2
METEOROLOGY — CHAPTER 2 · NOTES
Atmospheric Pressure
by Ghost Aviator
LECTURE PARTS
Table of Contents
- Static and Dynamic Pressure
- Pressure as Weight of Air Above
- Vertical Variation of Pressure
- Variation of Pressure – Warm vs Cold Air
- Semi-Diurnal Variation of Pressure
- Measurement Instruments
- Altimeter Correction (Alticor / D Value)
- Contours, Thickness and Pressure Gradient
- Pressure Tendency and Isallobars
- Altimetry — Definitions
- Pressure Settings (QFE, QFF, QNH, QNE)
- Relation between QNH, QFE and QNE
- Under-Reading / Over-Reading
- Pressure Patterns
- Practice Q&A (41 Questions)
- Master Reference Tables
1. Static and Dynamic Pressure
Definition: Pressure is the force per unit area exerted by molecules of air in random motion, acting uniformly in all directions. This is called Static Pressure or Barometric Pressure.
When air is in motion, an additional pressure is exerted in the direction opposite to the flow. This additional component is called Dynamic Pressure or Wind Pressure.
When air is in motion, an additional pressure is exerted in the direction opposite to the flow. This additional component is called Dynamic Pressure or Wind Pressure.
🎯 Exam Tip: "Static = air at rest. Dynamic = air in motion (opposes flow)." Think: Static Stops, Dynamic Does move.
2. Pressure as Weight of Air Above
The atmospheric pressure at any level is the weight of the column of air of unit cross-section extending vertically to the top of the atmosphere. As the amount of air in the column decreases with height, the pressure also decreases with height.
Units adopted by ICAO: Hectopascal (hPa)
1 hPa = 1000 dyn/cm²
Unit of force is Newton (N).
Standard Sea Level Pressure:
1013.25 hPa = 760 mm = 29.92 in
Also called 1 Bar = 100,000 N/m²
1 Bar = 1000 mb (millibars) = 1000 hPa
Heecto = 100; so 1 hPa = 100 Pa
1 hPa = 1000 dyn/cm²
Unit of force is Newton (N).
Standard Sea Level Pressure:
1013.25 hPa = 760 mm = 29.92 in
Also called 1 Bar = 100,000 N/m²
1 Bar = 1000 mb (millibars) = 1000 hPa
Heecto = 100; so 1 hPa = 100 Pa
Conversion: hPa → Inches
Multiply hPa value by 0.02953
Multiply hPa value by 0.02953
Example: 1013.25 hPa × 0.02953 = 29.92 inches
⚠️ Important: Air pressure changes cause ears to pop up when traveling over significantly varying topography and during rapid descent of an aircraft.
3. Vertical Variation of Pressure
The pressure decreases with height at a decreasing rate:
| Height Range | Rate of Decrease per 100 m |
|---|---|
| Sea level to 600 m | 4% |
| Up to 1.5 km | 3% |
| Up to 3 km | 2.5% |
| At 6 km | Reduces to half the sea level value |
| At 100 km | Negligible — regarded as vacuum |
Fig 2.1: Variation of Pressure with Height
Height Difference with 1 hPa Change
Formula:
Height difference = 96 × T / p feet
(where T = temperature in Kelvin, p = pressure in hPa)
Height difference = 96 × T / p feet
(where T = temperature in Kelvin, p = pressure in hPa)
Worked Example (ISA conditions):
At 1000 hPa, T = 300 K:
Height = 96 × 300 / 1000 = 28.8 ft
At 1000 hPa, T = 300 K:
Height = 96 × 300 / 1000 = 28.8 ft
In ISA conditions, 1 hPa pressure change corresponds to approximately:
| Level | MSL | 2,000 ft | 20,000 ft | 40,000 ft |
|---|---|---|---|---|
| Height equivalent | 27 ft | 30 ft | 50 ft | 100 ft |
⚠️ Critical Rule:
• If air is warmer than ISA → height change for 1 hPa will be MORE than above values
• If air is colder than ISA → height change will be LESS than above values
• If air is warmer than ISA → height change for 1 hPa will be MORE than above values
• If air is colder than ISA → height change will be LESS than above values
4. Variation of Pressure – Warm vs Cold Air
Cold air is denser than warm air. Pressure falls at a faster pace over a cold column of air than over a warm column.
At any given height, pressure at 850 hPa will be at a higher height over warm air than over cold air.
Key Rule: Pressure at any given height will be higher over warm air than over cold air.
