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In a METAR such as this one:

KGOK 161553Z AUTO 18011KT 4SM BR OVC007 07/05 A3011 RMK AO2 SLP201 T00670050

Translated to:

Jan 16, 1553Z, Automated. Mist, Wind from 180° at 11 knots, 4 statute miles visibility, Ceiling is Overcast at 700 feet, Temperature 7°C, Dewpoint 5°C, Altimeter is 30.11. Remarks: automated station with precipitation discriminator sea level pressure 1020.1 hectopascals hourly temp 6.7°C dewpoint 5.0°C

Why would I want/need to know, or what would I do with, this information in the remarks section:

sea level pressure 1020.1 hectopascals hourly temp 6.7°C dewpoint 5.0°C

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    $\begingroup$ Are you asking about why the values are present at all (07/05 A3011 is the first time, and absolutely essential information in the report), or why are they given twice (SLP201 T00670050 is the second time, as a remark, with a bit of extra precision)? $\endgroup$ – Jan Hudec Jan 16 '19 at 18:26
  • $\begingroup$ You forgot “baby rain” (aka mist) :) $\endgroup$ – bartonjs Jan 18 '19 at 4:45
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The sea level pressure allows pilots to calibrate their altimeters to make sure they are accurate. KGOK is 1069 feet elevation, if a pilot were to set their altimeter to 1020hpa (or 30.11 inches of mercury as used in the US, that's the A3011 in the METAR) then their altimeter would read 1069ft at ground level.

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    $\begingroup$ Ok so it is helpful to know that those two things are related (A3011 and the RMKs about sea level pressure) and I know what I would do with the A3011 to set my altimeter; what causes me to ask this question was: why the extra information if it is already communicated with A3011? I am not sure that it is at all useful beyond what A3011 tells me. $\endgroup$ – user2325243 Jan 16 '19 at 17:28
  • $\begingroup$ Whatever site is translating that METAR for you is converting the measurements @user2325243. $\endgroup$ – GdD Jan 16 '19 at 21:11
  • $\begingroup$ +1 answer Above Sea Level and Above Ground Level may apply differently. Local flight would set altimeter for AGL. Cross country would set ASL (or AGL of destination). Changing weather will also factor in. I believe (correct me if I'm wrong) flight levels are in ASL. $\endgroup$ – Robert DiGiovanni Sep 29 '19 at 14:23
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It is additional information for reference use. A pilot in the US will not typically use the additional information unless they have an altimeter calibrated in hPa, but a meteorologist might be accustomed to working in hectopascals (or millibars). The additional temperature/dewpoint data is also more precise -- to the tenth of a degree C.

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    $\begingroup$ The hPa is also useful for international pilots arriving from locations where they typically use hPa instead of inches of Hg. All airliners can switch between the two units, but pilots like what they use most often. $\endgroup$ – Gerry Jan 16 '19 at 18:16
  • $\begingroup$ Why not use Q1020 in the main section, as done in other countries, rather than putting it (in a different format!) in the remarks section? $\endgroup$ – StephenS Jan 16 '19 at 18:26
  • $\begingroup$ @StephenS, the remarks section has extra precision. I suspect it is there for the benefit of meteorologists, because pilots don't need it. $\endgroup$ – Jan Hudec Jan 16 '19 at 18:34
  • $\begingroup$ @StephenS, "Why not use Q1020 in the main section, as done in other countries..." Because we are Americentric! ;) $\endgroup$ – Michael Hall Jan 16 '19 at 21:10
  • $\begingroup$ @Gerry Not all airliners in all places... a common exception was the mechanical Airbus standby altimeters which only had one unit, so they often had a decal somewhere with the conversion. (Even more annoying if the plane originally came from the wrong place and the airline was too cheap to swap out the unit) $\endgroup$ – user71659 Feb 15 '19 at 22:54
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Note that A3011 (30.11 inHg) is 1019.6 hPa, which is lower than 1020.1 (SLP201).

The small difference is because SLP (unlike QNH) is corrected for the 12-hour mean temperature at the reporting station, giving more accuracy to the meteorologists.

So SLP is not a metric conversion of the altimeter setting in inHg to be used by pilots accustomed to hPa – it's for meteorology, not flying.

Likewise for the more precise temperature reading in the remarks.


Further reading: https://www.wingsbywerntz.com/520-metar-slp-sea-level-pressure

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