Sea-level pressure reduction
Three pressure terms:
| Term | Meaning | Cite |
|---|---|---|
| Station pressure | What barometer reads at station's own elevation | FAA AWH 8.2.3 |
| Sea-level pressure (SLP) | Station pressure reduced to sea level using day's actual temperatures, so stations at different heights compare | FAA AWH 8.2.5 |
| Pressure altitude | Altitude read with altimeter set to 29.92" (height that pressure has in the ISA; see pressure-altitude) | FAA AWH 8.4.1.4 |
- FAA calls the value dialed in the altimeter setting. QNH = ICAO label for it (NAV CANADA AIP GEN 12.3.1.8).
- Canada reports no QFE (NAV CANADA AIP GEN 4.6.7).
SLP vs altimeter setting (QNH):
| Term | Reduced with | Cite |
|---|---|---|
| SLP | Actual temperature | TC AIM MET 8.1 |
| Altimeter setting (QNH) | ISA | TC AIM MET 8.1 |
- Close, not identical: can disagree.
Station pressure to SLP:
- Standard rate: 1 inHg per 1000 ft (see atmosphere) (TC AIM AIR 1.5.3; worked figure FAA AWH 8.2.5).
- SLP ≈ station pressure +
elevation ÷ 1000inHg. - Example: station at 5000 ft reads 25" station pressure. 5000 ÷ 1000 = 5. SLP ≈ 25 + 5 = 30".
Trap: station pressure is the raw reading; SLP (actual temperature) is not the altimeter setting (ISA) and can disagree.
References
- TC AIM AIR 1.5.3
- TC AIM MET 8.1
- FAA Aviation Weather Handbook (FAA-H-8083-28) 8.2.3
- FAA Aviation Weather Handbook (FAA-H-8083-28) 8.2.5
- FAA Aviation Weather Handbook (FAA-H-8083-28) 8.4.1.4
- NAV CANADA AIP GEN 4.6.7
- NAV CANADA AIP GEN 12.3.1.8