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I've been listening to a lot of liveatc.net recently (mostly at CYVR).

What does it mean when the ATC give an "altimeter" reading (usually around the 3000 area).

Altimeter three-zero-one-one.

I'm assuming it's like a QNH setting? But not in kPa?

Addon Question:

Is inches of mercury specific to the Americas? North America or other places as well? I've only flown in South Africa where we use kPA (SI). We'd get information from tower along the lines of QNH 1013.

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    $\begingroup$ The units are inches of mercury. $\endgroup$ – Steve Sep 23 '16 at 15:16
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    $\begingroup$ U.S. and Canada use inches, rest of the world hectopascals/millibars. $\endgroup$ – ymb1 Sep 23 '16 at 15:33
  • $\begingroup$ Related and this one too, especially to your question about where inches are used $\endgroup$ – Pondlife Sep 23 '16 at 15:46
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    $\begingroup$ QNH 1013 is in hPa (not kPa). Generally we prefer to use 10^3 steps (10^-3/milli, 10^0/unit, 10^3/kilo, 10^6/mega, 10^9/giga, etc) but here 1013 hPa uses the hecto (10^2) prefix instead of 101.3 kPa just because many countries previously used the non SI unit bar, especially the millibar. It was easier to ask people to switch from 1013 mb to 1013 hPa, rather than to 101.3 kPa. For people using Hg inches, moving to the SI unit is more difficult. $\endgroup$ – mins Sep 23 '16 at 16:54
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    $\begingroup$ @ymb1, many places in South America also use inHg. So the limit seems to be the Americas. $\endgroup$ – Jan Hudec Sep 24 '16 at 17:19
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That is a QNH altimeter setting in inches of mercury, with the decimal omitted. So in the case given, that is a QNH of 30.11.

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    $\begingroup$ But the inch is supposed to be subdivided into powers of 2 rather than decimally, isn't it? So shouldn't it be $30\frac{7}{64}''$ instead of $30.11''$? $\endgroup$ – hmakholm left over Monica Sep 23 '16 at 18:01
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    $\begingroup$ @HenningMakholm No, never seen that convention in aviation in the US (or elsewhere, for that matter). Every altimeter I've used has had decimal settings. Some altimeters have both mb and inches, i.e. two scales that move simultaneously when the adjustment knob is turned. But I don't think I've ever seen one that worked in x/64" fractions. Nor heard it reported that way. $\endgroup$ – Ralph J Sep 23 '16 at 18:08
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    $\begingroup$ @LightnessRacesinOrbit: My measuring tape has centimeters on it. $\endgroup$ – hmakholm left over Monica Sep 23 '16 at 18:37
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    $\begingroup$ @HenningMakholm: Then find one that has inches on it. thetapestore.co.uk/media/wysiwyg/tape-with-markings-small.jpg $\endgroup$ – Lightness Races in Orbit Sep 23 '16 at 18:37
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    $\begingroup$ @HenningMakholm - when measuring a distance with a ruler or tape marked in inches, the inches will commonly be divided into powers-of-two units because this is traditional. However, when setting "inches of mercury" on an altimeter the setting will be in whole inches plus decimal partial-inches, e.g. "30.11". $\endgroup$ – Bob Jarvis - Reinstate Monica Sep 23 '16 at 19:14
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It is the barometric pressure at the airfield, in inches of mercury. It is to two decimals, so you got 30.11 inches of mercury. It is the same as QNH (which is the airfield pressure adjusted to sea level). From flightsim.com

... (it) is the barometric pressure, given in inches of mercury. If you look on a standard wall barometer, you'll see that it equates to "fair". In the UK, we adopted SI units (hectopascals/millibars) some years ago, and 29.92 inches is the same as 1013mb. Anything above this value is regarded as "high" pressure, and anything below it as "low". In the days before weather forecasts were dumbed down, you'd often see synoptic charts with pressure values on them.

In Europe, again, we don't say "altimeter", instead we use what are known as "Q codes", that go back to the early days of Radio Telephony and Morse Code. So instead of hearing "altimeter 2992", in the UK you would hear "QNH 1013". Effectively it means the same thing.

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  • $\begingroup$ Is is not the pressure at the airfield unless the airfield is at sea level. As I write this, Telluride Regional (KTEX) in CO with a field elevation of 9069 ft MSL has an altimeter reading of 30.16 inHg. The real pressure onsite is closer to 21.09 inHg. The same thing applies to IFR aircraft cruising in the flight levels and using a seal level STP altimeter setting of 29.92 inHg, when the outside air pressures are less than half that value. $\endgroup$ – Carlo Felicione Sep 23 '16 at 17:14
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    $\begingroup$ At least on amateur radio, with a few exceptions, using Q-codes on phone is discouraged. Basically, it's okay to state that your QTH is Springfield, but it's generally not recommended to ask for the QTR (because it's usually much easier to ask for the current time) or state that QSQ Bart Simpson (because it's usually easier to state that Bart is onboard or at the station). $\endgroup$ – a CVn Sep 23 '16 at 19:43
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That refers to an altimeter adjustment to compensate for local atmospheric variations in pressure. The reading given is what the atmospheric pressure would be on the field if it were at sea level in inches of Mercury (inHg). This is most often used in adjusting a sensitive barometric altimeter in an aircraft so that it reads an altitude which is within +/- 75ft of the actual value. These sensitive barometric altimeters are required equipment on IFR flights and much more useful on VFR flights as well.

ATC often give an altimeter readout when handing aircraft over to another ARTCC or approach controller, generally as a courtesy to the pilot and, in the case of IFR flights, to ensure the flight crew is using the correct altimeter settings. Other cases include at controllers at towered airports, in between the hourly ATIS broadcast update, when there has been a noticeable and fast change in the local barometric pressure at the field.

Off field and bush flying operations can make use of the same information in the form of portable weather stations such as Kestrel's series of handheld weather stations which provide barometric pressure readouts. Pilots also will just set their altimeters to the established field elevation listed on charts in lieu of an altimeter readout as well. The altimeter should then read to within +/-75 ft of the correct altitude, provided the unit was serviced and inspected within 24 calendar months of the flight.

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Altimeter settings are usually QNH (pressure at sea level), but they're occasionally QFE (pressure at field elevation).

The international standard for pressure is hectopascals (hPa), which is indicated by a Q in METARs. The US and some other countries use inches of mercury (inHg), which is indicated by an A in METARs. They're also in separate numerical ranges, e.g. Q1013 = A2992, so you can easily deduce which is being used even without that hint.

Many aircraft (particularly ones expected to make transoceanic flights) will have altimeters that can be set using either unit; for those that don't, pilots will need to carry a conversion table when flying in areas that use units not matching their equipment.

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