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PCAA Meteorology practice questions

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Question 1The Atmosphere: Composition, Structure and Standard Atmosphere

In the ICAO Standard Atmosphere, what is the assumed sea-level pressure?

  1. 1013.25 hPa Correct answer.
  2. 1025.00 hPa
  3. 998.20 hPa
  4. 1000.00 hPa

Explanation

The ICAO Standard Atmosphere adopts a mean sea-level pressure of 1013.25 hPa (29.92 inHg), which is the reference from which pressure altitude is defined. This value is fixed by convention so that altimeters and performance charts share a common datum. Any other figure would shift the entire pressure-altitude scale.

Question 2The Atmosphere: Composition, Structure and Standard Atmosphere

Approximately what proportion of the atmosphere's total mass lies below 5500 m (about 18000 ft)?

  1. One tenth
  2. Three quarters
  3. One half Correct answer.
  4. One quarter

Explanation

Because density decreases roughly exponentially with height, about half of the atmospheric mass is contained in the lowest 5.5 km, the layer that holds most weather and most of the water vapour. This is why aviation weather and aircraft performance are so sensitive to conditions in the lower troposphere. The remaining half is spread through a much greater depth of thin air above.

Question 3The Atmosphere: Composition, Structure and Standard Atmosphere

Which gas is the most abundant constituent of the dry atmosphere by volume?

  1. Carbon dioxide
  2. Oxygen
  3. Argon
  4. Nitrogen Correct answer.

Explanation

Nitrogen makes up roughly 78 percent of dry air by volume, with oxygen at about 21 percent and argon near 0.93 percent. Carbon dioxide, though radiatively important, is only around 0.04 percent. The dominance of nitrogen is a defining feature of the atmosphere's composition.

Question 4The Atmosphere: Composition, Structure and Standard Atmosphere

In which atmospheric layer does temperature increase with height due to absorption of ultraviolet radiation by ozone?

  1. Exosphere
  2. Stratosphere Correct answer.
  3. Mesosphere
  4. Troposphere

Explanation

The stratosphere contains the ozone layer, and ozone absorption of solar ultraviolet energy warms the upper stratosphere, producing a temperature inversion with height. That stable stratification suppresses vertical motion and gives commercial jets a smooth cruise environment. The other layers either cool with height or are too tenuous to matter here.

Question 5The Atmosphere: Composition, Structure and Standard Atmosphere

What is the standard lapse rate of the ICAO Standard Atmosphere in the troposphere?

  1. 3.00 degrees C per 1000 ft
  2. 1.98 degrees C per 1000 ft Correct answer.
  3. 1.50 degrees C per 1000 ft
  4. 2.00 degrees C per 1000 ft

Explanation

The standard atmosphere assumes a tropospheric temperature decrease of 6.5 degrees C per km, which converts to about 1.98 degrees C per 1000 ft. This value underpins pressure-altitude and density-altitude computations in performance planning. It is a defined convention, not a measured average for any particular day.

Question 6The Atmosphere: Composition, Structure and Standard Atmosphere

At the tropopause in the ICAO Standard Atmosphere, what are the assumed temperature and pressure?

  1. Minus 56.5 degrees C and 226.32 hPa Correct answer.
  2. Minus 65.0 degrees C and 150.00 hPa
  3. Minus 50.0 degrees C and 250.00 hPa
  4. Minus 40.0 degrees C and 300.00 hPa

Explanation

The standard atmosphere places the tropopause at 11 km (about 36089 ft) with a temperature of minus 56.5 degrees C and a pressure near 226.32 hPa. Above that level the standard assumes an isothermal layer up to 20 km. These values are used when converting altitudes above the tropopause.

Question 7The Atmosphere: Composition, Structure and Standard Atmosphere

An aircraft cabin is pressurised to a cabin altitude of 8000 ft. Using the standard atmosphere, which statement best describes the cabin pressure compared with sea level?

  1. It is nearly equal to sea-level pressure
  2. It is roughly one tenth of sea-level pressure
  3. It is roughly half of sea-level pressure
  4. It is roughly three quarters of sea-level pressure Correct answer.