• Isobars Dip going from High to Low pressure
• Isobars Rise going from Low to High pressure
• Isobar = line joining places of equal pressure
Key Rule: Pressure at any given height will be higher over warm air than over cold air.
• Isobars Dip going from High to Low pressure
• Isobars Rise going from Low to High pressure
• Isobar = line joining places of equal pressure
Isobaric levels are at lower height over cold column — higher over warm column
5. Semi-Diurnal Variation of Pressure
Atmospheric pressure follows a bimodal curve during a day:
• Maxima: 1000 hr and 2200 hr (local time)
• Minima: 0400 hr and 1600 hr (local time)
This semi-diurnal variation is due to solar influence.
Variations are: very small at the Poles and about 3–5 hPa at the Equator.
This is probably a natural oscillation of the atmosphere with a period of almost 12 hours.
• Maxima: 1000 hr and 2200 hr (local time)
• Minima: 0400 hr and 1600 hr (local time)
This semi-diurnal variation is due to solar influence.
Variations are: very small at the Poles and about 3–5 hPa at the Equator.
This is probably a natural oscillation of the atmosphere with a period of almost 12 hours.
🎯 Mnemonic – "Max at 10 and 10" (1000 hr & 2200 hr)
Min at 0400 (4 AM) and 1600 (4 PM). Remember: pressure is lowest in early morning and late afternoon.
Min at 0400 (4 AM) and 1600 (4 PM). Remember: pressure is lowest in early morning and late afternoon.
6. Measurement Instruments
| Instrument | Type | Description |
|---|---|---|
| Mercury Barometer | Accurate | Atmospheric pressure balanced by height of mercury column |
| Aneroid Barometer | Practical | Uses metal capsule; not as accurate as mercury; scale graduated in pressure (replaces mercury for ease of handling) |
| Altimeter | Aneroid type | Aneroid barometer with scale graduated in altitude instead of pressure; can be set to desired pressure value |
| Barograph | Recording | Continuously records pressure; can be Daily Barograph or Weekly Barograph |
7. Altimeter Correction (Alticor / D Value)
The correction applied to indicated altitude to obtain true altitude is called Altimeter Correction (also called Alticor).
This is frequently required for mountain flying and bombing operations.
D Value = True Altitude − Indicated Altitude
Correction to obtain true altitude = True Altitude − Indicated Altitude
D Value decreases when aircraft flies from High to Low pressure.
This is frequently required for mountain flying and bombing operations.
D Value = True Altitude − Indicated Altitude
Correction to obtain true altitude = True Altitude − Indicated Altitude
D Value decreases when aircraft flies from High to Low pressure.
Rough Calculations of Alticor
(a) Pressure Correction:
Add 30 ft for every 1 hPa when MSL pressure is higher than 1013.2 hPa
Subtract 30 ft per 1 hPa when MSL pressure is lower than 1013.2 hPa
(b) Temperature Correction:
Add 1% of indicated altitude for every 3°C temperature is higher than ISA
Subtract 1% per 3°C when temperature is lower than ISA
(c) Add these algebraically for final correction.
Add 30 ft for every 1 hPa when MSL pressure is higher than 1013.2 hPa
Subtract 30 ft per 1 hPa when MSL pressure is lower than 1013.2 hPa
(b) Temperature Correction:
Add 1% of indicated altitude for every 3°C temperature is higher than ISA
Subtract 1% per 3°C when temperature is lower than ISA
(c) Add these algebraically for final correction.
🎯 Memory Aid (H-L-H rule):
High to Low → Over Read (altimeter reads High = dangerous, you're lower than indicated)
Low to High → Under Read (altimeter reads Low; this alone does not establish terrain clearance)
Warm to Cold → Over Read | Cold to Warm → Under Read
If QNH is High → True Alt will be High
High to Low → Over Read (altimeter reads High = dangerous, you're lower than indicated)
Low to High → Under Read (altimeter reads Low; this alone does not establish terrain clearance)
Warm to Cold → Over Read | Cold to Warm → Under Read
If QNH is High → True Alt will be High
8. Contours, Thickness and Pressure Gradient
Contours
Lines joining places of equal height are called Contours. Contour lines may also be regarded as isobars.
In Contour Charts, contours are drawn at an interval of 40 gpm (geopotential metre) in 700 hPa and 500 hPa level charts; and at 80 gpm in 300 hPa and 200 hPa level charts.
1 gpm = 9.8 J/kg (gravitational potential energy per unit mass).
Contour lines numbered in geopotential decametres, e.g. 5280 gpm = 528.
