Atmospheric Pressure: IC Joshi Chapter 2, DGCA Notes and Practice Questions
Atmospheric Pressure is a High-weight chapter, and it's where the numericals live. QNH, QFE, QFF, pressure altitude, density altitude and altimeter errors turn up in almost every DGCA Meteorology paper. Get the definitions exact, understand why pressure behaves as it does with height and temperature, and practise the arithmetic until it's automatic.
What DGCA asks from this chapter
- Definitions. Height, altitude, elevation and flight level, and the four Q codes: QFE, QNH, QFF and QNE.
- Numericals. QFE to QNH, pressure altitude, true altitude after a pressure change, density altitude, and the effect of a wrong subscale setting.
- Altimeter errors. Whether the aircraft is higher or lower than indicated, and by how much.
- Transition procedures. Transition altitude, transition level, the transition layer, and how QNH moves the transition level.
- Pressure behaviour. How pressure falls with height and why temperature changes the rate, the semi-diurnal variation, pressure tendency, and the shapes on a pressure chart.
Pressure: what it is and how it's measured
- Static pressure is the weight of the column of air above a point, per unit area, acting equally in all directions. It's what a barometer and an altimeter measure.
- Dynamic pressure is the extra pressure from air in motion against a surface, ½ρV². The pitot tube senses total pressure (static plus dynamic). The airspeed indicator subtracts static to get dynamic.
- Units. Pressure is reported in hectopascals (hPa), equal to millibars (mb). 1013.25 hPa equals 29.92 inches of mercury and 760 mm of mercury.
- Instruments. The mercury barometer is the standard reference. The aneroid barometer uses a sealed, partly evacuated capsule and is the principle behind the altimeter. The barograph records pressure against time. See Chapter 23, Met Instruments.
ISA values worth knowing cold
| ISA value | Figure |
|---|---|
| Mean sea level pressure | 1013.25 hPa |
| Mean sea level temperature | +15°C |
| Mean sea level density | 1.225 kg/m³ |
| Temperature lapse rate to the tropopause | 1.98°C per 1,000 ft (6.5°C per km) |
| Tropopause | 36,090 ft (11 km), −56.5°C |
Vertical variation of pressure
Pressure falls with height, quickly at first and then more slowly, because the air is compressed by its own weight and is densest near the ground. Near mean sea level it falls by about 1 hPa for every 27 ft. This is the figure to use in DGCA calculations. Some books round it to 30 ft to make the arithmetic easier.
| Pressure level | Approximate ISA height | Approximate ft per hPa |
|---|---|---|
| 1013 hPa | Mean sea level | 27 |
| 850 hPa | 5,000 ft | 32 |
| 700 hPa | 10,000 ft | 37 |
| 500 hPa | 18,000 ft | 50 |
| 300 hPa | 30,000 ft | 73 |
| 200 hPa | 39,000 ft | 105 |
The heights are standard pressure levels used on upper-air charts, rounded. Note that pressure halves at roughly 18,000 ft.
Why temperature changes the rate
Cold air is denser than warm air, so a column of cold air is shorter than a column of warm air holding the same weight of air. That means pressure falls faster with height in cold air and more slowly in warm air. Two consequences matter for the exam:
- In cold air, a given pressure level, and so a given altimeter reading, sits lower than in ISA. In warm air it sits higher. That's the temperature error of the altimeter.
- On an upper-air (constant pressure) chart, high contour heights mark warm columns of air and low contour heights mark cold ones. Pressure systems therefore tilt with height: a cold-core low strengthens with height, and a warm-core high strengthens with height.
Pressure changes over time
Semi-diurnal variation
Even with no weather system moving through, surface pressure rises and falls twice a day, because of atmospheric tides driven by solar heating. It peaks around 10:00 and 22:00 local time and dips around 04:00 and 16:00. The swing is up to about 3 hPa in the tropics ⚑, and almost nothing at high latitudes, so it matters across India. In the tropics, the daily variation is often larger than the change caused by weather systems. A fall in pressure that goes against the daily pattern, or is larger than it, can be the first sign of a developing depression.
Pressure tendency
The change in surface pressure over the last three hours is the pressure tendency, reported in synoptic observations. Lines joining places with equal tendency are isallobars. A steady, rapid fall usually means an approaching depression or trough. A steady rise means a ridge or high building in.
