Aviation meteorology notes on atmospheric pressure, altimetry and pressure systems, covering QNH, QFE, QFF, pressure altitude, true and indicated altitude, altimeter errors, high- and low-pressure systems, troughs, ridges, Cols and constant-pressure charts. These notes are structured for quick revision and pilot examination preparation.

Consolidated Summary Notes

1. Atmospheric Pressure

Definition

• Atmospheric pressure is the force per unit area exerted by the weight of the atmosphere.
• It can also be understood as the weight of the column of air above a given surface.
• Gravity holds the air molecules close to Earth.
• Because air is compressible:
  • Air density is greatest near the surface.
  • A large proportion of the atmosphere’s mass is concentrated at lower levels.
• Pressure differences are fundamental to the generation of wind and weather systems.

Standard sea-level pressure

Pressure and altitude

• Pressure always decreases as altitude increases.
• The decrease is not uniform.
• Pressure decreases:
  • Rapidly near the surface
  • More slowly at higher altitudes
• The reason is decreasing air density with height.

Approximate lower-level relationship

• Near MSL:
  • 1 hPa ≈ 27 ft
• For numerical MCQs, use the conversion specifically stated in the question.

2. Pressure Lapse Rate and Temperature

The rate at which pressure decreases with height depends strongly on the density and temperature of the air column.

Cold air

• Cold air is denser.
• Pressure decreases more rapidly with height.
• Constant pressure surfaces (isobars) are lower/closer together.
• At a given indicated altitude:
  • True altitude is lower than indicated altitude.
  • TA < IA
• Altimeter therefore over-reads. ( reads Higher)

Warm air

• Warm air is less dense.
• The air column expands vertically.
• Pressure surfaces are higher/farther apart.
• At a given indicated altitude:
  • True altitude is higher than indicated altitude.
  • TA > IA
• Altimeter therefore under-reads. ( Reads Lower)

Memory aids

Warm to cold — don’t be bold!

High to low — beware below!

3. Pressure Gradient

Pressure gradient

• Pressure gradient describes the change in pressure over a horizontal distance.
• A steep/strong pressure gradient means pressure changes rapidly over a short distance.

Isobar spacing

• Closely spaced isobars produce a strong Pressure Gradient Force (PGF).
• Strong PGF produces stronger winds.
• Widely spaced isobars indicate weak PGF and lighter winds.

4. Important Pressure Lines

Isobars on surface charts

• Surface/synoptic charts use isobars to show the pressure pattern.
• The source identifies the pressure represented as QFF/MSL pressure.
• QFF is based on the actual temperature, whereas QNH uses ISA conditions.

5. High-Pressure System / Anticyclone

Basic characteristics

A high-pressure system is associated with:

• Surface divergence
• Descending air / subsidence
• Adiabatic compression and warming
• Increased atmospheric stability
• Generally fair weather

Surface wind

• Isobars are generally widely spaced.
• Pressure gradient is weak.
• Winds are therefore generally light or calm.

Vertical movement

• Air sinks from aloft.
• As it descends:
  • Pressure increases.
  • Air is compressed.
  • Temperature rises adiabatically.
  • Relative humidity decreases.
  • Stability increases.

Weather

• Cloud tends to dissipate.
• Precipitation is generally absent in the central high.
• Generally good/fair weather.
• However, visibility can become poor because of trapped pollutants and moisture.

6. Subsidence

Definition

Subsidence = large-scale downward/sinking movement of air.

Associated with

• High-pressure systems
• Anticyclones
• Ridges
• Upper-level convergence

Effects

Descending air:

1. Is compressed.
2. Warms adiabatically.
3. Becomes more stable.
4. Suppresses vertical convection.
5. Encourages cloud dissipation.
6. Can produce a subsidence inversion.

7. Subsidence Inversion

Formation

A subsidence inversion forms when:

• Air descends in a high-pressure system.
• Descending air is compressed.
• Compression causes adiabatic warming.
• Warmer air becomes positioned above relatively cooler air.

Characteristics

• Creates a very stable layer.
• Acts like a lid on vertical movement.
• Traps:
  • Smoke
  • Dust
  • Moisture
  • Pollution
  • Haze

Consequence

• Poor visibility is common below the inversion.
• The inversion may persist for several days or longer according to the source.

8. Visibility in High and Low Pressure

High-pressure visibility

• Summer: haze/smoke is common.
• Winter: radiation fog can occur.
• Light winds and inversion prevent effective dispersion.
• A Col can also produce poor visibility because of stagnant air.

9. Low-Pressure System / Cyclone / Depression

Basic characteristics

A low-pressure system is associated with:

• Surface convergence
• Ascending air
• Cyclonic circulation
• Cloud formation
• Precipitation
• Generally unsettled/bad weather

Sequence

Low pressure → surface convergence → ascent → expansion/cooling → condensation → clouds → precipitation

Visibility

• Rising air and mixing generally improve visibility.
• Visibility can nevertheless become poor inside precipitation.

10. Trough

Definition

A trough is an elongated area of relatively low pressure.

Important characteristics

• It is generally an extension of a low-pressure system.
• The trough axis represents:
  • Lowest pressure
  • Maximum cyclonic curvature
• Moving perpendicular away from the trough axis, pressure rises.

Northern Hemisphere

• Low-pressure circulation is counter-clockwise/cyclonic.
• Surface friction slows the wind.
• Reduced wind speed weakens Coriolis force.
• PGF becomes relatively more dominant.
• Wind crosses the isobars toward lower pressure.
• This produces a backing of the surface wind.

Remember

Trough → Low pressure → Convergence → Ascent → Backing in NH

11. Ridge

Definition

A ridge is an elongated area of relatively high pressure.

Characteristics

• Extension of a high-pressure system.
• Ridge axis represents maximum anticyclonic curvature/high pressure.
• Moving perpendicular away from the ridge axis:
  • Pressure falls.

Air movement

• Subsidence
• Surface divergence
• Adiabatic warming
• Increased stability
• Generally fine weather

Remember

Ridge → High pressure → Subsidence → Divergence

12. Trough vs Ridge

13. Col

Definition

A Col is a neutral region of very little pressure variation.

It is located:

• Between two highs and two lows
• At the intersection of a ridge and trough
• Analogous to a mountain pass.

Characteristics

• Very weak pressure gradient
• Very light winds
• Stagnant air
• Poor ventilation

Weather hazards

• Winter: fog/low stratus
• Summer: instability clouds/thunderstorms can develop

The source specifically associates Cols with poor visibility because of stagnant air.

14. Upper-Level Divergence and Surface Pressure

Upper divergence

When air diverges at upper levels:

• Air/mass is removed from the atmospheric column.
• Surface pressure falls.
• Surface convergence develops.
• Air rises.
• Clouds and precipitation may form.
• A surface low can develop/intensify.

Upper convergence

When air converges aloft:

• Mass accumulates in the column.
• Surface pressure rises.
• Surface divergence develops.
• Air sinks.
• Clouds tend to dissipate.
• A high-pressure system can develop/intensify.

Simplified chain

Upper divergence → Surface pressure falls → Surface convergence → Rising air → LPA

Upper convergence → Surface pressure rises → Surface divergence → Sinking air → HPA

15. Altimeter — Basic Principle

What is an altimeter?

• An aircraft altimeter is essentially an aneroid barometer.
• It measures atmospheric pressure, not geometric height directly.
• It converts pressure into an altitude/height indication using the pressure-height relationship.

Aneroid mechanism

• Uses a partially evacuated flexible aneroid cell.
• Changes in external pressure cause the cell to expand/contract.
• This movement is mechanically converted into an altitude indication.

Critical concept

The altimeter reads according to the pressure datum set on its subscale.

16. Altimeter Settings

QNH

• QFE reduced to MSL using ISA conditions.
• Used for altimetry.
• On the ground:
  • Altimeter reads aerodrome elevation.
• In flight:
  • Indicates altitude above MSL.