A geopotential height of 700 gpm is approximately 700 m. A chart label 700 in geopotential decametres represents 7000 gpm; always check the chart units.
Centres of Low and High are marked as in surface pressure charts.
In Contour Charts, contours are drawn at an interval of 40 gpm (geopotential metre) in 700 hPa and 500 hPa level charts; and at 80 gpm in 300 hPa and 200 hPa level charts.
1 gpm = 9.8 J/kg (gravitational potential energy per unit mass).
Contour lines numbered in geopotential decametres, e.g. 5280 gpm = 528.
A geopotential height of 700 gpm is approximately 700 m. A chart label 700 in geopotential decametres represents 7000 gpm; always check the chart units.
Centres of Low and High are marked as in surface pressure charts.
Thickness
The height interval between the lower and upper level is called Thickness. Low thickness = lower mean temperature; High thickness = higher mean temperature. The isopleths of thickness coincide with the isotherms of mean temperature of the layer.
Pressure Gradient
Pressure gradient is the horizontal rate of change of pressure perpendicular to the isobars, directed from high to low pressure.
• Isobar = line joining places of equal pressure.
• Steep pressure gradient → isobars are close together → strong winds.
• Flat/weak gradient → isobars far apart → light winds.
• Isobar = line joining places of equal pressure.
• Steep pressure gradient → isobars are close together → strong winds.
• Flat/weak gradient → isobars far apart → light winds.
9. Pressure Tendency and Isallobars
The change of pressure with time is called Pressure Tendency. In India, pressure tendency is worked out for the past 24 hr and in higher latitudes for the past 3 hours of the current observation.
Isallobars = lines joining places of equal pressure change. At a glance, isallobars indicate areas of rising or falling pressure tendency.
• Isallobaric Low = region of greatest fall enclosed by isallobars.
• Isallobaric High = region of highest rise.
Surface Lows tend to move towards Isallobaric Low.
Isallobaric High indicates weakening of a pressure system.
Isallobars = lines joining places of equal pressure change. At a glance, isallobars indicate areas of rising or falling pressure tendency.
• Isallobaric Low = region of greatest fall enclosed by isallobars.
• Isallobaric High = region of highest rise.
Surface Lows tend to move towards Isallobaric Low.
Isallobaric High indicates weakening of a pressure system.
10. Altimetry — Definitions
| Term | Definition |
|---|---|
| Mean Sea Level (MSL) | A reference level taking average of high and low tides. |
| Altitude | Vertical distance from MSL. |
| Height | Vertical distance from a specific datum (e.g., from the ground). |
| Elevation | Vertical distance of a point or level on the surface of the earth from MSL. |
| Transition Altitude (TA) | Highest altitude below which aircraft always fly on local QNH. Vertical position controlled with reference to height above aerodrome. |
| Transition Level (TL) | Lowest Flight Level above which aircraft always fly on standard QNH 1013.25 hPa. Above TL, vertical position expressed in hundreds of feet. |
| Transition Layer | The airspace between Transition Altitude (TA) and Transition Level (TL). |
| Flight Level (FL) | Levels of constant pressure at or above TL, separated by pressure intervals corresponding to MSL pressure. E.g., FL50 = 5,000 ft, FL300 = 30,000 ft, FL200 = 20,000 ft. |
| Pressure Altitude | When the altimeter sub-scale is set to 1013.2 hPa, the altimeter indicates Pressure Altitude. Expressed as FL35, FL190, FL400 (avoid last two zeros). |
11. Pressure Settings
Corrections applied to barometer readings to reduce to a common level (MSL) are called Pressure Setting. The corrections are: Index correction (instrumental error), Gravity correction (varies with latitude), and Temperature correction (varies with temperature).
