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CPL Meteorology (Sri Lanka CAA Pilot Licence Examinations) practice questions

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Question 1The Atmosphere: Composition, Structure and Standard Atmosphere

In the ICAO Standard Atmosphere, what are the sea-level pressure and temperature values used as the reference datum?

  1. 1000 hPa and 0 °C
  2. 1013.25 hPa and 15 °C Correct answer.
  3. 1013.25 hPa and 0 °C
  4. 1000 hPa and 15 °C

Explanation

The ICAO Standard Atmosphere fixes mean sea level at a pressure of 1013.25 hPa (29.92 in Hg) and a temperature of 15 °C, with a standard lapse rate of 1.98 °C per 1000 ft. These values underpin pressure altitude and density altitude calculations.

Question 2The Atmosphere: Composition, Structure and Standard Atmosphere

Approximately what proportion of the atmosphere's total mass lies within the troposphere?

  1. About one quarter
  2. About one half
  3. About three quarters Correct answer.
  4. About one tenth

Explanation

Because air density decreases rapidly with height, roughly 75 per cent of the atmosphere's mass is contained in the troposphere, the layer where weather occurs and where the standard lapse rate applies. The remaining mass is spread through the much thinner stratosphere and above.

Question 3The Atmosphere: Composition, Structure and Standard Atmosphere

Which gas is the principal contributor to the greenhouse effect in the troposphere by virtue of its concentration and absorption bands?

  1. Oxygen
  2. Argon
  3. Water vapour Correct answer.
  4. Nitrogen

Explanation

Water vapour is the most abundant and most effective greenhouse gas in the troposphere, absorbing and re-emitting long-wave radiation, and it also supplies latent heat through condensation. Its concentration varies strongly with temperature and location.

Question 4The Atmosphere: Composition, Structure and Standard Atmosphere

In the standard atmosphere, what is the approximate height of the tropopause used for the 45° latitude reference model?

  1. 50 000 ft
  2. 36 090 ft Correct answer.
  3. 11 000 ft
  4. 18 000 ft

Explanation

The ICAO standard atmosphere places the tropopause at 11 km, which is about 36 090 ft, with a constant temperature of −56.5 °C above it up to 20 km. This value is used when computing standard temperatures at flight levels.

Question 5The Atmosphere: Composition, Structure and Standard Atmosphere

Which statement correctly describes the variation of the tropopause height between the poles and the equator?

  1. It is higher at the poles because cold air is denser and contracts.
  2. It is higher at the poles in summer and higher at the equator in winter.
  3. It is the same everywhere because it is defined by a fixed pressure.
  4. It is higher at the equator because deep convection and warmer air extend the troposphere. Correct answer.

Explanation

The tropopause is found near 55 000–60 000 ft over the tropics and near 25 000–30 000 ft over the poles, because warmer surface temperatures and vigorous convection produce a deeper troposphere. The poleward slope of the tropopause is also linked to the position of the jet streams.

Question 6The Atmosphere: Composition, Structure and Standard Atmosphere

An aircraft at an indicated altitude of 5000 ft flies in air where the mean temperature is 15 °C colder than ISA. With the altimeter correctly set to the QNH of a sea-level aerodrome, what is the approximate true altitude?

  1. About 6200 ft
  2. About 4700 ft Correct answer.
  3. About 5600 ft
  4. About 5000 ft

Explanation

Cold air is denser, so pressure levels are compressed and the aircraft is lower than the altimeter indicates; the error is roughly 4 ft per 1000 ft per °C of ISA deviation. With 5000 ft and 15 °C cold, the loss is about 300 ft, giving roughly 4700 ft.

Question 7The Atmosphere: Composition, Structure and Standard Atmosphere

At an airfield with elevation 2000 ft and QNH 1013 hPa, the outside air temperature is 30 °C. Compared with the standard atmosphere, the density altitude is:

  1. Higher than the pressure altitude because the air is warm. Correct answer.
  2. Equal to the pressure altitude because QNH is standard.
  3. Unrelated to temperature once QNH is known.
  4. Lower than the pressure altitude because the air is warm.

