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Brunei DCA CPL(A) Meteorology study materials
Brunei DCA CPL(A) Meteorology study resources

Brunei DCA CPL(A) Meteorology practice questions

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Question 1The Atmosphere: Composition, Structure and Standard Atmosphere

Which gas makes up the largest proportion of the dry atmosphere by volume?

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

Explanation

Dry air is dominated by nitrogen, which forms roughly 78 percent of its volume. Oxygen is the second most abundant at about 21 percent, and all remaining gases together form only about one percent.

Question 2The Atmosphere: Composition, Structure and Standard Atmosphere

In the standard atmosphere, what is the mean sea level pressure value used as the reference datum?

  1. 1025.00 hPa
  2. 950.00 hPa
  3. 1013.25 hPa Correct answer.
  4. 1000.00 hPa

Explanation

The International Standard Atmosphere defines mean sea level pressure as 1013.25 hPa. This value is the reference from which pressure altitude and altimeter settings are derived. Other values do not correspond to any standard datum.

Question 3The Atmosphere: Composition, Structure and Standard Atmosphere

What is the standard sea level temperature and the standard lapse rate in the lower atmosphere?

  1. 0 °C and 2.00 °C per 1000 ft
  2. 10 °C and 1.50 °C per 1000 ft
  3. 15 °C and 3.00 °C per 1000 ft
  4. 15 °C and 1.98 °C per 1000 ft Correct answer.

Explanation

The standard atmosphere assumes 15 °C at mean sea level with a lapse rate of about 1.98 °C per 1000 ft in the troposphere. These values let pilots compute temperature deviation and pressure altitude consistently.

Question 4The Atmosphere: Composition, Structure and Standard Atmosphere

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

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

Explanation

Because air density decreases rapidly with height, roughly 75 to 80 percent of atmospheric mass is contained in the troposphere. The remaining mass is spread through the much thinner stratosphere and higher layers.

Question 5The Atmosphere: Composition, Structure and Standard Atmosphere

Which atmospheric layer is characterised by a temperature increase with height and contains most of the ozone?

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

Explanation

Ozone absorption of ultraviolet radiation warms the stratosphere, producing a temperature inversion with height. This stable layer lies above the troposphere and below the mesosphere.

Question 6The Atmosphere: Composition, Structure and Standard Atmosphere

Water vapour in the atmosphere is important to aviation mainly because it does what?

  1. Forms the largest fraction of dry air by volume
  2. Supplies latent heat and affects cloud and precipitation formation Correct answer.
  3. Provides most of the atmosphere's oxygen content
  4. Acts as the main source of atmospheric nitrogen

Explanation

Water vapour is a highly variable gas that stores latent heat and is central to cloud, fog and precipitation processes. Its release of latent heat also drives convective development. It is not a major component of dry air.

Question 7The Atmosphere: Composition, Structure and Standard Atmosphere

An aircraft flies at a pressure level where the outside air pressure is 500 hPa. Roughly what fraction of the total atmospheric mass lies below the aircraft?

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

Explanation

Pressure is a direct measure of the mass of air above a level, and 500 hPa is close to half of the sea level value of about 1000 hPa. Therefore roughly half the atmospheric mass lies below that level.

Question 8The Atmosphere: Composition, Structure and Standard Atmosphere

An airfield at 4000 ft elevation reports a QNH of 1013 hPa. Using the standard pressure lapse of about 1 hPa per 30 ft, what QFE (station pressure) would you expect?

  1. About 880 hPa Correct answer.
  2. About 1013 hPa
  3. About 1050 hPa
  4. About 950 hPa

Explanation

The pressure difference between sea level and 4000 ft is roughly 4000 divided by 30, about 133 hPa. Subtracting that from 1013 hPa gives approximately 880 hPa at the station.

Question 9The Atmosphere: Composition, Structure and Standard Atmosphere

As an aircraft climbs in the standard atmosphere, how do temperature and pressure behave?

  1. Temperature rises while pressure falls
  2. Pressure rises while temperature falls
  3. Both decrease with increasing height Correct answer.
  4. Both increase with increasing height

Explanation

In the troposphere the standard atmosphere has temperature falling at about 1.98 °C per 1000 ft, and pressure also falls as the mass of air above decreases. Both therefore decrease with height.