Pressure settings: QFE, QFF and QNH
Pressure settings: QFE, QFF and QNH
| Setting | Definition | Use |
|---|---|---|
| QFE | Pressure at the specified aerodrome or runway pressure datum. With QFE set, the altimeter reads approximately zero at that datum. | Height above aerodrome |
| QFF | Barometric pressure of aerodrome reduced to MSL using the actual temperature of the place (column of air extending up to MSL). | Plotting synoptic charts and drawing isobars |
| QNH | Station pressure reduced to mean sea level using the standard atmosphere. With QNH set, the altimeter reads the aerodrome elevation at its datum. Absolute altitude means height above ground and is a different quantity. | Vertical separation of aircraft and terrain |
| Regional QNH | Forecast value of the lowest pressure expected in an Altimeter Setting Region (ASR). Issued every hour, valid for one hour. | Ensuring adequate terrain clearance |
| QNE | Altitude indicated when sub-scale set to 1013.25 hPa (29.92 in). Used for high altitude airfields. | High altitude airfields; Pressure Altitude reference |
12. Relation between QNH, QFE and QNE
Station Elevation = (QNH − QFE) × 30 ft
Example (i): QNE > QNH → Sea level lower than MSL
QFE = 950 hPa, QNH = 1000 hPa, 1 hPa = 30 ft
Elevation = (QNH − QFE) × 30 = (1000 − 950) × 30 = 50 × 30 = 1500 ft
QFE = 950 hPa, QNH = 1000 hPa, 1 hPa = 30 ft
Elevation = (QNH − QFE) × 30 = (1000 − 950) × 30 = 50 × 30 = 1500 ft
Example (ii): Altimeter set to QNE = 1013 hPa
Station level would be = (QNE − QFE) × 30 ft = (63 hPa) × 30 = 1890 ft
Station level would be = (QNE − QFE) × 30 ft = (63 hPa) × 30 = 1890 ft
Example (iii): QFE 970 hPa, QNH 1020 hPa
Station elevation = (QNH − QFE) × 30 = (50 hPa) × 30 = 1500 ft
Station elevation = (QNH − QFE) × 30 = (50 hPa) × 30 = 1500 ft
Example (iv): QNE = 1013 hPa, Station elevation = 270 ft
270 / 27 = 10 hPa → QNH = 1000 + 10 = 1010 hPa
270 / 27 = 10 hPa → QNH = 1000 + 10 = 1010 hPa
Example (v): Airfield elevation 400 ft, QNH 1010 hPa, lands at elevation 600 ft without resetting, altimeter reads 700 ft. Correct QNH at B?
Height difference = 700 − 400 = 300 ft = 300/27 = 11 hPa
QNH at B = 1010 − 11 = 999 hPa
Height difference = 700 − 400 = 300 ft = 300/27 = 11 hPa
QNH at B = 1010 − 11 = 999 hPa
L-H-L Rule for QNH/QFE Relationship
Hotter than ISA: QFF < QNH
At MSL: QFF = QNH
Pressure set on altimeter < QNH → Indicated Altitude < True Altitude
Pressure set on altimeter > QNH → Indicated Altitude > True Altitude
At MSL: QFF = QNH
Pressure set on altimeter < QNH → Indicated Altitude < True Altitude
Pressure set on altimeter > QNH → Indicated Altitude > True Altitude
Standard Isobaric Levels
Corresponding to ISA pressure, the Pressure Altitude (Ft) and Flight Levels are:
| Level (hPa) | 850 | 700 | 500 | 400 | 300 | 200 | 100 |
|---|---|---|---|---|---|---|---|
| Pressure Altitude (ft) | 5,000 | 10,000 | 18,000 | 24,000 | 30,000 | 38,000 | 53,000 |
| Flight Level | FL50 | FL100 | FL180 | FL240 | FL300 | FL380 | FL530 |
13. Under-Reading / Over-Reading
When a pressure value is set on the sub-scale, the altimeter indicates height as per ISA specifications.
H-L-H (Over Read)
High to Low → Over Read
Warm to Cold → Over Read
If QNH is High → True Alt is High
Altimeter reads too high — you are lower than indicated!
High to Low → Over Read
Warm to Cold → Over Read
If QNH is High → True Alt is High
Altimeter reads too high — you are lower than indicated!
L-H-L (Under Read)
Low to High → Under Read
Cold to Warm → Under Read
Altimeter Setting is Low → Indicated Alt will be Low
You are higher than indicated for this pressure-error comparison. Terrain clearance still requires the correct setting and other applicable corrections.
Low to High → Under Read
Cold to Warm → Under Read
Altimeter Setting is Low → Indicated Alt will be Low
You are higher than indicated for this pressure-error comparison. Terrain clearance still requires the correct setting and other applicable corrections.
⚠️ Critical: When flying with winds from port, the altimeter will over-read. With winds from starboard, it will under-read. Error increases with strength of wind and length of route. In S hemisphere the opposite is true.
14. Pressure Patterns
To represent the spatial distribution of pressure at a given time, all pressures are reduced to MSL. Pressures at stations are plotted on a chart, and isobars drawn at 2 hPa intervals.
| Pattern | Description | Weather | Hemisphere Note |
|---|---|---|---|
| Low (L) / Cyclone | Area enclosed by isobars with lowest pressure at centre. Two or more closed isobars = Depression. Severe = Cyclone. | Cloud and precipitation can occur with rising moist air; visibility varies | N Hemisphere: winds blow anticlockwise, converging & upward |
| High (H) / Anticyclone | Region enclosed by isobars with highest pressure at centre. | Fair weather but visibility may be poor | N Hemisphere: winds blow clockwise, veer and diverge outwards |
| Trough | "V" shaped extension of isobars from a low. Wind direction abruptly changes and backs. | Bad weather, associated with monsoon trough along Indo-Gangetic plains | — |
| Ridge | Wedge-like extension of isobars from high. Isobars rounded, not "V" shaped. | Fair weather; winds veer at a ridge | — |
| Col | Between two highs and two lows; 2 hPa higher than lows, towards highs. | Mixed — light variable winds | Winds light variable; pressure uniform |
| Cold Anticyclone | Forms in cold region behind a cyclone, compresses air as it moves away. | Warms up after sometimes | Blocks movement of migratory cyclones = Blocking Anticyclone |
🎯 Quick Reference:
Low = Cyclone = Convergence = Rising air = Bad Wx
High = Anticyclone = Divergence = Subsidence = Good Wx (but poor vis)
Trough = V-shape = Bad Wx | Ridge = Rounded = Good Wx | Col = Uniform = Light winds
Low = Cyclone = Convergence = Rising air = Bad Wx
High = Anticyclone = Divergence = Subsidence = Good Wx (but poor vis)
Trough = V-shape = Bad Wx | Ridge = Rounded = Good Wx | Col = Uniform = Light winds
flowchart LR
A[Low/Depression] -->|winds converge| B[Upward Motion]
B --> C[Cloud & Precipitation]
C --> D[Bad Weather]
E[High/Anticyclone] -->|winds diverge| F[Downward Motion/Subsidence]
F --> G[Clear Sky]
G --> H[Fair Weather\nbut Poor Visibility]
Practice Q&A — Atmospheric Pressure
Practice questions with verified answer key.
Q1. Winds in a low pressure area
(a) Converge (b) Diverge (c) Go straight
(a) Converge (b) Diverge (c) Go straight
✅ Answer: (a) Converge
In a Low, air flows inward (converges) and rises.
In a Low, air flows inward (converges) and rises.
❌ (b) Diverge — This happens in a High pressure area. (c) Go straight — only in absence of pressure gradient.
💡 Instructor's Note: Low = Convergence = Rising air = Bad weather. High = Divergence = Subsidence = Fair weather.
Q2. Low pressure is associated with
(a) Good Weather (b) Bad Weather (c) None
(a) Good Weather (b) Bad Weather (c) None
✅ Answer: (b) Bad Weather
❌ (a) Good Weather — this is associated with High pressure areas. (c) None — wrong.
💡 Instructor's Note: Low → rising air → cloud & precipitation → bad weather. Classic DGCA question.
Q3. In a high pressure area winds are
(a) Normal (b) Strong (c) Weak
(a) Normal (b) Strong (c) Weak
✅ Answer: (c) Weak — In a high (anticyclone) the isobars are widely spaced, so the pressure gradient is slack and the winds are light.
❌ (a) 'Normal' is not a description of wind strength. (b) Strong winds go with closely packed isobars, as in a deep low.
💡 High = light winds, sinking air, fair weather. Low = stronger winds, rising air, bad weather.
Q4. Flying from Low to High, an altimeter would read
(a) Over (b) Under (c) constant
(a) Over (b) Under (c) constant
✅ Answer: (b) Under — Flying from low towards high pressure with the subscale unchanged, the altimeter under-reads: the aircraft is higher than the altimeter shows.
❌ (a) Over-reading is the error when flying from high to low pressure. (c) The reading does not stay correct: the error grows as the pressure changes.
💡 High to Low, look out below (the altimeter over-reads). Low to High, you are higher than indicated (it under-reads).
Q5. Isallobars are lines of equal
(a) Pressure (b) Temperature Tendency (c) Pressure Tendency
(a) Pressure (b) Temperature Tendency (c) Pressure Tendency
✅ Answer: (c) Pressure Tendency
❌ (a) Pressure — that's an Isobar. (b) Temperature Tendency — that would be Isalloterms.
💡 Mnemonic: Is-ALLO-bar = Isobar that ALTERS (changes) = Isallobar = equal pressure CHANGE.
Q6. What kind of a barometer is an altimeter?
(a) Aneroid (b) Mercury (c) Alcohol
(a) Aneroid (b) Mercury (c) Alcohol
✅ Answer: (a) Aneroid — An altimeter is an aneroid barometer: a sealed, partly evacuated capsule that expands and contracts with pressure, with its scale marked in height.