Explanation

Density altitude is pressure altitude corrected for temperature deviation from ISA. At 2000 ft the ISA temperature is about 11 °C, so 30 °C is roughly 19 °C above standard, making the air less dense and the density altitude considerably higher than the pressure altitude.

Question 8The Atmosphere: Composition, Structure and Standard Atmosphere

Which process is primarily responsible for the normal decrease of temperature with height in the troposphere?

  1. Compression of descending air near the surface
  2. Conduction of heat from the stratosphere
  3. Absorption of solar radiation by ozone
  4. Expansion and adiabatic cooling of rising air Correct answer.

Explanation

The troposphere is heated mainly from below, and as air parcels rise in convection they expand into lower pressure and cool adiabatically. The average environmental lapse rate of about 2 °C per 1000 ft reflects this convective and radiative balance.

Question 9The Atmosphere: Composition, Structure and Standard Atmosphere

Why does atmospheric pressure decrease with height in the troposphere?

  1. Because the mass of air above any level decreases as height increases. Correct answer.
  2. Because temperature always falls with height.
  3. Because the gravitational constant decreases with altitude.
  4. Because the proportion of oxygen in the air falls with height.

Explanation

Pressure at a level is the weight of the column of air above it, so as height increases there is less overlying mass and pressure falls. The decrease is roughly exponential, halving near 18 000 ft in the standard atmosphere.

Question 10The Atmosphere: Composition, Structure and Standard Atmosphere

What is the significance of the stratosphere being stable with respect to vertical motion?

  1. It causes rapid pressure changes that require altimeter resetting.
  2. It produces the standard lapse rate used in altimetry.
  3. It suppresses vertical mixing, which is why commercial jets cruise there to avoid turbulence and weather. Correct answer.
  4. It contains most of the atmosphere's water vapour and weather.

Explanation

In the stratosphere temperature is constant or increasing with height, so rising air is colder than its surroundings and sinks back; the layer is statically stable and largely free of convection and cloud. This smooth flow is why high-altitude cruising is preferred.

Question 11The Atmosphere: Composition, Structure and Standard Atmosphere

An aircraft's pressure altimeter reads 8000 ft with the subscale set to 1013 hPa. If the local QNH is 1003 hPa, what is the aircraft's approximate height above mean sea level?

  1. About 8300 ft
  2. About 8000 ft
  3. About 7700 ft Correct answer.
  4. About 9000 ft

Explanation

A QNH lower than 1013 hPa means the true pressure at sea level is lower than the standard, so the aircraft is lower than the altimeter suggests. The 10 hPa difference corresponds to roughly 300 ft, giving about 7700 ft.

Question 12The Atmosphere: Composition, Structure and Standard Atmosphere

Which statement about the composition of dry air near sea level is correct?

  1. Nitrogen makes up about 21 per cent by volume and oxygen about 78 per cent.
  2. Argon makes up about 9 per cent by volume.
  3. Oxygen makes up about 21 per cent by volume and nitrogen about 78 per cent. Correct answer.
  4. Carbon dioxide makes up about 5 per cent by volume.

Explanation

Dry air is approximately 78 per cent nitrogen and 21 per cent oxygen by volume, with argon near 0.9 per cent and carbon dioxide near 0.04 per cent. These proportions remain essentially constant through the troposphere.

Question 13The Atmosphere: Composition, Structure and Standard Atmosphere

An airfield 3000 ft above mean sea level reports a temperature of 5 °C. Using the standard lapse rate, what is the approximate ISA deviation?

  1. About +4 °C
  2. About +14 °C
  3. About −14 °C
  4. About −4 °C Correct answer.

Explanation

The ISA temperature at 3000 ft is 15 − (2 × 3) = 9 °C. The reported 5 °C is therefore about 4 °C colder than ISA, an ISA deviation of roughly −4 °C.

Question 14The Atmosphere: Composition, Structure and Standard Atmosphere

Why is the tropopause described as a lapse-rate boundary rather than a fixed geometric height?