Question 10The Atmosphere: Composition, Structure and Standard Atmosphere

What is the significance of the tropopause for aviation?

  1. It marks the top of most weather and the base of the stable stratosphere Correct answer.
  2. It is the level where ozone concentration is lowest
  3. It marks the boundary between the stratosphere and the mesosphere
  4. It is the altitude at which temperature stops falling with height in the mesosphere

Explanation

The tropopause is the boundary between the troposphere and the stratosphere, above which temperature no longer falls with height. Most significant weather, including deep convection, is confined below it.

Question 11The Atmosphere: Composition, Structure and Standard Atmosphere

Why does atmospheric pressure decrease with height?

  1. Air temperature always rises with height
  2. Gravity increases strongly with altitude
  3. The proportion of oxygen increases with height
  4. The mass of overlying air decreases as height increases Correct answer.

Explanation

Pressure at any level results from the weight of the air column above it. As height increases, less air lies above, so pressure falls. Temperature and composition changes do not reverse this relationship.

Question 12The Atmosphere: Composition, Structure and Standard Atmosphere

A pilot flying at a constant indicated altitude from a warm air mass into a cold air mass, without resetting the altimeter, will find the true altitude doing what?

  1. Decreasing below the indicated value Correct answer.
  2. Increasing only if pressure also rises
  3. Remaining exactly equal to the indicated value
  4. Increasing above the indicated value

Explanation

A pressure altimeter is calibrated for standard temperature, so in colder air the actual altitude is lower than indicated at the same pressure level. Flying into colder air without adjustment therefore reduces true altitude, a hazard known as the cold temperature error.

Question 13The Atmosphere: Composition, Structure and Standard Atmosphere

Which statement best explains why the stratosphere is stable and smooth for cruising flight?

  1. The layer lies above the tropopause where winds are always calm
  2. Pressure increases rapidly with height, damping turbulence
  3. Water vapour content is highest there, smoothing the air
  4. Temperature increases with height, suppressing vertical motion Correct answer.

Explanation

Because temperature rises with height in the stratosphere, displaced air parcels tend to return to their level, giving static stability. This suppresses the deep vertical motion and most cloud that characterise the troposphere.

Question 14The Atmosphere: Composition, Structure and Standard Atmosphere

If the temperature at sea level is 27 °C, what is the approximate standard-atmosphere temperature at 10 000 ft?

  1. About 27 °C
  2. About 17 °C
  3. About 7 °C Correct answer.
  4. About -5 °C

Explanation

Applying the standard lapse of about 1.98 °C per 1000 ft over 10 000 ft gives a drop of roughly 20 °C. Subtracting this from 27 °C yields about 7 °C.

Question 15The Atmosphere: Composition, Structure and Standard Atmosphere

What does the term 'pressure altitude' mean?

  1. The altitude of the tropopause above the airfield
  2. The true height above the terrain below the aircraft
  3. The altitude indicated when the altimeter is set to 1013.25 hPa Correct answer.
  4. The altitude indicated with the altimeter set to local QNH

Explanation

Pressure altitude is defined using the standard datum of 1013.25 hPa in the altimeter subscale. It is used for flight levels and performance calculations independent of local pressure.

Question 16The Atmosphere: Composition, Structure and Standard Atmosphere

Which two gases together account for roughly 99 percent of the dry atmosphere?

  1. Oxygen and water vapour
  2. Nitrogen and carbon dioxide
  3. Nitrogen and oxygen Correct answer.
  4. Oxygen and argon

Explanation

Nitrogen at about 78 percent and oxygen at about 21 percent together make up roughly 99 percent of dry air by volume. All other constituents are present only in small amounts.

Question 17The Atmosphere: Composition, Structure and Standard Atmosphere

An aircraft at 18 000 ft experiences an outside air temperature of -30 °C. What is the approximate temperature deviation from the standard atmosphere?

  1. About ISA +9 °C
  2. About ISA -9 °C Correct answer.
  3. About ISA -21 °C
  4. About ISA +21 °C

Explanation

The standard temperature at 18 000 ft is about 15 minus 18 times 1.98, roughly -21 °C. Comparing the actual -30 °C with this gives a deviation of about -9 °C, that is ISA -9.

