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.