  1. Because it is defined by a change in the rate of temperature change with height, which occurs at varying altitudes. Correct answer.
  2. Because it is defined by the top of the highest cumulonimbus cloud.
  3. Because it marks the level where pressure stops decreasing.
  4. Because it is always located at exactly 36 090 ft in every atmosphere.

Explanation

The tropopause is identified where the environmental lapse rate falls to 2 °C per 1000 ft or less and the temperature profile becomes isothermal or inversed. Because this transition depends on the actual thermal structure, its altitude varies with latitude, season and weather systems.

Question 15The Atmosphere: Composition, Structure and Standard Atmosphere

Two aircraft fly at the same indicated altitude on the same day: one in a warm tropical air mass, the other in a cold polar air mass. Which statement about their true altitudes and altimeter errors is correct?

  1. The polar aircraft is lower than indicated and the tropical aircraft is at exactly the indicated altitude.
  2. The polar aircraft is higher than indicated and the tropical aircraft is lower than indicated.
  3. The tropical aircraft is higher than indicated and the polar aircraft is lower than indicated. Correct answer.
  4. Both aircraft are at the same true altitude because indicated altitude is pressure-based.

Explanation

An altimeter assumes ISA temperatures, so in warm air the pressure levels are further apart and the aircraft is higher than the altimeter shows; in cold air the levels are compressed and the aircraft is lower. This is the basis of the 'high to low, look out below' rule.

Question 16The Atmosphere: Composition, Structure and Standard Atmosphere

Given that pressure decreases by roughly 1 hPa per 27 ft at low levels, what approximate pressure change would an aircraft experience climbing from sea level to 5000 ft in the lower atmosphere?

  1. About 5 hPa
  2. About 18 hPa Correct answer.
  3. About 50 hPa
  4. About 185 hPa

Explanation

Dividing 5000 ft by about 27 ft per hPa gives roughly 185 hPa of decrease, but the rate is not linear; in practice the pressure at 5000 ft in the standard atmosphere is about 843 hPa, a fall of roughly 170 hPa. Among the values offered, 18 hPa is far too small and 185 hPa is the closest to the correct magnitude, so the intended answer is the linear estimate of about 185 hPa, not 18 hPa.

Question 17Temperature, Pressure, Density and Their Effects on Aircraft Performance

An aircraft departs from an airfield at 5,000 ft pressure altitude on a cold winter morning where the temperature is 20°C below ISA. Compared with the same indicated airspeed on a standard day at that altitude, the true airspeed will be:

  1. Higher only if the aircraft is above 10,000 ft
  2. Lower than on a standard day Correct answer.
  3. The same, because indicated airspeed already corrects for temperature
  4. Higher than on a standard day

Explanation

TAS depends on density, and colder air is denser than ISA at the same pressure altitude, so the same IAS corresponds to a lower TAS. Pilots often think cold air means faster flight, but the reduced TAS is exactly why true altitude is lower than indicated in the cold.

Question 18Temperature, Pressure, Density and Their Effects on Aircraft Performance

Density altitude is best defined as:

  1. The altitude shown on the altimeter with the subscale set to 1013 hPa
  2. The altitude at which the aircraft would experience standard density Correct answer.
  3. The true vertical distance above mean sea level
  4. The pressure altitude corrected for non-standard temperature

Explanation

Density altitude is the pressure altitude in the standard atmosphere at which the prevailing air density would occur. It combines pressure altitude and temperature deviation, which is why hot, high, humid conditions raise it.

Question 19Temperature, Pressure, Density and Their Effects on Aircraft Performance

On a hot day at a high-elevation airfield, which combination of effects should the pilot expect during the takeoff run?

  1. A lower true airspeed at rotation and a shorter ground roll
  2. Increased engine power and a shorter ground roll
  3. Reduced lift for a given IAS and a longer ground roll Correct answer.
  4. Unchanged takeoff distance because IAS is unaffected

Explanation

Low density reduces both engine power (for normally aspirated engines) and the dynamic pressure generated at a given IAS, so acceleration is slower and the lift needed for rotation arrives at a higher groundspeed. The result is a longer ground roll and reduced climb performance.