Explanation

In the standard atmosphere, pressure at 8000 ft is approximately 356 hPa, about 74 percent of the 1013 hPa sea-level value, so roughly three quarters is the best description. Pressure falls off with height in a roughly exponential manner, so an 8000 ft cabin is noticeably but not drastically reduced. This matters for passengers with respiratory conditions.

Question 8The Atmosphere: Composition, Structure and Standard Atmosphere

Which statement correctly describes how water vapour is distributed in the atmosphere compared with the permanent gases?

  1. It is confined entirely to the stratosphere
  2. It is highly variable, ranging from near zero to about 4 percent by volume Correct answer.
  3. It is uniformly mixed at about 4 percent everywhere
  4. It is more abundant at high altitude than near the surface

Explanation

Water vapour is a variable constituent, ranging from almost nothing in cold dry air to around 4 percent by volume in warm humid tropical air. Unlike nitrogen and oxygen, it is not well mixed because it condenses, evaporates and is transported by weather systems. It is concentrated in the lower troposphere, where most evaporation occurs.

Question 9The Atmosphere: Composition, Structure and Standard Atmosphere

Why does atmospheric pressure decrease with altitude?

  1. The mass of overlying air decreases with height Correct answer.
  2. The proportion of oxygen increases with height
  3. Temperature always decreases with height
  4. Gravity weakens rapidly with height

Explanation

Pressure at any level is the weight of the air column above it, so as you climb there is less overlying mass and pressure falls. Density also decreases, which makes the decrease roughly exponential rather than linear. Gravity is effectively constant over the depths of the atmosphere used in aviation.

Question 10The Atmosphere: Composition, Structure and Standard Atmosphere

The standard atmosphere is best described as which of the following?

  1. A forecast of the atmosphere valid for 24 hours
  2. The average of all radiosonde observations worldwide
  3. A model used only above the tropopause
  4. A defined reference model with specified temperature, pressure and density profiles Correct answer.

Explanation

The standard atmosphere is a defined reference model, not a statistical average of observations, with specified sea-level values, lapse rate and tropopause parameters. It provides a common datum so that altimeters, performance charts and aircraft design share consistent assumptions. Real atmospheric conditions routinely differ from it.

Question 11The Atmosphere: Composition, Structure and Standard Atmosphere

At 20000 ft in the standard atmosphere, the temperature is approximately minus 25 degrees C. If the actual temperature at that level is minus 40 degrees C, what is the temperature deviation?

  1. Plus 65 degrees C
  2. Minus 65 degrees C
  3. Plus 15 degrees C
  4. Minus 15 degrees C Correct answer.

Explanation

Deviation is the actual temperature minus the standard temperature, so minus 40 minus minus 25 equals minus 15 degrees C. A negative deviation means the air is colder than standard, which increases density and improves some performance parameters while degrading others. Deviation is used to adjust performance calculations from standard charts.

Question 12The Atmosphere: Composition, Structure and Standard Atmosphere

Which atmospheric layer is characterised by temperature decreasing with height and contains most of the atmosphere's water vapour and weather?

  1. Thermosphere
  2. Mesosphere
  3. Troposphere Correct answer.
  4. Stratosphere

Explanation

The troposphere is heated mainly from below and cools with height, and it holds nearly all the water vapour and virtually all weather. Its depth varies from roughly 8 km at the poles to 16 km in the tropics. Aircraft operating in it are subject to turbulence, icing and convective hazards.

Question 13The Atmosphere: Composition, Structure and Standard Atmosphere

What is the primary significance of the tropopause for aviation?

  1. It is the level where ozone concentration is lowest
  2. It is the altitude at which altimeters are always set to 1013 hPa
  3. It marks the level above which jet aircraft may cruise with less turbulence and where the temperature stops falling Correct answer.
  4. It is the maximum certified altitude for all aircraft

Explanation

The tropopause is the boundary between the troposphere and the stratosphere, where the lapse rate becomes small or reverses. Its stable stratification reduces vertical motion and turbulence, which is why high-altitude cruise often occurs near or above it. It also acts as a lid on most convective weather.

