Understanding QNH: how the altimeter setting based on sea level pressure keeps you at the right altitude

What does "QNH" indicate in aviation weather reporting?

• A. Wind pressure at flight level
• B. Altimeter setting based on sea level pressure
• C. Temperature at ground level
• D. Visibility conditions

Answer: (B)

In aviation weather reporting, "QNH" specifically indicates the altimeter setting that is adjusted to sea level pressure. This measurement is critical for pilots, as it allows them to calibrate their altimeters correctly, ensuring that they have the most accurate altitude information relative to sea level. When a pilot sets their altimeter to the QNH value provided by weather reports or ATC, they are able to fly at the correct altitude above the ground, which is especially important for safety during takeoff and landing. Using QNH is vital in aviation as it helps in maintaining appropriate vertical separation between aircraft and avoiding terrain obstructions. An accurate understanding of altimeter settings ensures pilots know their aircraft's altitude in relation to mean sea level, which is essential for safe navigation and landing procedures. This is why B is the correct answer.

What is QNH, exactly? A simple, essential piece of aviation weather you’ll hear in weather reports, every flight. If you’ve ever scanned a METAR and spotted the letters QNH, you’re not alone. Let me break it down in a way that sticks, with enough detail to feel credible but not so much that it becomes a flight manual.

QNH: the short version

QNH is the altimeter setting based on mean sea level pressure. In plain terms, it’s the pressure value you dial into the aircraft’s altimeter so that, when the aircraft is on the ground at sea level, the readout shows zero altitude. When you set QNH, the altimeter’s readings align with the level of the sea, not with the terrain beneath your wheels or the airfield’s ground elevation.

If you want to contrast it with related terms, think of:

• QFE: the altimeter setting that makes the altimeter read zero at the airfield’s elevation. It’s altitude above the airfield, not above mean sea level.

• QNE: the standard pressure setting (usually 1013 hPa or 29.92 inHg) used at higher altitudes, regardless of local sea-level pressure.

So, QNH ties the pressure you read in the cockpit to mean sea level. It’s a small setting, but it has a big impact on what you actually see on the altimeter.

Why sea level pressure matters in the cockpit

Here’s the thing: air pressure falls as you climb. The further you rise, the lower the pressure you sense. The altimeter translates that pressure into a height, but to be meaningful for navigation, you need a reference point. That reference point is mean sea level.

When you set QNH, the altimeter is calibrated so that your altitude readout corresponds to your height above mean sea level, not your height above the terrain. That distinction matters because ATC assigns altitudes in feet (or meters) above mean sea level. If your altimeter is mis-set, your displayed altitude won’t match your actual height above sea level, and that creates a misleading picture of vertical separation from other aircraft and from terrain.

In practice, you’ll see QNH printed in METARs alongside wind, visibility, cloud, temperature, and dewpoint. It’s a weather value, but its effect is mechanical: it’s the knob you turn to keep your altitude information honest. Pilots use QNH to ensure they’re at the correct altitude for takeoff, cruise, and especially for approach and landing where terrain and obstacles become a real concern.

A quick way to picture it

Think of the altimeter like a height readout on a car’s odometer, but instead of miles driven, it’s height above sea level. If you want the car analogy to be even clearer, imagine you’re driving in a region with hills. If your speedometer were actually telling you your height in the hills rather than your speed, you’d need a fix to read correctly. QNH is that fix for altitude: it makes the altitude tell the truth about how high you are above the ocean, regardless of the land below.

Where you’ll encounter QNH in the real world

• METARs and ATIS: QNH is routinely included in weather broadcasts for airports. It tells pilots what pressure setting they should use to get an accurate altitude reading upon approach or departure.

• ATC coordination: Air traffic controllers reference QNH when giving altitude instructions. If you’re climbing or descending, you want your altimeter to reflect your true height above mean sea level to maintain safe separation from terrain and other aircraft.

• Cockpit practice in different regions: The units used can vary. In many parts of the world, QNH is given in hectopascals (hPa). In the United States and a few other places, you’ll also see inches of mercury (inHg). It’s the same idea, just in different measurement systems.

A small, practical nuance

Altitude readings need to be trustworthy to prevent collisions with terrain or with other aircraft, especially in poor visibility or busy airspace. If you fly into a region where the sea-level pressure is unusually high or unusually low, that value changes your altimeter setting. With the wrong QNH, you might think you’re at a safe altitude when you’re actually closer to the mountain tops or simply too low above the runway for a precise approach.