QFE

• Pressure at aerodrome datum/reference point.
• On ground:
  • Altimeter reads zero.
• In flight:
  • Indicates height above aerodrome level.

QFF

• QFE reduced to MSL using actual outside temperature.
• Used primarily for meteorological analysis/surface charts.
• Must not be used as an altimeter setting.

17. QNH vs QFE — Quick Rules

Aerodrome above MSL

QNH > QFE

Example from the source:

• Elevation = 160 m
• QNH = 1005 hPa
• Given conversion = 1 hPa / 8 m

Pressure difference:

160 ÷ 8 = 20 hPa

Therefore:

QFE = 1005 − 20 = 985 hPa

Another source example

• Elevation = 200 m
• QNH = 1015 hPa
• 1 hPa = 8 m

Pressure difference:

200 ÷ 8 = 25 hPa

Therefore:

QFE = 1015 − 25 = 990 hPa

Remember

AMSL → QNH higher than QFE

18. Aerodrome at MSL

If the aerodrome is exactly at MSL:

QNH = QFF = QFE

Reason:

• There is no elevation difference to correct.
• No reduction from aerodrome level to MSL is required.
• This remains true regardless of actual temperature.

19. QNH vs QFF

QNH

• Uses ISA conditions.
• QFE → MSL.
• Used for altimetry.

QFF

• Uses actual prevailing temperature.
• QFE → MSL.
• Used for meteorological analysis.
• Isobars on surface charts represent QFF according to the source.

20. QNH/QFF Relationship with Cold Air

The source repeatedly tests this concept.

Aerodrome ABOVE MSL + colder than ISA

Cold air is denser, so pressure decreases more rapidly with height.

Therefore:

QFF > QNH

Examples in the source:

• If QNH = 1009 hPa → QFF must be more than 1009 hPa.
• If QFF = 1030 hPa → QNH must be less than 1030 hPa.

Aerodrome BELOW MSL + colder than ISA

The source gives:

QNH > QFF

So:

21. Constant Indicated Altitude and Pressure Surfaces

When an aircraft maintains a constant indicated altitude/Flight Level:

• It is effectively flying along a constant pressure surface.
• The physical height of that pressure surface can change from one region to another.

High pressure

• Pressure surfaces are higher.
• Constant-pressure surface rises.
• True altitude becomes greater than indicated altitude.

Low pressure

• Pressure surfaces are lower.
• Constant-pressure surface dips.
• True altitude becomes less than indicated altitude.

22. Pressure Altimeter Error — Pressure Effect

Flying from LOW → HIGH pressure

If the altimeter is not reset:

• Pressure surface rises.
• Aircraft’s true altitude increases.
• TA > IA
• Altimeter under-reads.

Flying from HIGH → LOW pressure

If the altimeter is not reset:

• Pressure surface becomes lower.
• True altitude decreases.
• TA < IA
• Altimeter over-reads.

Memory aid

High to low — beware below!

23. Altimeter Error — Temperature Effect

Warm air

• Air expands.
• Pressure surfaces rise.
• True altitude > indicated altitude.

Cold air

• Air contracts.
• Pressure surfaces lower/compact.
• True altitude < indicated altitude.

24. Combined Pressure + Temperature Effect

Safest positive relationship

Hot/Warm + High pressure → TA > IA

Hazardous relationship

Cold + Low pressure → TA < IA

Memory aids:

High to low — beware below!

Warm to cold — don’t be bold!

The source specifically identifies Hot/High as producing the greatest positive true-altitude relationship and Cold/Low as the hazardous combination.

25. Wind Drift and Altimetry in the Northern Hemisphere

Starboard/right drift

If an aircraft experiences starboard drift:

• Wind is coming from the left/port side.

Buys Ballot relationship in NH

The source associates:

• Lower pressure/cooler air generally to the left of the wind.
• Therefore, when wind comes from the left, the aircraft can be associated with lower pressure/colder air to that side.

Altimetry consequence

When entering colder/lower-pressure conditions:

• Altimeter can over-read.
• True altitude becomes lower than indicated.

26. Constant Pressure Charts

Definition

A constant pressure chart shows meteorological conditions at a specific pressure level.

