Out of this confusing hodgepodge, some meteorologists now seem to think they have discovered the perfect unit in which to measure pressure: the hectopascal! This is especially common in aviation meteorology. They have grown accustomed to millibars, old metric units which don't fit into the modern metric system.
There is increasing pressure to use the modern units. So they came up with a scheme to hang onto these outdated units by giving them a pseudo-SI name.
That clearly is not the case with hectopascals, since the pascal as a special name for newtons per square metre was not introduced until the 1970s. Therefore hectopascals could not have been established before SI was introduced.
The proper units of atmospheric pressure are kilopascals (kPa), which are, for example, the units which have been used by Environment Canada in its public weather reports for many years (I live close to the border and hear these units used on Canadian radio and television stations). For an example, you can check out the current conditions and forecast for Estevan, SK. Or, from a different office of Environment Canada, Saskatchewan Current Weather Conditions and Les conditions actuelles pour T.N.O.
There are other examples of the use of the correct units of pressure in meteorology. Here in Crosby I cannot find many barometers, but the only ones I did find were some Sunbeam brand with dual scales in inches (of mercury) and kilopascals
| What are mb/hPa? |
|---|
| Every once in a while I see this formulation used, apparently to indicate that the number associated with this can be called either millibars or hectopascals. But it looks like a normal division of units, millibars per hectopascal, which would equal the dimensionless number 1. Using kilopascals would keep people from using confusing terminology. |
Another advantage of using kilopascals is that the numbers fit more easily and more legibly on the face of an aneroid barometer than millibars do. This is related to another preference in SI. Prefixes should be chosen, when possible, so that most of the numbers fall in the range from 0.1 to 1000. (See, e.g., International Standard ISO 1000-1981(E), SI units and recommendations for the use of their multiples and of certain other units, Rule 4.2: "The multiple can usually be chosen so that the numerical values will be between 0,1 and 1000.") Atmospheric pressures in hectopascals will often fall outside this range, but kilopascals are well within it.
Here is the way SP (Swedish National Testing and Research Institute) puts these rules on its web page:
A prefix combined with a unit indicates that the unit is multiplied by a particular power of ten. The new unit is referred to as a multiple unit. There are 20 standardised prefixes. The choice of prefix depends on purely practical considerations: in practice, it should be chosen such that the resulting numeral lies between 0,1 and 1000. Prefixes that indicate multiplication by 1000 or 1/1000 should preferably be chosen.
One of the main reasons for preferring the powers of one thousand is that the units that differ only by factors of ten are too close to the same size and can be confused. There is the additional problem that it is sometimes difficult to differentiate a handwritten h from a k. This problem showed up on The Weather Channel Forum on CompuServe, when someone asked about conversion factors for kilopascals. Someone answered the question, then one of the forum regulars who is knowledgeable about weather jumped in with a comment "I thought kilopascals were the same as millibars." This confusion is one of the hidden dangers of using hectopascals equal to millibars.
No real scientist uses hectopascals. This prefix with pascals is not used anywhere else in science or industry. Take a look at the numbers molded into the side of the tires on your automobiles it most likely includes a maximum inflation pressure in kilopascals. Even when I was in college 20-30 years ago, in my chemistry and physics classes the 'standard atmosphere' was often defined as 101.325 kilopascals or kilonewtons per square metre, though 760 mmHg was still quite common then.
To use hectopascals in aviation meteorology is to miss out on one of the major advantages of the International System of Units, not having to learn a whole new set of units for each field of endeavor.
| Divide millibars by 10 to get
kilopascals: 993.0 mb = 99.30 kPa |
Most people don't know what the prefix hecto- means. The only time it is commonly used in the variation without the "o" in hectares.
This unit was, of course, a part of the original metric system in the 1790s. There are only a few places where ares are used standing alone--Switzerland, perhaps in other places in Europe for the size of city lots. Hectares or dekares are more common. These area units do not take full advantage of SI, and should not be used outside of the established land area use. A hectare is 10000 square metres.
However, hecto- doesn't stand for 10000, but rather the square root of that number. Therefore, even though hectares are used, they provide no help at all in giving most people any kind of understanding of what the prefix "hecto-" means. I'm pretty sure that the hectare was originally designed as an abbreviated form of square hectometre, and that ares were backformed from this at the time the metric system was originally established.
| A special problem with the unfamiliarity of hectopascals
is the commonly seen misuse of capitalization in the name of this
unit. Even the U.S. National Weather Service and Environment
Canada come up with incorrect spellings such as
hectoPascals HectoPascals Hectopascals |
It would be nice if centibars had been the units established in meteorological usage, because then you could convert to the proper kilopascals without even having to move the decimal point. But even in the cgs systems, there was a preference for prefixes which are powers of 1000, and the same is true with bars, a unit so far out of date that it doesn't even fit in with the cgs systems.
The pascal (or the appropriate decimal multiple, ordinarily the kilopascal) is the preferred unit for AMS journals .... Although some authors and readers might prefer the hectopascal (hPa) because it is equal in size to the more familiar millibar (i.e., 1 mb = 1 hPa), the AMS prefers the kilopascal (kPa) as the unit of atmospheric pressure.The National Weather Service does use hectopascals in reference to sea level pressure in the latest version of their Federal Meteorological Handbook No. 1. But the office which publishes FMH-1, the Office of the Federal Coordinator for Meteorology, has known since at least 1991 that hectopascals are not proper units of pressure. They said in publication FCM- G2-1991 that "the kilopascal is more appropriate for atmospheric pressures" and that "the prefix 'hecto' is not a preferred multiplier and not recommended by the SI community."