Question 14The Atmosphere: Composition, Structure and Standard Atmosphere

A parcel of dry air at 3000 ft has a temperature of 15 degrees C. Using a dry adiabatic lapse rate of 3 degrees C per 1000 ft, what is its temperature after being lifted adiabatically to 8000 ft?

  1. 10 degrees C
  2. Minus 5 degrees C
  3. 5 degrees C
  4. 0 degrees C Correct answer.

Explanation

The parcel rises 5000 ft, and at 3 degrees C per 1000 ft that is a cooling of 15 degrees C. Starting at 15 degrees C, the parcel reaches 0 degrees C. This dry adiabatic process applies until saturation occurs and the saturated rate takes over.

Question 15The Atmosphere: Composition, Structure and Standard Atmosphere

Which statement about the composition of the atmosphere is correct?

  1. Nitrogen and oxygen together make up about 99 percent by volume Correct answer.
  2. Oxygen is the most abundant gas by volume
  3. Carbon dioxide is the third most abundant gas
  4. Argon is more abundant than oxygen

Explanation

Nitrogen at about 78 percent and oxygen at about 21 percent together account for roughly 99 percent of dry air by volume. Argon is the third most abundant at about 0.93 percent. Carbon dioxide is a minor constituent despite its importance for radiative transfer.

Question 16The Atmosphere: Composition, Structure and Standard Atmosphere

At an altitude of 18000 ft, standard pressure is about 506 hPa. If the local altimeter setting is 1013 hPa and the aircraft altimeter reads 18000 ft, which statement is correct?

  1. The aircraft is at a temperature altitude of 18000 ft
  2. The aircraft is at a true altitude of 18000 ft
  3. The aircraft is at a density altitude of 18000 ft
  4. The aircraft is at a pressure altitude of 18000 ft Correct answer.

Explanation

When the altimeter is set to the standard 1013 hPa, the indicated altitude is pressure altitude by definition. True altitude, density altitude and temperature altitude are separate quantities that depend on actual temperature and pressure distribution. Pressure altitude is therefore the value read directly from the altimeter with the standard setting.

Question 17The Atmosphere: Composition, Structure and Standard Atmosphere

Approximately how much of the atmosphere's mass lies below 11000 m (about 36000 ft)?

  1. About 90 percent
  2. About 30 percent
  3. About 50 percent
  4. About 70 percent Correct answer.

Explanation

Roughly 70 percent of the atmospheric mass lies below the tropopause at about 11 km. The mass distribution is exponential, so a large fraction is concentrated in the lowest few kilometres. This concentration explains why most weather and most of the water vapour are found in the troposphere.

Question 18The Atmosphere: Composition, Structure and Standard Atmosphere

Which statement best explains why density altitude increases on a hot day at a fixed pressure altitude?

  1. Warmer air is denser, so the density altitude is lower than the pressure altitude
  2. Density altitude depends only on pressure, not temperature
  3. Warmer air is less dense, so the density altitude is higher than the pressure altitude Correct answer.
  4. Density altitude is always equal to true altitude

Explanation

Density altitude is the altitude in the standard atmosphere at which the observed density would occur. Because warm air is less dense at a given pressure, the density altitude exceeds the pressure altitude on a hot day. This reduces lift and engine performance, which is why hot-and-high operations require careful planning.

Question 19The Atmosphere: Composition, Structure and Standard Atmosphere

Which gas in the atmosphere is primarily responsible for absorbing the majority of incoming solar ultraviolet radiation in the stratosphere?

  1. Carbon dioxide
  2. Water vapour
  3. Methane
  4. Ozone Correct answer.

Explanation

Ozone absorbs most of the harmful ultraviolet radiation in the stratosphere, protecting life at the surface and warming the upper stratosphere. This absorption creates the temperature inversion that stabilises the layer. Other gases absorb infrared radiation but play little role in ultraviolet absorption.

Question 20The Atmosphere: Composition, Structure and Standard Atmosphere

In the standard atmosphere, the temperature remains constant with height between 11 km and 20 km. What is the main consequence of this isothermal layer?