❌ (b) A mercury barometer is a fixed, fragile laboratory instrument, not used in aircraft. (c) Alcohol is not used as a barometer liquid in aviation.
💡 Altimeter = aneroid capsule. Mercury barometers are the station-level standard.
Q7. A region between two Lows and two Highs is called
(a) Depression (b) Secondary Low (c) Col
(a) Depression (b) Secondary Low (c) Col
✅ Answer: (c) Col
❌ (a) Depression — enclosed area of low pressure. (b) Secondary Low — a smaller low near main low.
💡 A Col is like a mountain pass in pressure topography — between two highs and two lows. Light variable winds.
Q8. Bad weather and better visibility is associated with
(a) High (b) Low
(a) High (b) Low
✅ Answer: (b) Low — A low brings cloud, rain and strong winds, yet the rain washes dust and haze out of the air, so visibility between showers can be good. A high gives fair weather but, with sinking air and light winds, haze and smoke are trapped under the inversion.
❌ (a) High: fair weather and often poor visibility, the opposite pairing.
💡 Low = bad weather, good visibility. High = fair weather, poor visibility.
Q9. The relationship between height and pressure is use in construction of
(a) Altimeter (b) ASI (c) V S I
(a) Altimeter (b) ASI (c) V S I
✅ Answer: (a) Altimeter — The altimeter senses static pressure and converts it to height using the pressure–height relationship of the standard atmosphere.
❌ (b) The ASI works from the difference between pitot and static pressure, not from height. (c) The VSI works from the rate of change of static pressure.
💡 Altimeter: pressure to height. ASI: pitot minus static. VSI: rate of change of static.
Q10. Altimeters always measure the height of aircraft above
(a) MSL (b) ASI (c) datum of 1013.2 hPa
(a) MSL (b) ASI (c) datum of 1013.2 hPa
✅ Answer: (c) datum of 1013.2 hPa
💡 Unless set to QNH/QFE, the altimeter measures height from 1013.25 hPa datum = Pressure Altitude.
Q11. Two aircraft flying at same indicated altitude with altimeters set to 1013.2 hPa. One is flying over cold air mass, other over warm air mass. Which has greater altitude?
(a) Ac flying over warm air mass (b) Ac flying over cold air mass (c) Both
(a) Ac flying over warm air mass (b) Ac flying over cold air mass (c) Both
✅ Answer: (a) Aircraft flying over warm air mass — Warm air is less dense, so pressure falls more slowly with height and a given pressure level lies higher. With 1013.2 hPa set, both aircraft are on the same pressure surface, but over warm air that surface is higher, so the aircraft over the warm air mass has the greater true altitude.
❌ (b) Over cold air the pressure levels are squashed downwards, so the same indication means a lower true altitude.
💡 Warm air: true altitude above indicated. Cold air: true altitude below indicated.
Q12. Rate of fall of pressure with height in cold air mass compared to warm air mass will be
(a) Same (b) More (c) Less
(a) Same (b) More (c) Less
✅ Answer: (b) More
Cold air is denser, so pressure falls more rapidly with height over a cold column.
Cold air is denser, so pressure falls more rapidly with height over a cold column.
💡 Cold air = denser = faster pressure decrease with height. Warm air = less dense = slower pressure decrease.
Q13. An increase of 1000 ft at MSL is associated with decrease of pressure of
(a) 100 hPa (b) 1000 hPa (c) 3 hPa (d) 33 hPa
(a) 100 hPa (b) 1000 hPa (c) 3 hPa (d) 33 hPa
✅ Answer: (d) 33 hPa
At MSL, 1 hPa ≈ 27–30 ft. For 1000 ft: 1000/30 ≈ 33 hPa.
At MSL, 1 hPa ≈ 27–30 ft. For 1000 ft: 1000/30 ≈ 33 hPa.
💡 Remember: at MSL, roughly 27 ft per hPa, so 1000 ft ≈ 33–37 hPa change.
Q14. Lines drawn through places of equal pressure are known as
(a) Isobars (b) Isotherms (c) Isoheights (d) Isoclinic
(a) Isobars (b) Isotherms (c) Isoheights (d) Isoclinic
✅ Answer: (a) Isobars
❌ (b) Isotherms = equal temperature. (c) Isoheights = equal height (contours). (d) Isoclinic = equal magnetic dip.
