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I was sitting around the other day, waiting to see if the weather would hold out for my flight lesson sooo... since weather was the topic at hand I was decoding the local METAR and there in the remarks section was SLP. Ok now the question - What is the significance of Sea Level Pressure in the remarks. Or in other words if the SLP is notated in the remarks what does it mean to us as pilots? Thanks, Jeff
SLP should be followed by some numbers, which indicated the altimeter setting in hectopascals (millibars) which is the metric standard used everywhere but the US.
so it is just what the altimeter at the weather station is reading, no more, no less, not really useful except to know what the reading at the station is?
Ok wait maybe I see a use for it, you can map all the connecting stations with isogonic lines and as long as you stay in that area then your altimeter will be "correct"
OK... So here's an exercise. Check the SLP issued on a METAR tag, convert the hPa into in/mg, and compare it to the Altimeter Setting in the METAR. Guess what... their close, but do they agree????? Unless its 15 deg Celcius outside, I doubt it.
The SLP is the Barometeric Pressure from the Weather Office. It's what they use to draw the isobars on the weathermap with.. but it's corrected for the height of the observing station (obviously) AND things like air temp. This way the weatherman can draw the isobars on the map and find the H or L pressure systems accurately. Cold air is much more dense than warm air. If the Wx office didn't correct for temps, the isobars would be off on the map and wouldn't show the true position of the H or L pressure system.
The Altimeter setting is the setting used to get your altimiter altitude to agree with elevation above Sea Level, so you'll be accurate in trying to figure out how much room you've got between you and the ground.
So if you're sitting on the runway waiting to take off at an airport who's elevation is say 3000ft and it's 15 deg C outside, the weather office SLP and the Altimeter setting would agree and you're altimiter would read 3000ft. But if it's -20 deg Celcius (and that's common in Canadian winters) the SLP and Altimeter setting in the METAR tag would NOT agree and using SLP to set your altimeter elevation would give an inaccurate elevation. Cold Air is much more dense and exerts more pressure, so less inches of mercury. That's why there's an "altimeter setting" in the 1st place.... instead of just giving the barometric pressure.
If you're curious one day, try it out when it's cold (on the ground and AFTER your flight of course). Convert the SLP hPa to in/mg and set the altimeter to that. You'll see the alt elevation will not agree with the stated elevation of the field.
At least that's how I understand it. And please correct me if I'm wrong.... but that's how I understand it.
Significance of SLP
Linear Mode