  1. Temperature increases rapidly, so the layer is unstable
  2. The layer contains most of the atmospheric water vapour
  3. Density increases with height, so pressure rises
  4. Pressure and density both decrease exponentially with height Correct answer.

Explanation

With constant temperature, hydrostatic balance and the ideal-gas relation make pressure and density decrease exponentially with height. At the tropopause, density's fractional decrease is faster just above the boundary than just below it, because tropospheric cooling partly offsets the density decrease caused by falling pressure.

Question 21Atmospheric Thermodynamics and Stability

In a parcel of unsaturated air that is lifted adiabatically, which quantity remains constant?

  1. Relative humidity
  2. Potential temperature Correct answer.
  3. Mixing ratio
  4. Equivalent potential temperature

Explanation

For an adiabatic process in unsaturated air, potential temperature is conserved because entropy is constant. Relative humidity and mixing ratio change when the parcel is lifted, and equivalent potential temperature is conserved only in saturated or moist-adiabatic processes where latent heat release matters.

Question 22Atmospheric Thermodynamics and Stability

What does the environmental lapse rate describe?

  1. The vertical temperature profile of the surrounding atmosphere Correct answer.
  2. The rate at which a rising parcel cools
  3. The rate of moisture loss in a cloud
  4. The rate of pressure decrease with height

Explanation

The environmental lapse rate is the actual change of temperature with height measured in the ambient atmosphere. It is distinct from the dry and saturated adiabatic lapse rates, which describe how a parcel's temperature changes as it rises.

Question 23Atmospheric Thermodynamics and Stability

Which condition indicates an absolutely unstable atmosphere?

  1. Environmental lapse rate equal to zero
  2. Environmental lapse rate less than the saturated adiabatic lapse rate
  3. Environmental lapse rate between the dry and saturated adiabatic lapse rates
  4. Environmental lapse rate greater than the dry adiabatic lapse rate Correct answer.

Explanation

Absolute instability occurs when the environmental lapse rate exceeds the dry adiabatic lapse rate, so a lifted parcel remains warmer than its surroundings even without condensation. Values between the dry and saturated rates give conditional instability, and values below the saturated rate give absolute stability.

Question 24Atmospheric Thermodynamics and Stability

A parcel of air at 20 °C with a mixing ratio of 10 g/kg is lifted until it becomes saturated. Which quantity is most directly conserved during the dry ascent before saturation?

  1. Equivalent potential temperature
  2. Density
  3. Wet-bulb potential temperature
  4. Mixing ratio Correct answer.

Explanation

In a closed parcel with no condensation and no mixing, the water vapor mixing ratio remains constant during dry ascent. Density decreases as the parcel expands, and the potential temperatures that involve latent heat are conserved only through moist processes.

Question 25Atmospheric Thermodynamics and Stability

Which process releases latent heat into an air parcel?

  1. Sublimation of ice
  2. Melting of ice crystals
  3. Evaporation of cloud droplets
  4. Condensation of water vapor Correct answer.

Explanation

Condensation converts water vapor to liquid water and releases latent heat, warming the parcel. Evaporation, melting, and sublimation all absorb latent heat from the environment, cooling the parcel.

Question 26Atmospheric Thermodynamics and Stability

Which lifted condensation level (LCL) calculation is most appropriate for a surface parcel with temperature T and dew point Td?

  1. LCL height ≈ 800 m per °C of (T − Td)
  2. LCL height ≈ 500 m per hPa of surface pressure
  3. LCL height ≈ 125 m per °C of (T − Td) Correct answer.
  4. LCL height ≈ 10 m per °C of (T + Td)

Explanation

A common rule of thumb gives the LCL about 125 m above the surface for each degree Celsius of dew-point depression. This reflects the dry adiabatic cooling rate and the slower dew-point decrease with height.

Question 27Atmospheric Thermodynamics and Stability

At a constant temperature, adding water vapor to a fixed volume of air does what to the air density?

  1. First increases then decreases it
  2. Decreases it
  3. Leaves it unchanged
  4. Increases it Correct answer.

Explanation

Adding water vapor without removing air increases the mass in the fixed volume, so density increases because density equals mass divided by volume. The lower density of moist air at equal temperature and total pressure refers to vapor replacing heavier dry-air molecules, not simple addition.

