QNH Question

[fonawah]

Just a curiosity.. hope you don't mind my asking.
I was flying in the Leeds area on Sunday and the Leeds QNH was 1030..In my (short and inexperienced) flying time, I have never had such a high QNH. I just would like to know from those of you out there with several more hrs than me, what are the highest & lowest QNH values you have come across?
Many thanks.
F

[ST4G]

As for the lowest just ask Mr Fish, late of the BBC, for the lowest.
From my aged memory a range of 990 to 1035 mb. (Approx)

Just for interest, British altimeters used to have a range of 800 to 1050 mb. Don't know if they still do.

ST4G

[Flypuppy]

I remember there was an Atlantic storm in '93 or '94 where the pressure dropped to below 940mb (it may even have gone lower).

Anyone got any better memory of that?

[Jinkster]

1013 tends to be quite common ;)

[Notso Fantastic]

In Alaska, it is not tremendously rare for the QNH to rise above 1050. I was there once when the authorities were close to grounding all aviation as it meant altimeters were overreading as the maximum setting was usually 1050. Coincidental with such weather patterns, it gets diabolically cold, presumably exacerbating the altimeter indication problem.

[HEATHROW DIRECTOR]

I've certainly seen in excess of 1040..

[fireflybob]

Take a look in the Guiness Book of Records - max/min values of atmospheric pressure are there together with other interesting records - maybe they have a website too?

[fonawah]

:D
Thanks everyone for your replies.

[slim_slag]

Sec. 91.144 Temporary Restriction on Flight Operations During Abnormally High
Barometric Pressure Conditions.

(a) Special flight restrictions. When any information indicates that
barometric pressure on the route of flight currently exceeds or will exceed
31 inches of mercury, no person may operate an aircraft or initiate a flight
contrary to the requirements established by the Administrator and published
in a Notice to Airmen issued under this section.
(b) Waivers. The Administrator is authorized to waive any restriction
issued under paragraph (a) of this section to permit emergency supply,
transport, or medical services to be delivered to isolated communities, where
the operation can be conducted with an acceptable level of safety.


Federal Register Notes

59 FR 17450, No. 70, Apr. 12, 1994

SUMMARY: This final rule amends part 91 of the Federal Aviation Regulations
(FAR) to provide for the issuance of temporary flight restrictions on certain
operations when accurate altitude information is not available. The rule is
warranted because barometric pressure higher than 31.00 inches of mercury
(inHg) (1049.8 millibars) exceeds the capability of standard aircraft
pressure altimeters and prevents the display of accurate altitude
information. The rule provides restrictions on certain flight operations
during periods of abnormal atmospheric pressure conditions and is necessary
to promote flight safety during certain operations for which accurate
altitude information is critical.

EFFECTIVE DATE: May 12, 1994.

[JantinusDaling]

I can add to the last message that according to the British Aviation Authorities an altimeter should have a subscale setting of 800 - 1050 mBar and according to JAR 950 - 1050 mBar.

If you want to check out local lows and highs you can also examine the significant weather charts.

Real low pressures are rare. For example I added a table with the pressures in the 5 different classifications of hurricanes.

Saffir-Simpson Maximum sustained Minimum surface
Category: wind speed (mph) pressure (mb)

1 74-95 mph >= 980mb
2 96-110 979-965
3 111-130 964-945
4 131-155 944-920
5 > 155 < 920

This must explain why we are not allowed to fly outside the range of the subsetting of the altimeter!

Hope to have helped you with your question.

[767bill]

Hi Fonowah,

Seems like you've had the highest and lowest. All I'll say is that like you I have limited flying experience but remember having set above 1030 on a number of occasions, including last Saturday, 1033 I think, of course you may be doing that less over Leeds than we do over S-E England !!! (but that's just my opinion of regional weather differences).

[PPRuNe Radar]

The lowest recorded around the British Isles is 925Mb with the highest being 1050Mb.

(According to a sailing book I have)

[reynoldsno1]

938 was the lowest I can recall when on a SAR call-out in mid-Atlantic.... 'twas very windy, and there was a lot of coffee dripping off the cabin roof.....

[B Fraser]

Each winter, there are normally a few North Atlantic systems that have a central pressure drop to around 950 millibars. I used to hate drawing up the charts and referred to those systems as zebra's :mad: rseholes.

:}

[G-ANDY]

Last Saturday and Sunday, the QNH at Gloucestershire Airfield was 1036mb.

Does change in pressure have an effect upon us humans?? If the pressure is realllly low, do lots of people get headaches??

G-ANDY

[Pub User]

I recall a particularly windy day at MPA, when the QFE was 932, which would equate to a QNH of around 940.

[parkfell]

The highest QNH I can recall was 1049mbs, in 1992/3 [?] when I was working at Prestwick.

A siberian high was dominating the winter weather at that time with the pressure rising continuously. It reached 1049 for a few hours before declining.

Lowest QNH - probably 925mbs at Glasgow in 1968 - also in winter.

:cool: