METAR SLP – Sea Level Pressure

Many airports have an entry SLPxxx in the remarks section of their METAR. What is it?  SLP stands for “Sea Level Pressure”. As an example, let’s look at a Burbank METAR:

KBUR 151753Z 14007KT 9SM CLR 23/14 A2994 
RMK AO2 SLP127 T02280144 10228 20167 50000 $

We see the A2994, which is our (in the US) normal altimeter setting in the units of “inches of mercury”, so that translates to 29.94″ Hg.

The SLP is the sea level pressure, showing the tens, units, and tenths of hectopascals (hPa, aka millibars). It’s left to you to determine what the thousands and hundreds are. It helps to know that the standard sea level pressure is 1013.2 hPa; for practical purposes 950-1049 is a reasonable range for this value. [The highest pressure ever recorded was 1085.7 and the lowest was 870, so that’s not absolutely true; the 870 was recorded in the eye of a typhoon and the high in winter in Mongolia in extreme cold, so not normal conditions]. Given that normal range, it’s easy to determine whether to append a 9 or 10 to the beginning of the provided value. In our case, we end up translating SLP127 to 1012.7 hPa or mbar.

Now, the sharp reader will notice that the altimeter setting of 29.94 is slightly above the standard of 29.92 and the 1012.7 is slightly below. What’s going on? While both values are meant to be pressure one might expect at sea level, they are calculated in different ways and for different purposes.

The altimeter setting is “the pressure value to which an aircraft altimeter scale is set so that it will indicate the altitude above mean sea level of an aircraft on the ground at the location for which the value was determined.”. In other words, it works the way we would at an airport without any weather reporting – figure out what altimeter setting gives you the right altitude. Our altimeters don’t correct for nonstandard temperature, so in many cases the actual pressure at sea level would be different. For examples where the altimeter setting can give you erroneous data that can be a problem read “How Hi Am I?

The sea-level pressure is “a pressure value obtained by the theoretical reduction of barometric pressure to sea level. Where the Earth’s surface is above sea level, it is assumed that the atmosphere extends to sea level below the station and that the properties of that hypothetical atmosphere are related to conditions observed at the station”. In more detail, “sea-level pressure shall be computed by adjusting the station pressure to compensate for the difference between the station elevation and sea-level. This adjustment shall be based on the station elevation and the 12-hour mean temperature at the station.” This is a value that might be more accurate for meteorological purposes and attempts to correct for non-standard temperatures.

You can read more on METAR Coding and the use of pressures in METARS.

22 thoughts on “METAR SLP – Sea Level Pressure

  1. Great job explaining SLP in METAR reports. My question is, why is it shown? Are there some aircraft systems that use that information? You dial in the barometric pressure on the altimeter in inches. Could it be displayed for non aviation use?

    • CYTZ 162130Z AUTO 30021G33KT 280V340 9SM -RA BKN038 OVC060 13/08 A2975 RMK PRESRR SLP075 DENSITY ALT 300FT

      I’m trying to understand what Sea Level Pressure would be for 075, “SLP075”. Can’t seem to find a value this low given in any METAR report reading examples.

      • Your going to put either “9” in front, or “10”. 907.5mbar would be really low. 1007.5mbar is very reasonable – slightly below the standard pressure of 1013.2, so 1007.5mbar is the answer.

  2. Thanks! I can find no aviation use for it. It would be useful for trying to draw isobars. METAR is a general purpose weather reporting format – you will often find non-airports reporting in METAR format. I’ve had students ask me about the “airport” at Mt. Wilson, because some systems show KMWS weather. It’s interesting to note that much of what we get reported in METARs is not the directly measured numbers, but derived items. Nowhere in most METARs do you find the actual observed pressure – we only get the altimeter or SLP (in both cases, the weather station is taking the actual pressure and making calculations to create the reported pressures).

    • I understand that. To be clear, the SLP field in the METAR is NOT what you would use to set your altimeter, as that entry includes temperature compensation. As explained, it is not simply the translation of 29.92″ = 1013.25 mbar, but is a _slightly_ different value.