Question 28Atmospheric Thermodynamics and Stability

A parcel is lifted along a saturated adiabat. Why is its cooling rate smaller than the dry adiabatic rate?

  1. Because condensation releases latent heat that partly offsets expansion cooling Correct answer.
  2. Because pressure decreases more slowly with height
  3. Because water vapor has a higher heat capacity than dry air
  4. Because the parcel mixes with warmer environmental air

Explanation

As the saturated parcel rises and cools, water vapor condenses and releases latent heat, partially compensating for adiabatic expansion cooling. This makes the saturated adiabatic lapse rate smaller than the dry adiabatic rate.

Question 29Atmospheric Thermodynamics and Stability

What is the primary effect of entrainment on a rising cumulus cloud?

  1. It warms and moistens the cloud
  2. It raises the cloud base
  3. It increases the cloud's liquid water content
  4. It cools and dries the cloud, reducing buoyancy Correct answer.

Explanation

Entrainment mixes cooler, drier environmental air into the cloud, evaporating some droplets and cooling the cloud. This reduces buoyancy and can limit vertical development.

Question 30Atmospheric Thermodynamics and Stability

Which statement best describes conditional instability?

  1. The environmental lapse rate lies between the dry and saturated adiabatic lapse rates Correct answer.
  2. The environmental lapse rate exceeds the dry adiabatic lapse rate
  3. The environmental lapse rate is less than the saturated adiabatic lapse rate
  4. The environmental lapse rate is exactly zero

Explanation

Conditional instability exists when the environmental lapse rate falls between the saturated and dry adiabatic rates. In that case, a saturated parcel is buoyant while an unsaturated parcel is not, so instability depends on whether condensation occurs.

Question 31Atmospheric Thermodynamics and Stability

Which quantity is conserved during a saturated adiabatic process in which condensate falls out of the parcel?

  1. Relative humidity
  2. Virtual temperature
  3. Equivalent potential temperature Correct answer.
  4. Liquid water mixing ratio

Explanation

For a pseudoadiabatic process, in which condensate is removed, equivalent potential temperature is approximately conserved. Relative humidity stays near saturation, and liquid water is removed rather than conserved.

Question 32Atmospheric Thermodynamics and Stability

Why does the wet adiabatic lapse rate decrease as temperature increases?

  1. Warmer air conducts heat away faster
  2. Warmer air contains less water vapor
  3. Warmer air has a lower saturation vapor pressure
  4. Warmer air has a higher saturation vapor pressure, so more latent heat is released per degree of cooling Correct answer.

Explanation

At higher temperatures the saturation vapor pressure is larger, so a given cooling step produces more condensation and more latent heat release. That extra latent heat offsets more of the expansion cooling, lowering the saturated adiabatic lapse rate.

Question 33Atmospheric Thermodynamics and Stability

A parcel has a temperature of 10 °C and a dew point of 6 °C at the surface. Using the 125 m per °C rule, at approximately what height is its LCL?

  1. 1000 m
  2. 250 m
  3. 500 m Correct answer.
  4. 750 m

Explanation

The dew-point depression is 4 °C, and multiplying by 125 m per °C gives about 500 m. This is the approximate height where the parcel reaches saturation.

Question 34Atmospheric Thermodynamics and Stability

Which sounding profile would most favor deep moist convection?

  1. A dry boundary layer with a strong stable layer above
  2. A shallow moist layer over a strong low-level inversion
  3. A nearly isothermal profile through the troposphere
  4. A steep lapse rate with abundant low-level moisture and a capping inversion that weakens Correct answer.

Explanation

Deep convection requires conditional instability, sufficient moisture, and a trigger to break any cap. A steep lapse rate supplies instability, low-level moisture supplies latent heat, and a weakening cap allows parcels to reach their level of free convection.

Question 35Atmospheric Thermodynamics and Stability

Which quantity captures the combined effect of temperature and moisture on air density?