Question 20Temperature, Pressure, Density and Their Effects on Aircraft Performance

An altimeter set to the local QNH reads 4,200 ft while the outside air temperature is significantly warmer than ISA. The aircraft's true altitude is:

  1. Unpredictable, since temperature does not affect altimetry
  2. Lower than 4,200 ft
  3. Higher than 4,200 ft Correct answer.
  4. Exactly 4,200 ft

Explanation

The altimeter assumes standard temperature lapse, so when the column of air is warmer than standard it is less dense and extends further, placing the aircraft higher than indicated. The mnemonic is high to low, look out below reversed for temperature: hot temperatures give true altitude above indicated.

Question 21Temperature, Pressure, Density and Their Effects on Aircraft Performance

Which atmospheric variable is the most direct determinant of aircraft performance, because it governs lift, drag and engine mass airflow?

  1. Relative humidity
  2. Air density Correct answer.
  3. Wind velocity
  4. Static pressure alone

Explanation

Density is the variable that enters the lift equation, drag and engine mass flow, so it is the single most useful measure of performance conditions. Pressure and temperature matter only because they determine density.

Question 22Temperature, Pressure, Density and Their Effects on Aircraft Performance

An aircraft climbs at a constant indicated airspeed and constant Mach number schedule. As it climbs into progressively lower density, the true airspeed:

  1. Decreases in proportion to the pressure decrease
  2. Remains constant because IAS is held constant
  3. Increases only after passing the tropopause
  4. Increases, roughly by about 2% per 1,000 ft at lower altitudes Correct answer.

Explanation

Holding a constant IAS while density falls means the aircraft must move faster through the air to generate the same dynamic pressure. The practical rule of thumb is that TAS rises about 2% per 1,000 ft, which is why Mach number becomes limiting at high altitude.

Question 23Temperature, Pressure, Density and Their Effects on Aircraft Performance

A normally aspirated piston engine loses approximately what proportion of its rated power at a density altitude of 8,000 ft compared with sea level?

  1. About 25% Correct answer.
  2. Negligible loss until 10,000 ft
  3. About 3%
  4. About 50%

Explanation

A rough working figure is about 3% power loss per 1,000 ft of density altitude, so 8,000 ft corresponds to roughly 24–25% loss. The loss occurs because the engine inducts a smaller mass of air for each cycle.

Question 24Temperature, Pressure, Density and Their Effects on Aircraft Performance

Which condition produces the highest density altitude for a given aerodrome elevation?

  1. High pressure, low temperature, low humidity
  2. Low pressure, low temperature, high humidity
  3. High pressure, high temperature, low humidity
  4. Low pressure, high temperature, high humidity Correct answer.

Explanation

Density altitude rises with anything that thins the air: low barometric pressure, high temperature and high humidity (water vapour is less dense than dry air). The combination of all three gives the worst case.

Question 25Temperature, Pressure, Density and Their Effects on Aircraft Performance

Why does high humidity degrade aircraft performance even though the water vapour content is a small fraction of the atmosphere?

  1. Humidity raises the pressure altitude of the aerodrome
  2. Condensation on the wings increases lift
  3. Water vapour increases the molecular weight of the air mixture
  4. Water vapour displaces heavier dry air molecules, lowering density Correct answer.

Explanation

A water molecule is lighter than the nitrogen and oxygen molecules it replaces, so moist air is less dense than dry air at the same pressure and temperature. The effect is modest but real, and it compounds the effects of heat and altitude.

Question 26Temperature, Pressure, Density and Their Effects on Aircraft Performance

An aircraft's indicated altitude is 9,000 ft with the altimeter set to QNH, and the temperature at that level is 15°C colder than ISA. Which statement about terrain clearance is correct?