That’s why a good pilot habit is to verify and update QNH promptly when weather shifts. In a fast-changing system, QNH can drift, and a stale setting is a recipe for altitude ambiguity. You don’t want to be “thinking” you’re at one height while you’re really at another. The risk isn’t theoretical—modern cockpits rely on accurate pressure settings to preserve safe vertical separation.

How QNH differs from other settings in practice

• QNH vs QFE: If you’re on the ground at the airfield and you dial QFE, your altimeter reads zero. That’s convenient for precise height above the airport’s elevation. But the moment you take off, that zero doesn’t tell you much about your height above sea level, which ATC uses for safe separation. QNH keeps everybody aligned on sea level references, which is what national and international airspace geometry depends on.

• QNH vs standard pressure (QNE): When you fly at very high altitudes or in airspace where standardization is enforced (like flight levels above certain transition levels), you switch to QNE, typically 1013 hPa or 29.92 inHg. The altimeter then reads height relative to a fixed standard pressure rather than local sea-level pressure. This simplification helps en route separation, but you’ll still need to know how local weather is changing when you descend toward an airport.

Real-world effects you can feel

• Takeoff in a high-tep terrain area: If the airport sits in a bowl with mountains nearby, accurate QNH helps you ensure you’re not still “too low” once you turn toward a lateral path. Even a small mis-match can shorten your margin before you reach safe climb gradients.

• Landing in changing weather: A front moving in can push sea-level pressure up or down. If you’re landing with the wrong setting, your approach altitude might be off, affecting the descent angle and the threshold crossing height.

• Mountain flying and island approaches: In rugged terrain or island airports, the difference between true altitude and indicated altitude becomes more noticeable because the terrain shapes the risk profile. QNH helps you keep the vertical picture honest.

Common misconceptions

• QNH is not a mystical number that only weather nerds care about. It’s practical. It’s the value you set to ensure your altitude reads correctly relative to sea level, which is what air traffic control expects and terrain clearance depends on.

• QNH is not the same as the airport elevation. They are connected, but different references. QNH ties your altitude to mean sea level; the airport elevation is a fixed ground height. The two interact through the altimeter’s setting, and the interplay matters most during approach and landing.

A few quick, memorable takeaways

• QNH = altimeter setting tied to sea level pressure, so altitude reads AMSL (above mean sea level).

• QFE = altitude above the airfield (airport elevation) at zero on the altimeter.

• QNE = standard pressure setting (1013 hPa / 29.92 inHg) used at higher, non-local altitudes.

• In practice, pilots update QNH to reflect local pressure changes and ensure the altimeter shows the true height above sea level, which keeps terrain and traffic separation honest.

A practical mental model to carry forward

Picture the cockpit as a cockpit-sized weather detective. The altimeter doesn’t know the terrain or the weather by itself; it needs a reference—QNH—to translate pressure into height. When you set QNH, you’re telling the instrument, “Use today’s sea level pressure as the baseline.” With that baseline in place, every lift-off, every turn, and every landing reflects a height that makes sense against the world you’re flying over.

If you’re the kind of student who appreciates a touch of realism, here’s a tiny aside: modern airliners and many light aircraft rely on digital displays that continually cross-check pressure data with GPS and other sensors. Still, the venerable altimeter remains the anchor. It’s a reminder that even in a high-tech cockpit, something as simple as “set the right QNH” can make a world of difference in safety and precision.

Putting it into practice (without turning this into a manual)

• When you read a METAR, notice the QNH value. If you’re planning a cross-country leg or an approach, you’ll want to have that value ready to dial in as you prepare for the next phase of flight.

• If you fly to unfamiliar airports, take a moment to compare QNH across neighboring fields. Local pressure differences can tell you a little about the terrain and weather patterns you’ll meet.

• In training flights or simulations, practice the habit of updating QNH with every new weather update. It’s a small discipline with big payoff in accuracy and safety.

A final thought

QNH isn’t glamorous, but it is foundational. It’s one of those “invisible threads” that holds the fabric of safe flight together: the knowledge that your altitude is what you think it is, relative to mean sea level, even as the weather outside changes and the terrain rises or falls around you. That is the quiet power of QNH.

If you want to keep digging, you can explore METARs, aviation weather resources, and cockpit manuals from reputable sources like aviation weather services and flight training libraries. They’ll show you actual examples of QNH in action, from the little numbers on a weather card to the way an altimeter’s needle dances as a plane climbs through a front. The more you see QNH in real life, the more intuitive this part of weather lore becomes—and the safer your flight planning and execution will feel.