  1. Equivalent potential temperature
  2. Potential temperature
  3. Wet-bulb temperature
  4. Virtual temperature Correct answer.

Explanation

Virtual temperature is the temperature dry air would need to have the same density as the moist air. It accounts for the lower molecular weight of water vapor, so buoyancy calculations use it.

Question 36Atmospheric Thermodynamics and Stability

What happens to the relative humidity of an unsaturated parcel as it is lifted dry adiabatically?

  1. It decreases
  2. It increases Correct answer.
  3. It first decreases then increases
  4. It stays constant

Explanation

As the parcel cools during lifting, its saturation vapor pressure falls while the mixing ratio stays constant. The ratio of vapor pressure to saturation vapor pressure therefore rises, so relative humidity increases toward saturation.

Question 37Atmospheric Thermodynamics and Stability

Which statement about potential temperature is correct?

  1. It equals the actual temperature at all heights
  2. It is the temperature a parcel would have if brought adiabatically to a reference pressure Correct answer.
  3. It is conserved only in saturated processes
  4. It always decreases with height in a stable atmosphere

Explanation

Potential temperature is defined as the temperature an air parcel would reach if moved adiabatically to a standard reference pressure, usually 1000 hPa. It is conserved in dry adiabatic processes and is used to compare air masses at different levels.

Question 38Atmospheric Thermodynamics and Stability

A layer has a potential temperature that increases sharply with height. What does this indicate?

  1. Absolute instability
  2. Neutral stability
  3. Strong static stability Correct answer.
  4. Strong conditional instability

Explanation

A rapid increase of potential temperature with height means a parcel displaced upward becomes colder than its environment and sinks back. That is the signature of strong static stability.

Question 39Atmospheric Thermodynamics and Stability

Which combination of processes can produce a saturated adiabatic lapse rate close to the dry adiabatic rate?

  1. Very warm temperatures with abundant condensate
  2. Strong vertical wind shear
  3. High pressure with dry air
  4. Very cold temperatures with little condensate Correct answer.

Explanation

At very low temperatures the saturation vapor pressure and moisture content are small, so little latent heat is released during ascent. The saturated adiabatic lapse rate then approaches the dry adiabatic rate.

Question 40Atmospheric Thermodynamics and Stability

Which quantity is most useful for comparing the buoyancy of parcels that differ in both temperature and moisture?

  1. Potential temperature
  2. Relative humidity
  3. Mixing ratio
  4. Virtual potential temperature Correct answer.

Explanation

Virtual potential temperature combines the effects of temperature, pressure, and moisture on density. Comparing virtual potential temperatures of a parcel and its environment gives a direct measure of buoyancy.

Question 41Atmospheric Thermodynamics and Stability

A parcel is warmer than its environment at its starting level but cooler than the environment after being lifted. What is the stability of that layer?

  1. Stable Correct answer.
  2. Conditionally unstable
  3. Neutral
  4. Unstable

Explanation

If a displaced parcel becomes cooler than its surroundings, it is negatively buoyant and tends to return to its original level. That restoring behavior defines a stable layer.

Question 42Atmospheric Thermodynamics and Stability

In a saturated parcel rising through a pseudoadiabatic process, which pair of quantities is approximately conserved?

  1. Density and pressure
  2. Potential temperature and mixing ratio
  3. Relative humidity and virtual temperature
  4. Equivalent potential temperature and wet-bulb potential temperature Correct answer.

Explanation

In a pseudoadiabatic process, condensate is removed and equivalent potential temperature and wet-bulb potential temperature are both approximately conserved. These conserved variables are used to trace saturated air masses.

Question 43Atmospheric Thermodynamics and Stability

Why can a layer with a lapse rate slightly less than the dry adiabatic rate still support deep convection when moisture is present?

  1. Latent heat release in saturated ascent can make the parcel warmer than the environment Correct answer.
  2. The environmental lapse rate increases with height automatically
  3. Pressure decreases faster than temperature
  4. The parcel's mixing ratio increases with height

Explanation

When the lapse rate lies between the saturated and dry adiabatic rates, the atmosphere is conditionally unstable. A saturated parcel that releases latent heat can become warmer than the environment and continue rising even though a dry parcel would not.