Examples:

• 850 hPa
• 700 hPa
• 500 hPa
• 400 hPa
• 300 hPa
• 200 hPa

Why they are important

Aircraft at higher levels often operate using Flight Levels, which are pressure-based levels.

Therefore, constant-pressure charts are particularly useful for:

• Upper winds
• Upper temperatures
• Pressure patterns
• Flight planning
• Meteorological analysis

27. Contours / Isohypses on Constant Pressure Charts

On a constant-pressure chart:

• Lines are contour lines / isohypses.
• They join points where the same pressure occurs at the same height.
• Therefore, they show the height of that pressure surface.

High contour values

• Pressure surface is higher.
• Associated with high pressure/ridge aloft.
• Generally associated with warmer air.

Low contour values

• Pressure surface is lower.
• Associated with low pressure/trough aloft.
• Generally associated with colder air.

28. Standard ISA Pressure Levels

Additional values appearing in the source

• 250 hPa ≈ 34,000 ft
• 150 hPa ≈ 53,000 ft
• One earlier source entry gives 100 hPa ≈ 53,000 ft.

⚠️ Source note: The material contains a conflicting approximation for the 100/150 hPa level. Do not merge those two values as though they were identical; use the value supplied with the specific question/source.

29. 850 hPa

Key facts

• Approximately 5,000 ft AMSL
• Approximately FL050
• Used for lower-level upper-air weather analysis.

The source also gives a more precise ISA height of about 4,781 ft, while operational questions commonly round this to 5,000 ft.

Example

At approximately 1620 m / 5315 ft, the closest standard pressure level is:

850 hPa

because 850 hPa corresponds approximately to 5,000 ft / 1524 m.

30. 700 hPa

Key facts

• Approximately 10,000 ft
• Approximately FL100
• Approximately 3,048 m

ISA temperature example

At 10,000 ft:

• ISA MSL temperature = +15°C
• Using 2°C/1,000 ft:
  • Temperature decrease = 20°C
  • ISA temperature at FL100 = −5°C

If observed temperature = −15°C:

ISA deviation = −15 − (−5) = −10°C

Therefore, the observed air is 10°C colder than ISA.

31. 500 hPa

Key facts

• Approximately 18,000 ft
• Approximately FL180
• Approximately 5.5 km

Atmospheric significance

• Approximately half of the atmospheric mass lies below this level.
• It is an important upper-air analysis level.

Chart

• 500 hPa chart uses contours/isohypses.
• Source gives 60 geopotential metres (gpm) as a standard plotting interval.

32. 400 hPa

• Approximately 24,000 ft
• Approximately FL240
• Used for upper wind and temperature charts.

33. 300 hPa

• Approximately 30,000 ft
• Approximately FL300
• Approximately 9,144–9,160 m
• Important for upper-level wind and temperature analysis.

Example

A contour height around 9160 m is associated with approximately:

300 hPa

34. 200 hPa

• Approximately 39,000–40,000 ft
• Approximately FL390
• Important high-altitude/jet-level chart.

The source gives both ~38,662 ft and the rounded operational value of ~40,000 ft.

35. Selecting the Correct Constant-Pressure Chart

FL060 / 6,000 ft

Known levels:

• 850 hPa ≈ 5,000 ft
• 700 hPa ≈ 10,000 ft

For FL060, the source identifies:

800 hPa

as the closest representative chart among the given options.

FL170 / 17,000 ft

Known levels:

• 700 hPa ≈ FL100
• 500 hPa ≈ FL180

Closest standard chart:

500 hPa

36. Tropopause

Definition

The tropopause is the boundary between:

• Troposphere
• Stratosphere

It marks the level where the normal decrease of temperature with height ceases.

Temperature

• Troposphere: temperature generally decreases with height.
• At/near tropopause: temperature stops decreasing.
• Lower stratosphere: temperature may remain approximately constant or begin increasing.

Latitude variation

The tropopause is not at the same height everywhere.