💡 Iso = equal; Bar = pressure (like barometer). Standard meteorological lines: Isobar, Isotherm, Isohyet (rainfall), Isoneph (cloudiness).
Q24. 300 hPa in ISA corresponds to the level
(a) 20,000 ft (b) 30,000 ft (c) 35,000 ft
(a) 20,000 ft (b) 30,000 ft (c) 35,000 ft
✅ Answer: (b) 30,000 ft
Standard: 300 hPa = 30,000 ft = FL300
Standard: 300 hPa = 30,000 ft = FL300
💡 Learn the standard isobaric levels table. 300 hPa → 30,000 ft is frequently tested.
Q36. QNH of an aerodrome 160 m AMSL is 1005 hPa. QFE? Assuming 1 hPa = 8m
(a) 1010 hPa (b) 985 hPa (c) 1005 hPa (d) 990 hPa
(a) 1010 hPa (b) 985 hPa (c) 1005 hPa (d) 990 hPa
✅ Answer: (b) 985 hPa — Height difference 160 m ÷ 8 m per hPa = 20 hPa. QFE = QNH − 20 = 1005 − 20 = 985 hPa. The aerodrome is above sea level, so the pressure at the aerodrome is lower than QNH.
❌ (c) 1005 hPa is the QNH itself; QFE equals QNH only at sea level. (d) 990 hPa would be correct for an elevation of 200 m. (a) 1010 hPa is above QNH, which cannot be right for an elevated aerodrome.
💡 QFE = QNH − (elevation ÷ 8 m per hPa), using the 1 hPa = 8 m the question gives.
Q37. Steep pressure gradient would mean
(a) Contours far apart, weak wind
(b) Contours far apart, strong wind
(c) Isobars far apart, strong wind
(d) Isobars closely packed and strong wind
(a) Contours far apart, weak wind
(b) Contours far apart, strong wind
(c) Isobars far apart, strong wind
(d) Isobars closely packed and strong wind
✅ Answer: (d) Isobars closely packed and strong wind
❌ (a) Far apart = weak gradient = light winds. (c) Far apart = weak, not strong.
💡 Mnemonic: "Close isobars = strong gradient = strong winds." Like contour lines on a steep mountain.
Q38. What type of inversion occurs when a stable layer lies in a high pressure area?
(a) Negative (b) Radiation (c) Subsidence (d) Airmass
(a) Negative (b) Radiation (c) Subsidence (d) Airmass
✅ Answer: (c) Subsidence — In a high-pressure area the air sinks, is compressed and warms, forming a warm layer above cooler air below: a subsidence inversion. It traps haze and smoke and is a key cause of poor visibility.
❌ (a) 'Negative' is not a type of inversion. (b) A radiation inversion forms on clear nights by ground cooling, not by sinking air.
💡 High pressure + sinking air = subsidence inversion = haze and low cloud under it.
Q39. Which of the following would cause true altitude to decrease when the altimeter indicates a constant altitude? (a) Warm/Low (b) Cold/High (c) Cold/Low (d) Warm/High
✅ Answer: (c) Cold/Low — True altitude is less than indicated when the air is colder than standard (the pressure levels are squashed downwards) and when you fly into lower pressure than the subscale setting. Cold and Low together lower the true altitude.
❌ Warm and High both raise the true altitude above the indication; Warm/Low and Cold/High have one effect each in opposite directions.
💡 Cold/Low: true below indicated, the dangerous direction over high ground.
Q41. An aerodrome is at MSL. Its QNH is 1014.0 hPa. Its QFF will be
(a) 1014.0 hPa (b) 1013.25 (c) Difficult to tell (d) More than QNH
(a) 1014.0 hPa (b) 1013.25 (c) Difficult to tell (d) More than QNH
✅ Answer: (a) 1014.0 hPa
If the aerodrome is at MSL, then QNH = QFF = 1014.0 hPa (no height correction needed, both reduce to the same level — MSL).
If the aerodrome is at MSL, then QNH = QFF = 1014.0 hPa (no height correction needed, both reduce to the same level — MSL).
💡 At MSL: QNH = QFF. The difference arises only when there is elevation, because QFF uses actual temperature while QNH uses ISA.
