** Caution: EXTERNAL Sender ** The reported pressure is always station pressure reduced to sea level, not the actual station pressure.
However, two different algorithms are used, and results may disagree slightly. For aviation, the reduction is the basis of the altimeter equation (or standard atmosphere model). In the US, this is always and only reported in inches of mercury. It may be wrong, but on the runway, it makes the altimeter of the aircraft read the height above sea level of the runway. At other altitudes, it may disagree a bit if temperature at sea level and/or lapse rate disagree from the standard atmosphere (which is always). A different model of the air column from station to sea level is used for weather forecasting. The US only reports this in millibars. Particularly at higher elevations and extreme weather, the two numbers may not "convert" in the expected way because they are based on different model assumptions. Note that aviation METAR and TAF report altimeter setting in inches Hg, and meteorological sea level pressure in millibars in a comments section. If they were reported in the same units, they would still differ (at least a bit). Maybe a pilot could explain better. On Friday, May 2, 2025 at 07:11:46 AM EDT, Martin Vlietstra via USMA <usma@lists.colostate.edu> wrote: ** Caution: EXTERNAL Sender ** Mike, An understanding of what is meant by 950 mb depends on where you live. If you are in Denver (elevation 1600 metres), then 950 mb is a very high pressure (typical pressure is, I believe, about 830 mb). If you are in New York, then 950 mb is a very low pressure. It would furthermore not surprise me if certain people got confused by the words "minibar" and "millibar". If you visit https://nam10.safelinks.protection.outlook.com/?url=https%3A%2F%2Fcommons.wikimedia.org%2Fwiki%2FFile%3ABaragraph_Storm_October_1987.svg&data=05%7C02%7Cusma%40lists.colostate.edu%7C341964295e5f4d10bd8008dd896d22ef%7Cafb58802ff7a4bb1ab21367ff2ecfc8b%7C0%7C0%7C638817824021099356%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=UoNI2IOhrlZE6PJ16SRxnAmjdK1DbS30by5Nyr0a1Ss%3D&reserved=0, you will see that during the Great (UK) Storm of 1986, the pressure dropped to 960 hPa at a location which was very close to sea level (certainly less than 50 metres). Martin Vlietstra -----Original Message----- From: Metricmike <metricmik...@gmail.com<mailto:metricmik...@gmail.com>> Sent: 02 May 2025 06:59 To: vliets...@btinternet.com; 'USMA List Server' <usma@lists.colostate.edu<mailto:usma@lists.colostate.edu>> Subject: Re: [USMA 2103] Use of Celsius only in USA There are far too many people who should not be in the piloting profession, and this was one of them unfortunately. I remember I was in Van Nuys north of L.A. early one morning and the temperature was about +1ºC, there was frost on the wing. So I asked them to bring a deice truck. They didn't have one, it being LA where it never snows. We had to get buckets of water thrown on the wing to get rid of all the frost. Because it was above freezing it was not going to re-freeze. Yes, it's a pity the US uses inches of mercury, as does aviation in Canada, the weather service in Canada and TV weather gives the pressure in kPa. The US weather service uses millibars for hurricane pressure and the rest of the world uses hPa, same unit, different name. I have to wonder how the general population in the US has any clue what 950 mb is anyway, let alone inches of mercury to 2 decimal places. I've always found whole number to be more meaningful. Mike Payne. On 01/05/2025 22:32, vliets...@btinternet.com<mailto:vliets...@btinternet.com> wrote: > Hi Mike, > > I read the report - deep in the report it does mention that -1°C is 30°F. The > report also mentioned that the hanger was heated and had a temperature of > about 60°. I assume that this was Fahrenheit! > > A few other points that I noticed, apart from the use of feet throughout as > opposed to metres, is that the pressure reading was in "in Hg", something > that I believe he rest of the world phased out years ago. > > I also notice that the vertical load factor was described as 1.1g’s. If they > wish to use "g" (acceleration due to gravity) as the unit of measure, then > they should have written "1.1 g" (with the context suggestion that "g" > referred to the acceleration due to gravity rather than gams). > > This report shows a very good case for the aviation industry to standardise > on degrees Celsius - the accident was caused by ice on the wings and the > critical temperature for the formation of ice is 0°C. If there is a positive > temperature, ice melts, if the temperature is negative, ice forms. > > Martin Vlietstra > > -----Original Message----- > From: USMA > <usma-boun...@lists.colostate.edu<mailto:usma-boun...@lists.colostate.edu>> > On Behalf Of Metricmike > via USMA > Sent: 01 May 2025 20:24 > To: USMA List Server > <usma@lists.colostate.edu<mailto:usma@lists.colostate.edu>> > Subject: [USMA 2103] Use of Celsius only in USA > > ** Caution: EXTERNAL Sender ** > > Here is an accident report issued by the NTSB in the USA describing the cause > of an business aircraft accident where the temperature was -1ºC. I like the > fact there is no Fahrenheit included. > > https://nam10.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww%2F&data=05%7C02%7Cusma%40lists.colostate.edu%7C341964295e5f4d10bd8008dd896d22ef%7Cafb58802ff7a4bb1ab21367ff2ecfc8b%7C0%7C0%7C638817824021114415%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=1S8gEL55xNecLTNCfNKzo7j68f0KmV5hnWv5TSmH5qE%3D&reserved=0. > avweb.com%2Faviation-news%2Ffailure-to-deice-cited-in-fatal-phenom-300 > -crash%2F%3Futm_medium%3Demail%26utm_source%3Drasa_io%26utm_campaign%3 > Dnewsletter&data=05%7C02%7Cusma%40lists.colostate.edu%7Cd00651a507ca45 > 153a2008dd88e5cc3d%7Cafb58802ff7a4bb1ab21367ff2ecfc8b%7C0%7C0%7C638817 > 242736754466%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwL > jAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C% > 7C&sdata=bL3Gk4LKyGxoZ38djzvkXF2mNQ8cRq3i3xeHsoH6I20%3D&reserved=0 > > Mike Payne > > > _______________________________________________ > USMA mailing list > USMA@lists.colostate.edu<mailto:USMA@lists.colostate.edu> > https://lists.colostate.edu/cgi-bin/mailman/listinfo/usma > _______________________________________________ USMA mailing list USMA@lists.colostate.edu<mailto:USMA@lists.colostate.edu> https://lists.colostate.edu/cgi-bin/mailman/listinfo/usma
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