Weather reports nowadays use millibars instead of inches of mercury to indicate barometric pressure. I know that <30 iom is dry and >30 iom means rain. So what is the conversion from iom to millbars? I need a reference point. And what the hell is a millibar anyway? (jokes welcome after the question is answered)
Must be a northern hemisphere thing, around here low pressure tends to mean rain.
:smack: typo
Typo or not, it’s more the direction the barometer is taking than the absolute reading. Falling pressure foretells bad weather, rising = good weather.
Of course, we have better tools nowadays than just a barometer, and barometric pressure isn’t the only factor in weather.
Download “This” and you can do all that nifty conversion stuff, on your own. (Well, unless you need to know how firkins there are in a hogshead.)
Tris
Thanks for the link. Here’s what we got:
1 iom=approximately 35 millibars
29 inches of mercury = 981.94 millibars
30 inches of mercury = 1 015.8 millibars
31 inches of mercury = 1 049.66 millibars
I’m still more comfortable with iom but now I at least have some point of reference.
As well, Google’s calculator/converter function can convert inches of mercury to millibars. To wit.
Millibars are a bit outdated now. Hectopascals are the in thing! Same size as a millibar, different name.
As mentioned above, the absolute reading of the atmospheric pressure is pretty much meaningless. As an example, the pressure here at Broome rarely gets above 1015 millibars, yet the weather for six months of the year is near perfect.
Low pressure relative to the surrounding area means air will be flowing into the low pressure area (a “low” on the weather map) and getting pushed up as it converges. The upgoing air is unstable and encourages the formation of clouds, rain, and other weather. A bigger change in pressure over distance results in a stronger “pressure gradient” which produces stronger winds and possibly more weather.
“Bar” in this context means “atmospheric pressure”, which is what’s read by a barometer. Of course one bar is a rather nominal value. But a millibar is then just one thousandth of a bar.
Check. “One bar” equals “one atmosphere” in the sense that the surface pressure on Mars is 0.03 atm. (or something like that; I didn’t look it up), carbon dioxide liquefies under at least 20 atm. of pressure, etc. For reasons I’m not totally clear on, this is equated to an arbitrary value for average sea level pressure of 29.533 inches of mercury. “Millibars” (as well as “hectopascals”) simply uses the metric system’s scaleability to give units of measure that can be worked with, instead of fractions of a giant whole – just as “farad” as a value for capacitance needs to be enormously high, so typical measurements of capacitance are in milli- and microfarads.
It is important to note that barometric pressure is adjusted to a sea level reading. The actual air pressure in Denver, CO is seldom above 25"Hg, but the barometric pressure will often be reported as greater than 30"Hg for that location. Dial type barometers are just adjusted for the altitude offset, but when using a real mercury barometer (closed end manometer) the correction must be made explicity, consulting a table, which is frequently engraved on the instrument itself. For modest altitudes (Denver would qualify, Mt. Everest would not) the correction amounts to very close to 1"Hg / 1000’MSL.
Historic reasons, of course. I realize my Chemistry teachers were Chemists and ChemEng, not historians, but they said that 1atm = 760mmHg because that was the value measured originally. We asked whether the person taking the measurement took it once and thought that was “The Value” or they’d already noticed it actually varies and thus the 760mm was an average… the teacher didn’t know but guessed it might have been the first, given the stories behind other units and key values.
The “inches” and “mm” in this case refer to the length of a column of mercury of a given cross-surface. The pressure is the pressure needed to “push” that column of mercury that far up; if the surface is smaller, the column is taller for the same pressure. The factors given in conversion programs and tables already take the value of the “given” surface into account.
[QUOTE=NavaThe “inches” and “mm” in this case refer to the length of a column of mercury of a given cross-surface.[/QUOTE]
No, it doesn’t. the cross-sectional area doesn’t matter at all
I’m pretty sure that’s not right. As long as the cross-section is the same on both sides of the tube, the height will remain constant. The downward force on the open side is equal to the atmospheric pressure * area. On the closed side, the force down is equal to the weight of the mercury, which equals volume of mercury (area * height) times the density of mercury. Since “area” is on both sides of the equation (and assumed to be identical), it cancels out, whether it’s one square mm, one square inch, or one square foot.
Of course, with really big cross sections, you’d run into structural problems, since the amount the mercury would get to the point where the tube would have problems holding it without breaking under the strain. So while there are practical limits, the math doesn’t require a given cross sectional area.
[sub]Normal people use kilopascals…[/sub]
What works for me is to say that the weight of the column of mercury in the barometer is equal to the weight of a column of atmosphere with the same cross-sectional area.
For mercury barometers, cross-sectional area is a compromise between minimizing the volume of mercury used and minimizing the effect of the meniscus on the reading.