Master Reference Tables — Chapter 2
All Numerical Values
| Parameter | Value |
|---|---|
| Standard sea level pressure | 1013.25 hPa = 760 mm Hg = 29.92 in |
| Atmospheric pressure at sea level | 100,000 N/m² = 1 Bar |
| 1 hPa | 1000 dyn/cm² |
| 1 Bar | 1000 mb = 1000 hPa |
| hPa to inches conversion factor | × 0.02953 |
| Height formula per 1 hPa | 96T/p feet |
| At 1000 hPa, 300K: height per hPa | 28.8 ft |
| 1 hPa at MSL | 27 ft |
| 1 hPa at 2,000 ft | 30 ft |
| 1 hPa at 20,000 ft | 50 ft |
| 1 hPa at 40,000 ft | 100 ft |
| Pressure halves at altitude | 6 km |
| Vacuum (negligible pressure) | 100 km |
| Semi-diurnal maxima | 1000 hr & 2200 hr |
| Semi-diurnal minima | 0400 hr & 1600 hr |
| Equatorial diurnal pressure variation | 3–5 hPa |
| Alticor pressure correction | 30 ft per hPa |
| Alticor temperature correction | 1% per 3°C |
| Contour interval (700 & 500 hPa charts) | 40 gpm |
| Contour interval (300 & 200 hPa charts) | 80 gpm |
| 1 gpm | 9.8 J/kg |
| Isobar drawing interval | 2 hPa (surface charts) |
| Station elevation formula | (QNH − QFE) × 30 ft |
| 850 hPa level | 5,000 ft = FL50 |
| 700 hPa level | 10,000 ft = FL100 |
| 500 hPa level | 18,000 ft = FL180 |
| 400 hPa level | 24,000 ft = FL240 |
| 300 hPa level | 30,000 ft = FL300 |
| 200 hPa level | 38,000 ft = FL380 |
| 100 hPa level | 53,000 ft = FL530 |
Mnemonics / Memory Aids
H-L-H (Over Read): High to Low → Over Read, Warm to Cold → Over Read
L-H-L (Under Read): Low to High → Under Read, Cold to Warm → Under Read
LOW = BAD (Lows = convergence = cloud = bad weather)
HIGH = FAIR (Highs = divergence = subsidence = fair weather, poor visibility)
"Max at Ten and Ten" = Semi-diurnal maxima at 1000 & 2200 hr
TROUGH = V shape = Bad; RIDGE = Rounded = Good
COL = between 2H and 2L = Light variable winds
L-H-L (Under Read): Low to High → Under Read, Cold to Warm → Under Read
LOW = BAD (Lows = convergence = cloud = bad weather)
HIGH = FAIR (Highs = divergence = subsidence = fair weather, poor visibility)
"Max at Ten and Ten" = Semi-diurnal maxima at 1000 & 2200 hr
TROUGH = V shape = Bad; RIDGE = Rounded = Good
COL = between 2H and 2L = Light variable winds
Quick Revision Summary
Top 10 Exam Points — Atmospheric Pressure:
1. Standard sea level pressure = 1013.25 hPa = 760 mm = 29.92 in
2. 1 hPa = 1000 dyn/cm²; 1 Bar = 1000 hPa = 100,000 N/m²
3. Pressure halves at 6 km; vacuum at 100 km
4. Height per 1 hPa = 96T/p feet (28.8 ft at 1000 hPa, 300K)
5. Semi-diurnal maxima: 1000 & 2200 hr; minima: 0400 & 1600 hr
6. D Value = True Alt − Indicated Alt; decreases flying High → Low
7. QFE = zero on runway; QNH = ISA corrected to MSL; QFF = actual temp correction
8. Station elevation = (QNH − QFE) × 30 ft
9. 300 hPa = 30,000 ft; 500 hPa = 18,000 ft; 700 hPa = 10,000 ft; 850 hPa = 5,000 ft
10. Isobars close = strong wind; H-L-H = over-read; Low = bad weather; High = fair wx
1. Standard sea level pressure = 1013.25 hPa = 760 mm = 29.92 in
2. 1 hPa = 1000 dyn/cm²; 1 Bar = 1000 hPa = 100,000 N/m²
3. Pressure halves at 6 km; vacuum at 100 km
4. Height per 1 hPa = 96T/p feet (28.8 ft at 1000 hPa, 300K)
5. Semi-diurnal maxima: 1000 & 2200 hr; minima: 0400 & 1600 hr
6. D Value = True Alt − Indicated Alt; decreases flying High → Low
7. QFE = zero on runway; QNH = ISA corrected to MSL; QFF = actual temp correction
8. Station elevation = (QNH − QFE) × 30 ft
9. 300 hPa = 30,000 ft; 500 hPa = 18,000 ft; 700 hPa = 10,000 ft; 850 hPa = 5,000 ft
10. Isobars close = strong wind; H-L-H = over-read; Low = bad weather; High = fair wx
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