Topic: Sea Level Pressure – pressure reduction equations?

[gszlag]

Sea Level Pressure – pressure reduction equations?

Many METAR reports (almost all in Canada) contain a sea level pressure (SLP) reading. But really, there are two sea level pressures. Altimeter (setting) is also a reduced sea level pressure. Both Altimeter and SLP in a METAR report are sea level pressures.  But where’s the pressure reduction? Pressure goes up at sea level so how can pressure be possibly reduced?

Who says meteorologists don’t have a sense of humour. They are actually referring to sea level elevation not pressure. Therefore, the equations that calculate Altimeter or SLP are actually reducing station pressure at your location’s elevation down to mean sea level elevation. It is therefore reducing elevation – not pressure.

Why is this important?

In setting up their barometers, newcomers to weather stations are often concerned about their airport’s elevation being different than their weather station’s elevation. After all, the manual says to compare and calibrate your barometer with a close-by airport. How can you do that if your airport is a lot higher (or lower) than your weather station?

It is important to note that station pressure values are reduced down to mean sea level elevation so that one can make make valid pressure comparisons between weather stations at different elevations. Mean sea level elevation is the common denominator that is used. All the fancy algorithms and equations do is convert your station pressure to what it would be at sea level elevation. Therefore, the isobars you see on weather maps are station pressures that are converted to sea level pressure at sea level elevation.
Or even simpler still – your station pressure is recalculated down to sea level.

The differences between Altimeter and SLP will be discussed in a future article.

The above article was originally meant to be part of a rather long post called: "How to set up the Ecowitt barometer – a tutorial for new weather station owners"; but it proved to be way too long and bordering on TL;DR territory. Instead I thought I would break it up into more bite-sized chunks.

[Gyvate]

I admire your dedication to the topic 

[gszlag]

I think this might fall into a specialist definition: "someone who knows a lot about something but nothing about anything else."

I am still far behind the number of posts about haptic rain gauges. I will try to fill the gap.

[TheBushPilot]

I think this might fall into a specialist definition: "someone who knows a lot about something but nothing about anything else."

Boy if that ain't the truth. You and me both.

Your discussions have been incredibly helpful even if I don't own an Ecowitt. I won't speak on behalf of others but I enjoy the longer write ups. So thank you.

Cheers