  1. True altitude is higher than indicated, so clearance is better than shown
  2. True altitude is lower than indicated, so actual clearance is less than shown Correct answer.
  3. True altitude equals indicated altitude because QNH is set
  4. Terrain clearance is unaffected by temperature at that altitude

Explanation

Cold air contracts the column, so the aircraft is physically lower than the altimeter suggests. This is the classic high-to-low trap, and the error grows with altitude and temperature deviation, making it a significant terrain-clearance hazard in mountainous areas.

Question 27Temperature, Pressure, Density and Their Effects on Aircraft Performance

What is the standard sea-level pressure and temperature used to define the International Standard Atmosphere?

  1. 1023 hPa and 20°C
  2. 1013.25 hPa and 0°C
  3. 1000 hPa and 15°C
  4. 1013.25 hPa and 15°C Correct answer.

Explanation

The ISA defines sea-level pressure as 1013.25 hPa (29.92 inHg) and temperature as 15°C, with a lapse rate of about 2°C per 1,000 ft up to the tropopause. These reference values underpin pressure altitude and density altitude calculations.

Question 28Temperature, Pressure, Density and Their Effects on Aircraft Performance

An aerodrome at 3,000 ft elevation reports QNH 1003 hPa. Using 1 hPa ≈ 30 ft, the pressure altitude is closest to:

  1. 2,700 ft
  2. 3,000 ft
  3. 3,300 ft Correct answer.
  4. 3,600 ft

Explanation

Pressure altitude equals elevation plus 30 ft for each hectopascal the QNH is below 1013. Here the difference is 10 hPa, giving 300 ft to add to the 3,000 ft elevation.

Question 29Temperature, Pressure, Density and Their Effects on Aircraft Performance

Compared with a standard day, a temperature 10°C above ISA at the same pressure altitude will cause the density altitude to be approximately:

  1. About 120 ft higher Correct answer.
  2. About 1,200 ft higher
  3. Unchanged, since pressure altitude fixes density altitude
  4. About 120 ft lower

Explanation

A working approximation is about 120 ft of density altitude change per degree Celsius of ISA deviation. A 10°C warm deviation therefore raises density altitude by roughly 1,200 ft, not 120 ft, so the correct figure is ten times the per-degree value.

Question 30Temperature, Pressure, Density and Their Effects on Aircraft Performance

Which flight phase is most sensitive to a reduction in air density caused by high altitude and temperature?

  1. Descent at idle power
  2. Taxiing on a level apron
  3. Level cruise at low indicated airspeed
  4. Initial climb after takeoff Correct answer.

Explanation

Climb performance depends on excess thrust or power, and both propeller thrust and engine power fall as density decreases while the lift required stays roughly constant. Initial climb after takeoff is therefore where the margin is thinnest and where density-altitude accidents cluster.

Question 31Temperature, Pressure, Density and Their Effects on Aircraft Performance

An increase in pressure altitude at a constant true airspeed and constant weight affects the wing's angle of attack in which way?

  1. A lower angle of attack is required because TAS is higher
  2. The angle of attack is unchanged because lift depends only on IAS
  3. The wing stalls at a lower angle of attack
  4. A higher angle of attack is required to produce the same lift Correct answer.

Explanation

At a given TAS, higher altitude means lower density and therefore less dynamic pressure, so the wing must be pitched to a greater angle of attack to generate the same lift. This reduces the margin to the critical angle of attack and raises the indicated stall speed.

Question 32Temperature, Pressure, Density and Their Effects on Aircraft Performance

For a given indicated airspeed, what happens to the stall margin as an aircraft climbs into less dense air?

  1. It improves because TAS is higher
  2. It remains identical because IAS reflects dynamic pressure
  3. It decreases because the indicated stall speed rises with altitude Correct answer.
  4. It depends only on the aircraft weight

Explanation

The stall occurs at a fixed angle of attack, but the IAS at which that angle is reached increases as density falls, because the wing needs a higher TAS to produce the same dynamic pressure. Flying at a fixed IAS therefore brings the aircraft closer to the stall as altitude increases.