Question 44Atmospheric Thermodynamics and Stability

Two parcels have the same temperature but different mixing ratios. How do their virtual temperatures compare?

  1. The parcel with the lower mixing ratio has the higher virtual temperature
  2. The parcel with the higher mixing ratio has the higher virtual temperature Correct answer.
  3. Virtual temperature depends only on pressure
  4. Both have identical virtual temperatures

Explanation

Water vapor lowers the mean molecular weight of air, so moist air is less dense than dry air at the same temperature and pressure. The parcel with more vapor therefore has a higher virtual temperature, all else equal.

Question 45Pressure, Density and Altimetry

In the ICAO Standard Atmosphere, what is the sea-level pressure value used as the reference for pressure altimetry?

  1. 1000.00 hPa
  2. 1013.25 hPa Correct answer.
  3. 1015.00 hPa
  4. 960.00 hPa

Explanation

The ICAO Standard Atmosphere fixes mean sea-level pressure at 1013.25 hPa (29.92 inHg), together with 15 °C and a standard lapse rate. This value is the datum against which pressure altitude is referenced, so an altimeter set to 1013 must indicate aerodrome elevation when the aerodrome is at standard pressure.

Question 46Pressure, Density and Altimetry

An aircraft on the ground at an aerodrome reports a pressure altitude of 2000 ft while the aerodrome elevation is 1500 ft. What does this indicate about the actual sea-level pressure compared with 1013.25 hPa?

  1. It is higher than standard Correct answer.
  2. It is equal to standard
  3. It cannot be deduced from these figures
  4. It is lower than standard

Explanation

Pressure altitude higher than true elevation means the altimeter, referenced to 1013.25 hPa, reads too high, which happens when the real sea-level pressure exceeds the standard value. Roughly 1 hPa corresponds to about 30 ft, so a 500 ft excess indicates a QNH around 1030 hPa.

Question 47Pressure, Density and Altimetry

Which statement correctly describes the relationship between air density, pressure, and temperature in the lower atmosphere?

  1. Density rises when temperature rises at constant pressure
  2. Density is independent of both pressure and temperature
  3. Density rises when humidity rises at constant pressure and temperature
  4. Density falls when pressure falls at constant temperature Correct answer.

Explanation

From the gas law, density is proportional to pressure and inversely proportional to absolute temperature. Therefore, at constant temperature, a decrease in pressure produces a decrease in density. This is why aircraft performance degrades at high-altitude aerodromes.

Question 48Pressure, Density and Altimetry

An aerodrome has elevation 4000 ft and reports QNH 1000 hPa. Using the approximation that 1 hPa equals 30 ft, what is the approximate pressure altitude?

  1. 4300 ft Correct answer.
  2. 3700 ft
  3. 4400 ft
  4. 4000 ft

Explanation

Pressure altitude equals elevation plus 30 ft for each hectopascal by which QNH falls below 1013.25. The difference is about 13 hPa, giving roughly 390 ft, so the pressure altitude is close to 4390 ft, which rounds to about 4300 ft.

Question 49Pressure, Density and Altimetry

Compared with dry air at the same pressure and temperature, moist air is:

  1. Denser only above the tropopause
  2. Of identical density
  3. More dense
  4. Less dense Correct answer.

Explanation

Water vapour has a lower molecular mass than the nitrogen and oxygen it displaces, so humid air is less dense than dry air at the same pressure and temperature. This is why high humidity slightly increases density altitude and reduces aircraft performance.

Question 50Pressure, Density and Altimetry

What is the definition of density altitude?

  1. The altitude indicated on the altimeter with QNH set
  2. The altitude at which the outside air temperature reaches 15 °C
  3. The altitude of the aircraft above mean sea level
  4. The altitude in the standard atmosphere at which the actual air density occurs Correct answer.

Explanation

Density altitude is the pressure altitude corrected for temperature deviation from the standard atmosphere. It expresses the actual density as the standard-atmosphere altitude where that density would occur. High density altitude means degraded engine, propeller, and wing performance.