The source gives approximately:

• Equator: 16–18 km
• Poles: ~8 km
• ISA average: ~11 km

37. Temperature Calculation Around Tropopause

Example in the source:

Given:

• Temperature at FL300 = −54°C
• Tropopause = FL330
• FL350 is above the tropopause.

FL300 → FL330

Difference:

3,000 ft

Using approximately 2°C/1,000 ft:

Temperature decrease = 6°C

Therefore:

Temperature at FL330 = −54 − 6 = −60°C

FL330 → FL350

• FL350 is in the lower stratosphere.
• Temperature is assumed approximately constant.

Therefore:

Temperature at FL350 ≈ −60°C

38. Barometer and Barograph

Barometer

• Measures atmospheric pressure.
• Can be:
  • Mercurial
  • Aneroid

Barograph

• A recording aneroid barometer.
• Provides a continuous record of atmospheric pressure over time.
• Usually uses a pen/arm and rotating chart drum.
• Useful for determining pressure tendency.

Other instruments

39. Diurnal / Semi-Diurnal Pressure Variation

The atmospheric pressure has a 12-hour/semi-diurnal oscillation.

Tropical and subtropical regions

• Variation is approximately 3 hPa between high and low values.

Temperate regions

• Variation is approximately 1 hPa.

Pressure maxima

• Around 1000 local time
• Around 2200 local time

Pressure minima

• Around 0400 local time
• Around 1600 local time

Quick memory

High: 1000 / 2200
Low: 0400 / 1600

40. Northern vs Southern Hemisphere Low Pressure

Northern Hemisphere

• Low/cyclone circulation:
  • Counter-clockwise
• Surface wind crosses inward toward low pressure.
• Friction contributes to the inward crossing.

Southern Hemisphere

• Low/cyclone circulation:
  • Clockwise
• Coriolis deflection is toward the left.
• Surface air spirals inward toward the low.

41. Important MCQ Relationships

Pressure system

Isobar / pressure gradient

Altimeter

42. High-Yield Memory Chain

High pressure

High → Divergence → Subsidence → Compression → Warming → Stability → Cloud dissipation

But:

Subsidence → Inversion → Trapped pollution/moisture → Haze/Fog → Poor visibility

Low pressure

Low → Convergence → Ascent → Expansion → Cooling → Condensation → Clouds → Precipitation

And:

Ascent/mixing → Better visibility outside precipitation

43. Most Important Numerical Values to Memorize

44. Final Rapid-Revision Sheet

Pressure

• Pressure = weight of air column.
• Pressure decreases with height.
• Decrease is fastest near surface.
• Cold/dense air → pressure falls faster with height.
• Warm/less dense air → pressure falls slower.

Pressure systems

• High: divergence + subsidence.
• Low: convergence + ascent.
• Ridge: elongated high.
• Trough: elongated low.
• Col: weak/stagnant pressure area.

Weather

• High → stable, generally fair, light winds, possible haze/fog.
• Low → unstable, clouds, precipitation, generally better visibility outside precipitation.
• Subsidence inversion traps pollutants/moisture.

Charts

• Isobar → equal pressure.
• Isotherm → equal temperature.
• Isallobar → equal pressure tendency.
• Isohypses/contours → equal height on constant-pressure chart.
• Close isobars → strong wind.
• Wide isobars → light wind.

Altimeter

• Altimeter measures pressure, displays height/altitude.
• QNH → altitude above MSL.
• QFE → height above aerodrome.
• SPS 1013.25 → pressure altitude/Flight Level.
• QFF → meteorological chart pressure, not altimetry.

Altimeter errors

• High → Low = true altitude decreases → beware below.
• Low → High = true altitude increases.
• Warm → true altitude higher.
• Cold → true altitude lower.

QNH/QFE

• Aerodrome AMSL → QNH > QFE
• Aerodrome at MSL → QNH = QFF = QFE
• AMSL + cold → QFF > QNH
• BMSL + cold → QNH > QFF

Pressure levels

850 → 5,000 → 700 → 10,000 → 500 → 18,000 → 400 → 24,000 → 300 → 30,000 → 200 → 39/40,000 ft