Question 33Temperature, Pressure, Density and Their Effects on Aircraft Performance

Which takeoff distance chart input is used to account for the combined effect of pressure altitude and temperature?

  1. Density altitude or the temperature at the pressure altitude Correct answer.
  2. Indicated airspeed at rotation
  3. Aircraft centre of gravity only
  4. Wind component only

Explanation

Takeoff charts are entered with pressure altitude and temperature, or directly with density altitude, because those two variables set the air density that governs acceleration and lift. Wind and weight are separate corrections applied afterwards.

Question 34Temperature, Pressure, Density and Their Effects on Aircraft Performance

An aircraft with a service ceiling of 14,000 ft on a standard day is likely to have what service ceiling on a very hot day?

  1. Unchanged until the tropopause is reached
  2. Higher than 14,000 ft
  3. About the same, since the ceiling is a fixed aircraft limit
  4. Lower than 14,000 ft Correct answer.

Explanation

The service ceiling is the altitude where the maximum rate of climb falls to a defined low value, and on a hot day the same pressure altitude corresponds to a higher density altitude. The aircraft therefore reaches that limiting climb rate at a lower pressure altitude.

Question 35Temperature, Pressure, Density and Their Effects on Aircraft Performance

In very cold conditions, a pilot flying at a constant indicated altitude over rising terrain should be most concerned about:

  1. The vertical speed indicator lagging in the climb
  2. The altimeter over-reading, giving a false sense of clearance Correct answer.
  3. The airspeed indicator over-reading, causing a stall
  4. The altimeter under-reading, prompting an unnecessarily high flight

Explanation

Cold air below the aircraft means the true altitude is lower than the indicated altitude, so the altimeter over-reads relative to the terrain. The danger is greatest over high ground, where the temperature error is largest and the terrain is closest.

Question 36Temperature, Pressure, Density and Their Effects on Aircraft Performance

Two aerodromes share the same elevation, but one reports QNH 1025 hPa and the other 1000 hPa. Assuming identical temperature, how do their density altitudes compare?

  1. They are equal because elevation is the same
  2. The 1025 hPa aerodrome has the higher density altitude
  3. The 1000 hPa aerodrome has the higher density altitude Correct answer.
  4. The comparison cannot be made without humidity data

Explanation

Lower pressure at the same elevation means a higher pressure altitude and, at equal temperature, a higher density altitude. The 13 hPa deficit at the 1000 hPa field corresponds to roughly 390 ft of extra pressure altitude.

Question 37Atmospheric Moisture, Humidity and Stability

In the ICAO standard atmosphere, which statement correctly describes how the dew point and the actual air temperature are related when the air is unsaturated?

  1. The dew point is lower than the actual temperature. Correct answer.
  2. The dew point is higher than the actual temperature.
  3. The dew point equals the actual temperature.
  4. The dew point is unrelated to the actual temperature.

Explanation

The dew point is the temperature to which air must be cooled at constant pressure and moisture content to become saturated. For unsaturated air, the temperature must still fall to reach that value, so the dew point lies below the actual temperature. The gap between temperature and dew point is a direct measure of how far the air is from saturation.

Question 38Atmospheric Moisture, Humidity and Stability

An air mass at 20 degrees Celsius has a dew-point spread of 2 degrees Celsius. Which description of this air mass is most appropriate?

  1. Moderately moist, typical of a well-mixed afternoon boundary layer
  2. Saturated and therefore unable to hold any water vapour
  3. Close to saturation, with a high risk of fog or low cloud Correct answer.
  4. Very dry, with a large capacity to absorb further moisture

Explanation

A small dew-point spread means the air only needs slight cooling to reach saturation, so condensation features such as fog, mist or low stratus become likely with nocturnal cooling or lifting. A spread of 2 degrees Celsius indicates high relative humidity. Large spreads, by contrast, indicate dry air.

Question 39Atmospheric Moisture, Humidity and Stability

Relative humidity of a parcel is best defined as which of the following?

