What is the difference between QNH, QFE and 1013?
On the ground, with QFE set the altimeter reads zero, with QNH it reads the aerodrome elevation and with 1013.25 hPa it reads a flight level.
A student's question, answered from the rotatestudy 050 notes.
Three settings, one altimeter
- QFE. The pressure at the aerodrome reference level. Set on the subscale, the altimeter reads zero on the ground.
- QNH. QFE reduced to mean sea level using the standard atmosphere. Set on the subscale, the altimeter reads the aerodrome elevation on the ground.
- 1013.25 hPa, the standard setting. Set on the subscale, the altimeter reads a flight level. A fourth name, QFF, is QFE reduced to sea level using actual conditions: it is used on surface analysis charts, not in aviation.
- Which is higher. At an aerodrome above sea level, QNH is higher than QFE: pressure falls with height; reducing it to sea level adds the difference back. Near the surface pressure drops about 1 hPa per 27 ft. In the exam, use the figure the question gives, or 30 ft per hPa when it gives none.
- When to switch. The transition altitude is the altitude above which the standard setting is used, in the climb. The transition level is the lowest flight level usable above it: QNH is reset there, in the descent. No level flight is conducted in the transition layer between the two. The lower the QNH, the higher the transition level.
- Apply it. You climb through 6 000 ft and the transition altitude is 5 000 ft. You are above it, so the altimeter is on 1013, not on QNH and not on QFE.
- Why it matters. From high to low, look out below: flying towards lower pressure or colder air, your true altitude is below the indicated altitude. At ISA − 30 and 3 000 ft, the temperature correction already reaches about 360 ft.
Sources: the rotatestudy 050 notes, § 1.3 (pressure and density) and § 1.5 (altimetry). Appendix E of the notes lists the texts they were checked against and the verification date.
The ten free questions below are from 050 and cover the whole subject. Two of them, the lapse rate and an ISA deviation, are on the standard atmosphere.
Ten free 050 Meteorology questions
No account needed.
Pick an answer and the explanation opens.
The notes come with 1 month of all 447 questions and the mock exams.
1Which of the following processes can cause the air temperature to decrease?
1. Adiabatic cooling
2. Expansion of air
3. Radiation cooling
4. Greenhouse effect
5. Conduction coolingShow the answer
Answer A. Rising air expands as the surrounding pressure falls and cools adiabatically, so statements 1 and 2 describe the same cooling mechanism. At night the ground loses heat by long-wave radiation, and the air in contact with it then cools by conduction, which covers 3 and 5. The greenhouse effect is the odd one out: water vapour and CO2 absorb outgoing infrared and slow the cooling, so it warms rather than cools.
2In the Northern Hemisphere, a pilot flying at 1000 ft AGL straight towards the centre of a LOW pressure area will most likely find the wind blowing from...
Show the answer
Answer B. Around a low in the northern hemisphere the air turns anticlockwise, and friction at 1000 ft adds a component across the isobars towards the centre. Flying straight at the centre, the circulation puts the wind on the aircraft's left, and the inflow towards the centre gives it a component from behind. Buys Ballot's law says the same thing: with your back to the wind, low pressure is on your left.
3If a parcel of humid air is forced to rise from the ground in summer, (1) ___ may occur and unstable conditions for (2) ___ are created.
Show the answer
Answer D. Humid air forced to rise cools, reaches its dew point and the vapour condenses into cloud. Condensation releases latent heat, which warms the parcel relative to its surroundings and makes it more buoyant. That is how a moist summer air mass becomes unstable and builds convective cloud; sublimation plays no part in this.
4Which option lists only main cloud genera that indicate an unstable atmosphere?
Show the answer
Answer A. Instability shows itself in heaped, convective cloud at whatever level it occurs: cumulus low down, altocumulus in the middle, cirrocumulus high up. The layered forms (stratus, altostratus, cirrostratus, nimbostratus) are the signature of stable air spreading out instead of rising, so any list containing one of them is wrong.
5A trough is an area of (1) ______ causing widespread (2) ______, with (3) ______ along the line of the trough.
Show the answer
Answer A. In a trough the isobars bend out from a low, and the surface flow converges along its axis. Air that converges at low level has to go up, so the trough is a line of lifting, often marked by cumuliform cloud, showers or squalls. Divergence and descent are the features of a ridge or high, which gives clear skies and light winds.
6Which statement about an aircraft being struck by lightning is true?
Show the answer
Answer C. The flash of a nearby strike, especially at night, can dazzle the pilots for several seconds, making the instruments hard to read, so the crew may briefly struggle to determine the aircraft's attitude. That is why cockpit lights are turned fully up near thunderstorms. A metal airframe acts as a Faraday cage and protects the occupants better than a non-conductive one, not worse, and a charged aircraft can itself trigger a discharge.
7Can TREND and TAF reports be found in scheduled HF VOLMETs and continuous VHF VOLMETs?
Show the answer
Answer B. VOLMET broadcasts carry the METARs or SPECIs of a group of aerodromes, each followed by its TREND forecast when one is issued, on both the scheduled HF and continuous VHF services. TAFs and SIGMETs may be added to HF VOLMET only when regional air navigation agreements provide for it, so they are not a standard content of both. Nothing prohibits trend forecasts in either broadcast.
8In the standard atmosphere, what is the temperature lapse rate in the troposphere?
Show the answer
Answer A. ISA temperature falls by 6.5 °C per km, which is 1.98 °C per 1000 ft, rounded to 2 °C per 1000 ft, from +15 °C at mean sea level up to the tropopause at 36 090 ft. The tempting -0.65 °C is the same lapse rate expressed per 100 m, not per 1000 ft.
9At FL370 the OAT is -45°C. What is the ISA deviation?
Show the answer
Answer D. The ISA tropopause is at 36 090 ft, so FL370 is already in the lower stratosphere where the standard temperature is constant at -56.5 °C. The deviation is -45 - (-56.5) = +11.5 °C. Extrapolating the 2 °C per 1000 ft lapse rate to FL370 gives -59 °C and the +14 trap.
10In which of the following changes of state is latent heat absorbed?
1. Evaporation
2. Condensation
3. Sublimation
4. Deposition
5. Freezing
6. MeltingShow the answer
Answer A. Evaporation, sublimation and melting all move water to a more energetic state, so they absorb latent heat and cool their surroundings. Condensation, deposition and freezing are the reverse changes and release that heat. Any option containing 2, 4 or 5 therefore mixes in a heat-releasing process.
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