from optimal comfortable temperature". You wouldn't say "It was perfect out yesterday, and twice as cold today", but you might say "It was uncomfortably cold yesterday, and twice as cold today".I've never heard of this, but it's an elegant and meaningful way to think about "twice as hot (or cold)." I love it!