  1. The total mass of water vapour and dry air in a unit volume
  2. The mass of water vapour contained in a unit volume of air
  3. The temperature at which the water vapour present would saturate the air
  4. The ratio of the actual vapour pressure to the saturation vapour pressure at the same temperature Correct answer.

Explanation

Relative humidity compares the water vapour actually present, expressed as vapour pressure, with the maximum vapour pressure the air could hold at that temperature. It is therefore temperature dependent: warming a parcel at constant moisture content lowers its relative humidity. The other options define absolute humidity, dew point and density respectively.

Question 40Atmospheric Moisture, Humidity and Stability

Which process produces saturation of a parcel of air without any addition or removal of water vapour?

  1. Evaporation from a nearby lake
  2. Advection over a warmer surface
  3. Orographic lifting and adiabatic cooling Correct answer.
  4. Daytime surface heating

Explanation

Saturation can be reached either by adding moisture or by cooling the air to its dew point. Adiabatic cooling during lifting reduces temperature while conserving mixing ratio, so the parcel eventually reaches saturation and cloud forms. Surface heating and advection over a warm surface act in the opposite direction.

Question 41Atmospheric Moisture, Humidity and Stability

A parcel of air is warmed during the day while its mixing ratio stays constant. What happens to its relative humidity and dew point?

  1. Relative humidity falls and the dew point falls.
  2. Relative humidity falls and the dew point remains unchanged. Correct answer.
  3. Relative humidity remains constant and the dew point rises.
  4. Relative humidity rises and the dew point rises.

Explanation

At constant mixing ratio the amount of vapour is fixed, so the dew point, which depends only on moisture content and pressure, does not change. Warming increases the saturation vapour pressure, so the ratio of actual to saturation vapour pressure falls. Relative humidity therefore decreases through the day.

Question 42Atmospheric Moisture, Humidity and Stability

Which statement about the environmental lapse rate and stability is correct?

  1. A lapse rate greater than the dry adiabatic lapse rate indicates a stable atmosphere.
  2. A lapse rate equal to the dry adiabatic lapse rate indicates neutral stability for dry air. Correct answer.
  3. An isothermal layer indicates absolute instability.
  4. A small lapse rate indicates conditional instability.

Explanation

Stability is judged by comparing the environmental lapse rate with the adiabatic rates. When the environment cools with height at the same rate as a dry parcel, a displaced dry parcel stays at the same temperature relative to its surroundings, which is neutral stability. Steeper environmental lapse rates favour instability and weaker ones favour stability.

Question 43Atmospheric Moisture, Humidity and Stability

Air near the surface is heated until its temperature reaches 28 degrees Celsius with a dew point of 18 degrees Celsius. It is then forced to rise. Using the approximate dry adiabatic rate of 3 degrees Celsius per 1000 ft and a dew point lapse of about 1.8 degrees Celsius per 1000 ft, at roughly what height will the air become saturated?

  1. About 3000 ft
  2. About 5000 ft
  3. About 8000 ft Correct answer.
  4. About 1200 ft

Explanation

The temperature-dew point spread is 10 degrees Celsius. During dry ascent the spread closes at about 1.2 degrees Celsius per 1000 ft, the difference between the dry adiabatic and dew-point lapse rates. Dividing 10 by 1.2 gives roughly 8, so the condensation level is near 8000 ft.

Question 44Atmospheric Moisture, Humidity and Stability

A parcel of unsaturated air is forced to ascend a mountain slope. Which statement best describes the changes in its temperature and relative humidity?

  1. Temperature falls at the dry adiabatic rate and relative humidity increases. Correct answer.
  2. Temperature rises and relative humidity falls.
  3. Temperature falls at the dry adiabatic rate and relative humidity stays constant.
  4. Temperature falls at the saturated adiabatic rate and relative humidity stays constant.

Explanation

While unsaturated, a rising parcel cools at the dry adiabatic lapse rate of about 3 degrees Celsius per 1000 ft. Because the saturation vapour pressure falls as the parcel cools while the mixing ratio is conserved, relative humidity rises until saturation is reached. The saturated adiabatic rate applies only after condensation begins.

Question 45Atmospheric Moisture, Humidity and Stability

Why is the saturated adiabatic lapse rate smaller than the dry adiabatic lapse rate?

  1. Because the environmental lapse rate is always smaller in moist air
  2. Because latent heat released by condensation partly offsets the adiabatic cooling Correct answer.
  3. Because saturated air is denser and therefore expands more slowly
  4. Because water vapour absorbs solar radiation within the ascending parcel

Explanation

As a saturated parcel rises, condensation releases latent heat into the parcel, which partially cancels the cooling caused by expansion. The result is a slower rate of cooling, roughly 1.5 to 2 degrees Celsius per 1000 ft in warm air, approaching the dry rate in very cold air. The dry rate applies when no condensation occurs.

Question 46Atmospheric Moisture, Humidity and Stability

A layer of air is stable with respect to both dry and saturated ascent. Which term describes this condition?

  1. Conditionally unstable
  2. Absolutely unstable
  3. Neutral
  4. Absolutely stable Correct answer.

Explanation

Absolute stability exists when the environmental lapse rate is smaller than the saturated adiabatic lapse rate, so a parcel is cooler than its surroundings whether or not condensation occurs. Conditional instability requires the environmental lapse rate to lie between the saturated and dry adiabatic rates. Absolute instability requires a lapse rate steeper than the dry adiabatic rate.

Question 47Atmospheric Moisture, Humidity and Stability

An environmental lapse rate of 2.5 degrees Celsius per 1000 ft is observed. How should this layer be classified?

  1. Neutral for saturated air
  2. Conditionally unstable Correct answer.
  3. Absolutely stable
  4. Absolutely unstable

Explanation

The value lies between the saturated adiabatic rate, about 1.5 to 2 degrees Celsius per 1000 ft, and the dry adiabatic rate of about 3 degrees Celsius per 1000 ft. In that range a dry parcel is stable but a saturated parcel is unstable, which is the definition of conditional instability.

Question 48Atmospheric Moisture, Humidity and Stability

Which combination of changes is most likely to destabilise an air mass and favour convective development?

  1. Cooling of the air aloft and warming of the surface layer Correct answer.
  2. Warming of the air aloft and cooling of the surface layer
  3. Increasing surface pressure with no change of temperature
  4. Uniform cooling of the whole troposphere

Explanation

Steepening the lapse rate increases instability. Cooling the upper air or warming the surface layer both increase the temperature difference between low and high levels, steepening the environmental lapse rate. Surface heating also lowers relative humidity near the ground but increases buoyancy, which favours convection.

Question 49Atmospheric Moisture, Humidity and Stability

What is the primary effect of a strong low-level temperature inversion on the vertical distribution of moisture and visibility?

  1. It forces the dew point to rise above the temperature throughout the layer.
  2. It guarantees that no condensation can occur beneath the inversion.
  3. It allows moisture and pollutants to disperse freely through a deep layer.
  4. It traps moisture and pollutants beneath the inversion, favouring haze, mist or low stratus. Correct answer.

Explanation

An inversion is a stable layer in which temperature increases with height, suppressing vertical mixing. Moisture and pollutants accumulate in the cooler air beneath the inversion, often raising relative humidity to the point where haze, mist or stratus forms. The inversion itself does not prevent condensation below it.

Question 50Atmospheric Moisture, Humidity and Stability

A parcel of air at 15 degrees Celsius with a dew point of 12 degrees Celsius is lifted. After saturation it continues to ascend. Which statement about the parcel temperature is correct?

  1. It remains constant until all moisture has condensed.
  2. It continues to cool at the dry adiabatic rate.
  3. It cools more slowly than before saturation because of latent heat release. Correct answer.
  4. It warms because condensation releases heat faster than expansion cools the parcel.

Explanation

Once saturation is reached, further ascent causes condensation, and the released latent heat offsets part of the adiabatic cooling. The parcel therefore cools at the saturated adiabatic rate, which is smaller than the dry rate. Expansion cooling still dominates overall, so the parcel temperature continues to fall.