Look at any modern encyclopedia and see what units they use
for atmospheric pressure. You will often find kilopascals used,
but you will never find hectopascals mentioned. See, for
example, Grolier's Academic American Encyclopedia, widely
available in print or on CD-ROM or in an electronic version on
places such as CompuServe (GO AAE). It says under
barometer:
This is equivalent to 101.3 kilopascals, the pressure unit meteorologists now use, besides millibars.
Hectopascals do not conform with Federal Standard 376B, Preferred Metric Units for General Use by the Federal Government. This standard says:
NOTE: The bar and its submultiples are accepted for limited use in meteorology only. It is not accepted for use in the U.S. for other applications, e.g., as the unit of fluid pressure in pipes and containers. The appropriate SI multiples, e.g., kilopascal or megapascal, should be used instead.This acceptability for limited use applies to millibars under their own name, and does not change the fact that kilopascals are to be preferred over millibars. In addition, hectopascals are not an "appropriate SI multiple".
millibar kilopascal (kPa) 0.1
Millibars are obsolete units. They don't fit into SI even as well as they fit into earlier cgs systems of units and they did not fit in with the cgs systems very well either. It is the barye which is the coherent cgs unit of pressure, equal to one dyne per square centimetre. This barye is equal to a microbar; the bar is a non-cgs unit equal to a million dynes per square centimetre.
There is finally increasing pressure to use the International System of Units and abandon old units such as these time-worn millibars. But the scheme to hang on to these obsolete units by disguising them in a pseudo SI name is a crackpot idea. Millibars are outdated, but at least they were once proper units of pressure. The same cannot be said about their alias; hectopascals never were proper.
I said above that U.S. Federal Meteorological Handbook Number
1 uses hectopascals. That isn't quite true. Every time they
appear there it is in the phrase "tenths of hectopascals." This
handbook deals with the METAR aviation reports, and the only time
these units are used in the U.S. mutant variation of this format
is for sea level pressure in the remarks column, where the is
coded as these examples:
SLP745 SLP331
When these numbers are decoded, you must add the one or two most significant digits at the left of the number. Then you still must add a decimal marker to get the pressure in units of hectopascals. You could just as easily put two digits after the decimal marker instead of one, and get the pressure in the proper units, viz. kilopascals.
Example: Decoding SLP945
add 9 to the front of the number 9745
insert decimal point 97.45 kilopascals
Decoding SLP331
add 10 to the front of the number 10331
insert decimal point 103.31 kilopascals
If one deprecated prefix is okay, why aren't these "tenths of hectopascals" called dekapascals?
This example also tends to refute any argument someone might want to make that hectopascals are the perfect size for atmospheric pressures.
Note that the same argument applies to Canadian manuals for METAR reports as well. Canada doesn't really use hectopascals in their coding either. So why didn't their decoding instructions tell you how to get the much more familiar (to Canadians at least) kilopascals, the units used for many years on Canadian radio and television and newspapers in their public weather reports?
The short answer is that no one seems to know why hectopascals are used, other than as an excuse to hang onto millibars. Everyone just passes the buck to someone else. I have run across nothing that indicates that the propriety of hectopascals vs. kilopascals was ever considered. A favorite ploy of the U.S. National Weather service, when they don't want to answer a question, is to say "that is an FAA requirement." The meteorologists in the field say its just something that the NWS (or the appropriate national agency) tells them to do. The NWS passes the buck to the World Meteorological organization, relying on a footnote in what Howard Diamond, U.S. METAR Implementation Manager, described to me as "the 1995 edition of the meteorological codes handbook No. 306." But even in this footnote, the WMO just passes the buck again, saying "the unit prescribed by the International Civil Aviation Organization (ICAO) Annex 5 for pressure is the hectopascals."
Mr. Diamond's counterpart in Canada, Gary Cormick, relied on some other oscure comment hidden away in an attachment to a WMO manual saying that "the hectopascal (hPa), equal to 100 pascals (Pa), shall be the unit in which pressures are reported for meteorological purposes." He gave no explanation why this did not need to be followed in Canadian public weather reports, which use kilopascals. When I pointed out to him that this wasn't limited to sea level pressures, but if it must be followed then it must be followed for altimeter setting pressures in the Canadian reports as well, which are not reported in hectopascals, he ignored me.
But an important thing to remember in all this is the following. It wouldn't matter if it were true that every meteorologist in the world used hectopascals all the time for atmospheric pressure--they would still be WRONG! The interdisciplinary nature of the International System of Units leads inevitably to this conclusion.
Contact your local meteorologist. Let him or her know that kilopascals are the proper units for atmospheric pressureand that millibars are obsolete and trying to hang onto them by calling them hectopascals is wrong. (In the U.S., you may first have to get over the hurdle of people thinking that going metric would involve changing from inches of mercury to millimetres of mercury, another once-but-no-longer acceptable unit that doctors hang onto as tenaciously as weathermen to millibars.)
Contact your national agency in charge of the weather observations. I haven't tried to list them here (though the links on the U.S. METAR page will lead you to places where you can leave messages for these offices in the U.S. and Canada). If you would like help finding an office somewhere else, leave me a message at the address below.
Use a forms-capable browser to leave a message for the World Meteorological Organization (WMO) at http://www.wmo.ch/
International System of Units My page on units and prefixes in SI, and some conversion factors and other information.
Canada/U.S. METAR -- Mutations of the International Standards
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