      • Hi, I’m prepping for the CPAER exam and just for fun (or to procrastinate) I started converting the nearby Metars from InHg to mbar and was startled when my figures were not matching the SLP in the remarks section… thought maybe I missed something. I’m up in Canada where its getting pretty cold and your explanation makes me realize I’m not going crazy.Thanks! Stephen.

        • That’s much the same way I found the difference, researched, and decided to document it. We’re inundated with data and learn to ignore many pieces of data, but its good to know why it’s safe to ignore certain data

          • I’m transiting from ICAO/EASA world to FAA 121 world. Indoc training material makes me realize a whole new world of terms for measuring visibility in statute miles in stead of meters and “hg instead of hpa. Is there a thumb rule for determining visibility from M to SM?

          • Yeah – we have the craziest mixture of units in the US for aviation. 1 statute mile is approximately 1600 meters. That pops up in TAFs – why do we make it P6SM (better than 6 statute miles of visibility forecast)?… because the standard is 10,000 meters and 6 sm is a good approximation (9656 m)

  3. These three airports were reporting an altimeter setting of 29.92 inches of mercury tonight, but the Sea-Level Pressure at each airport was different, and none equaled 1013.2 mb.

    KENW 090453Z AUTO 02006KT 8SM SCT021 BKN027 17/15 A2992 RMK AO2 SLP130 T01670150

    KPWK 090452Z AUTO 32009KT 10SM CLR 18/15 A2992 RMK AO2 SLP127 T01830150

    KARR 090352Z AUTO 26005KT 7SM CLR 15/14 A2992 RMK AO2 SLP129 T01500139

    I see that each airport had a slightly different temperature, but the two airports with the most similar temperatures had the least similar Sea-Level Pressures. Does this situation likely involve differences in the “12-hour mean temperature” among the stations?

    • We’re talking really tiny differences here:
      0.01/29.92 = 0.0003
      .5/1013.2 = 0.0005 (.5 being the difference between 1013.2 and the lowest SLP of 1012.7)
      So, the total difference in the SLP would only make a max of .02″ difference anyway. It wouldn’t take much temperature deviation over 12 hrs to make that difference, or heck one could actually be 29.924 and the other 29.916 and that would be undetectable.

  4. thank you very much but i have a question…..SLP 102 means 1010.2. very good so i added 10 to the left to now my sub-scale.
    what about if the SLP is less than 1000 like 995 how would SLP would express that?

    • Here’s a METAR from this morning:
      KCIU 241356Z AUTO 17017G25KT 5SM RA BR OVC005 09/08 A2899 RMK AO2 PK WND 17026/1339 SLP846 T00890083 PNO $

      SLP846 is translated to 984.6 mbar – basically if the SLP numbers are high (500-999), put a 9 in front. This system wouldn’t work for really low pressures, as might be seen near a hurricane or tropical storm

  5. The purpose of SLP? You ask? Why, its purpose is perfectly clear: To impress other pilots of your superior knowledge when you casually state “Sea level pressure Ten Twelve point 7” (1012.7 mb). Geesh! This guy knows EVERYTHING! Thats why I’m learning it!

  6. And it is useless information to pilots – the equivalent of spam in the METAR. The FAA would do better to focus on giving us information that is useful in planning a safe flight.

    • METAR is not just for pilots. Meteorologists use it to calibrate their weather stations and to compare weather stations. So it isn’t spam.

  7. Thanks for putting my mind at ease. 🙂 I was concerned because I could not figure out the discrepancy in altitudes and was afraid I was missing something important. Yours was the only detailed explanation of this Metar puzzle I could find on the web yet I’m sure a lot of pilots wonder about this.

    • Thanks. Glad you found it helpful. It’s funny that this article is the one which so many pilots/students come to my site. I hope you find other articles useful as well.

  8. Excellent explanation, seriously….and the responses in the threads are equally informative! Thank you for clearing this up for me.

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