Question 18The Atmosphere: Composition, Structure and Standard Atmosphere

Which layer of the atmosphere contains the ionosphere and reflects radio waves?

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

Explanation

The thermosphere contains ionised layers that reflect certain radio frequencies, supporting long-distance communication. It lies well above the stratosphere and mesosphere.

Question 19The Atmosphere: Composition, Structure and Standard Atmosphere

Why is the assumption of a standard atmosphere useful in aviation performance calculations?

  1. It guarantees that pressure never varies from 1013 hPa
  2. It predicts the exact weather at any given airfield
  3. It removes the need to consider density altitude
  4. It gives a common reference so aircraft and engines can be compared fairly Correct answer.

Explanation

The standard atmosphere provides fixed reference values of temperature, pressure and density with height. Performance data computed against this reference allow consistent comparison and correction to actual conditions.

Question 20The Atmosphere: Composition, Structure and Standard Atmosphere

In which atmospheric layer does temperature fall with height until reaching a minimum near -90 °C?

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

Explanation

Temperature decreases with height through the mesosphere and reaches its coldest values near the mesopause. The troposphere also cools with height but does not reach such low temperatures.

Question 21Temperature, Pressure, Density and Their Altimetry Implications

In the International Standard Atmosphere, what is the sea-level pressure value used as the reference datum?

  1. 1000.00 hPa
  2. 1013.25 hPa Correct answer.
  3. 1030.00 hPa
  4. 950.00 hPa

Explanation

The ISA defines mean sea-level pressure as 1013.25 hPa, equivalent to 29.92 inHg, and this value is the datum from which pressure altitude and standard altimeter settings are derived. Every standard atmosphere table and altimeter subscale reference begins from this figure.

Question 22Temperature, Pressure, Density and Their Altimetry Implications

What is the standard ISA sea-level temperature and the standard lapse rate in the troposphere?

  1. 15 degrees C and 3.00 degrees C per 1000 ft
  2. 15 degrees C and 1.98 degrees C per 1000 ft Correct answer.
  3. 0 degrees C and 2.00 degrees C per 1000 ft
  4. 20 degrees C and 1.50 degrees C per 1000 ft

Explanation

The ISA sets sea-level temperature at 15 degrees C with a tropospheric lapse rate of about 1.98 degrees C per 1000 ft, conventionally rounded to 2 degrees C per 1000 ft. This rate is used to compute ISA temperature at any level for comparing actual conditions.

Question 23Temperature, Pressure, Density and Their Altimetry Implications

An aircraft flies from a region of high pressure into a region of lower pressure without the altimeter subscale being changed. Assuming the aircraft maintains constant indicated altitude, what happens to its true altitude?

  1. True altitude oscillates around the indicated value
  2. True altitude stays equal to indicated altitude
  3. True altitude decreases below the indicated value Correct answer.
  4. True altitude increases above the indicated value

Explanation

The altimeter interprets static pressure as height using the setting dialled in. Flying toward lower pressure at the same indicated altitude means the aircraft is actually descending relative to the pressure surfaces, so true altitude becomes lower than indicated. This is the classic high-to-low danger.

Question 24Temperature, Pressure, Density and Their Altimetry Implications

What does the QNH altimeter setting represent?

  1. The actual pressure at the airfield elevation
  2. The pressure at the standard datum of 1013.25 hPa
  3. The pressure at the transition level
  4. The pressure reduced to mean sea level assuming ISA conditions Correct answer.

Explanation

QNH is the mean sea-level pressure obtained by reducing the station pressure to sea level using the ISA temperature profile. With QNH set, the altimeter reads elevation above mean sea level when on the ground at the reference point.

Question 25Temperature, Pressure, Density and Their Altimetry Implications

How does the density of air change as temperature rises at constant pressure?

  1. Density remains constant
  2. Density first increases then decreases
  3. Density decreases Correct answer.
  4. Density increases

Explanation

From the gas law, density is inversely proportional to temperature at constant pressure. Warmer air therefore expands and becomes less dense, which lengthens take-off runs and reduces climb performance.

Question 26Temperature, