galfert wrote: ↑Sat 02 May 2020 10:23 am
Tip - don't look at EGXC (Coningsby(RAF) pressure in mb units. Looks like they are rounding to whole hecto Pascal.
It is part of the instructions for reporting METAR, that these pressure (or Q values) are truncated down to an integer. METAR dates back to communication by teletype days and aims to use very short messages.
galfert wrote: ↑Sat 02 May 2020 10:23 am
Instead look at the inHg units and then convert to hPa (mb).
Do NOT do that.
The conversion to inHG on
https://aviationweather.gov/metar/data? ... e=&hours=0 is no more accurate, it is based on the conversion from the value shown in brackets, and the true value might be .9 hPa higher. Both are taken from the entry in the raw METAR beginning with Q which is I believe being interpreted by that USA site as altimeter pressure QFE, i.e. at level for a aircraft cabin sitting on the runway. It may actually be a SLP, as outside North America the Q element is used for QNH (nautical height to a pilot being sea level to you or me). Believe me, I spent a lot of months studying METAR several years ago, and I don't trust sites that convert RAW METAR, they are not consistent in the assumptions they make.
The latest complete METAR at time of typing is
2020/05/02 10:50 EGXC 021050Z 28010KT 9999 SCT035 14/05 Q1009 RMK BLU
and that tells me the Q1009 is reporting SLP.
I believe there is already a link to
https://cumuluswiki.org/a/Pressure_Measurement#Datum earlier in this thread, where it mention that the pressure reported is not always sea level. Some RAF stations follow UK guidance and report SLP in the international standard part of METAR for pressure that is prefixed by Q, but some were (or still are) staffed by USAF, particularly in East Anglia, and they follow US guidance reporting airfield level pressure (actually it is a standard cabin height above the runway) in that standard METAR pressure section. (US put sea level pressure in the non-standard Remark part of METAR). Because of these problems and other variations between METAR reporting stations, a proposal to replace METAR was made many years ago, and the new standard is in use; but this replacement is in XML/Geo code designed for machines to read (e.g computers flying aircraft), and it is not made available to public.