Pressure patterns
Surface pressure charts plot QFF (pressure reduced to sea level), with lines of equal pressure called isobars, usually drawn every 2 or 4 hPa.
| Pattern | Features | Typical weather |
|---|---|---|
| Low (depression) | Closed isobars, lowest pressure in the centre. Air converges at the surface and rises. | Cloud, precipitation, stronger winds. Anticlockwise circulation in the northern hemisphere. |
| High (anticyclone) | Closed isobars, highest pressure in the centre. Air subsides and diverges at the surface. | Usually settled. Subsidence can cause an inversion, trapping haze, smoke and fog. Clockwise in the northern hemisphere. |
| Trough | A V-shaped extension of low pressure. | Similar to a low, often with a line of cloud and showers along its axis. |
| Ridge | A U-shaped extension of high pressure. | Similar to a high, usually fair. |
| Col | The neutral area between two highs and two lows. | Light, variable winds. Fog in winter, thunderstorms in summer. |
The pressure gradient is the rate of change of pressure with horizontal distance, measured at right angles to the isobars. Isobars close together mean a steep gradient and strong winds. That's the link to Chapter 6, Winds.
Height, altitude, elevation and flight level
| Term | Measured from | Altimeter setting |
|---|---|---|
| Height | A specified datum, usually the aerodrome | QFE |
| Altitude | Mean sea level | QNH |
| Elevation | Mean sea level, for a point or level on the ground | None. It's a fixed property of the place. |
| Flight level | The 1013.25 hPa pressure surface, in hundreds of feet | 1013.25 hPa (standard) |
| Pressure altitude | The 1013.25 hPa pressure surface | 1013.25 hPa. FL 75 means a pressure altitude of 7,500 ft. |
The four Q codes
| Code | What it is | On the ground, the altimeter reads |
|---|---|---|
| QFE | Pressure at aerodrome level (officially, at the runway threshold or aerodrome reference point) | Zero |
| QNH | Aerodrome pressure reduced to mean sea level using the ISA temperature | Aerodrome elevation |
| QFF | Aerodrome pressure reduced to mean sea level using the actual temperature. Plotted on synoptic charts, not set on an altimeter. | Not used for altimetry |
| QNE | The standard setting, 1013.25 hPa, set above the transition altitude so the altimeter reads flight levels. Strictly, in the Q code, QNE is the altimeter reading on the ground at an aerodrome with 1013.25 set, which is the aerodrome's pressure altitude. Both uses appear in exam questions. | With it set on the ground, the altimeter reads the aerodrome's pressure altitude. |
QNH versus QFF: which is bigger?
Both reduce aerodrome pressure to sea level, and the only difference is the temperature assumed for the imaginary column of air between the aerodrome and sea level. This makes a popular technical question:
- Colder than ISA: the actual column is denser, so pressure increases faster going down. QFF is greater than QNH.
- Warmer than ISA: the column is less dense. QFF is less than QNH.
- ISA conditions, or an aerodrome at sea level: QFF equals QNH.
Transition procedures
- Transition altitude: at or below it, vertical position is given as altitude on QNH. Climbing through it, set 1013.25 hPa.
- Transition level: the lowest flight level available above the transition altitude. Descending through it, set QNH.
- Transition layer: the airspace between the two. Level flight within it isn't normally allowed.
The transition altitude is fixed and published. The transition level moves with QNH. When QNH is low, flight levels sit lower in true altitude, so the transition level has to be raised to keep the transition layer at least the required depth. The lower the QNH, the higher the transition level.
The depth of the transition layer itself also depends on the QNH:
| QNH | Transition layer depth |
|---|---|
| 1013 hPa or above | 1,000 ft |
| 995 to 1013 hPa | 1,500 ft |
| Below 995 hPa | 2,000 ft |
Altimeter errors
| You fly from | Without resetting, the altimeter | So the aircraft is |
|---|---|---|
| High pressure to low pressure | Over-reads | Lower than indicated |
| Low pressure to high pressure | Under-reads | Higher than indicated |
| Warm air to cold air (colder than ISA) | Over-reads | Lower than indicated |
| Cold air to warm air (warmer than ISA) | Under-reads | Higher than indicated |
Subscale setting errors
Turning the subscale up increases the altimeter reading, at 27 ft per hPa. If the correct QNH is 1005 but 1015 is set, the altimeter reads 270 ft too high, so the aircraft is 270 ft lower than indicated. A higher setting than the true value always makes the aircraft lower than it thinks.
Worked numericals
1. QFE to QNH
An aerodrome has an elevation of 540 ft and a QFE of 995 hPa. At 27 ft per hPa, 540 ft is 20 hPa. QNH is higher than QFE for an aerodrome above sea level, so QNH = 995 + 20 = 1015 hPa.
2. Pressure altitude
The same aerodrome, with QNH 1003 hPa. The difference from standard is 1013 − 1003 = 10 hPa, which is 270 ft. Lower pressure means a higher pressure altitude, so pressure altitude = 540 + 270 = 810 ft. With the standard setting (QNE, 1013.25 hPa) set on the ground, the altimeter reads 810 ft, the aerodrome's pressure altitude.
3. True altitude after a pressure change
An aircraft departs with QNH 1013 set and holds 3,000 ft on the altimeter into an area where QNH is 993. The altimeter is now over-reading by 20 × 27 = 540 ft. True altitude ≈ 2,460 ft.
4. Density altitude
Add about 120 ft to the pressure altitude for every 1°C the temperature is above ISA ⚑. An aerodrome has a pressure altitude of 2,000 ft and a temperature of +36°C. ISA at 2,000 ft is 15 − 4 = 11°C, so it's ISA +25. Density altitude ≈ 2,000 + (25 × 120) = 5,000 ft. The aircraft performs as if it were at 5,000 ft: longer take-off run and poorer climb. See Chapter 4, Air Density.
Common traps
Practice questions
Three questions from the 295 on this chapter. Each one cites the book it comes from.
1. When flying from a region of high pressure into a region of lower pressure at a constant indicated altitude, what happens to the aircraft's true altitude?
Show answer
Answer: D. True altitude decreases
One of the most significant hazards in altimetry arises from variations in sea-level pressure along the route of flight. When an aircraft flies from a region of high pressure into a region of lower pressure while maintaining a constant indicated altitude (with no change to the altimeter sub-scale setting), the actual pressure at any given geometric height is lower than the altimeter assumes. As a result, the altimeter over-reads — it indicates a higher altitude than the aircraft is actually at — and the true altitude of the aircraft is therefore less than the indicated altitude. This phenomenon is captured by the well-known adage 'from high to low, beware below.' The danger is particularly acute in mountainous terrain, where reduced terrain clearance due to this effect can be catastrophic if the pilot does not update the altimeter setting regularly along the route.
Source: Aviation Meteorology — IC Joshi · Chapter 2 — Atmospheric Pressure · Page 79
2. An aircraft flying from a warm airmass into a colder airmass at the same indicated altitude and with no change in altimeter setting will experience which of the following effects on its true altitude?
Show answer
Answer: C. True altitude decreases because colder air is denser, compressing the pressure levels closer to the surface
The relationship between temperature, air density, and the vertical spacing of pressure levels is fundamental to altimetry. In a cold airmass, the air is denser, causing isobaric surfaces (levels of equal pressure) to be located geometrically closer to the Earth's surface than in a warmer airmass. An altimeter measures pressure and converts it to altitude using the ISA pressure-altitude relationship; it has no direct knowledge of the actual temperature of the air column below the aircraft. When an aircraft transitions from a warm to a cold airmass at constant indicated altitude, it is in reality descending to a lower true altitude even though the altimeter reading remains unchanged. This phenomenon, summarised by the memory aid 'High to Low, Look Out Below', is especially hazardous during instrument approaches in cold conditions and mandates the application of cold-temperature altitude corrections to all procedure altitudes.
Source: Aviation Meteorology — IC Joshi · Chapter 2 — Atmospheric Pressure · Page 35
3. What is the term for the altimeter sub-scale setting that causes the altimeter to read zero when the aircraft is on the ground at an aerodrome?
Show answer
Answer: A. QFE
QFE is the atmospheric pressure at aerodrome level, and when this value is set on the altimeter sub-scale, the instrument reads zero feet when the aircraft is on the ground at that particular aerodrome. During flight, the altimeter then indicates the height above that aerodrome reference datum, which is directly useful for visual circuit operations. QFE is particularly favoured at some aerodromes in certain countries because it gives the pilot an immediate indication of height above the aerodrome, simplifying visual approaches. However, it provides no direct information about height above surrounding terrain or obstacles, which can be hazardous in hilly or mountainous areas. Unlike QNH, which is referenced to mean sea level and is more universally used in modern operations, QFE is a local reference tied to the specific aerodrome elevation.
Source: Aviation Meteorology — IC Joshi · Chapter 2 — Atmospheric Pressure · Page 146
Frequently asked questions
Which chapter of IC Joshi covers atmospheric pressure?
Chapter 2, Atmospheric Pressure, in the 2019 edition of Aviation Meteorology by IC Joshi. It covers static and dynamic pressure, how pressure varies with height and through the day, pressure patterns, and altimetry.
What is the difference between QNH and QFE?
QFE is the pressure at aerodrome level. With it set, the altimeter reads zero on the ground and shows height above the aerodrome. QNH is that pressure reduced to mean sea level using the ISA temperature. With it set, the altimeter reads the aerodrome elevation on the ground and shows altitude above mean sea level.
How many feet is 1 hPa for altimeter calculations?
About 27 ft per hPa near mean sea level, which is the figure to use in DGCA calculations. It is sometimes rounded to 30 ft. The figure increases with height, to about 50 ft per hPa at 18,000 ft.
What is the difference between transition altitude and transition level?
Transition altitude is where a climbing aircraft changes from QNH to 1013.25 hPa. Transition level is the lowest usable flight level above it, where a descending aircraft changes from 1013.25 hPa to QNH. The space between them is the transition layer.
Related chapters
Practise Chapter 2: Atmospheric Pressure
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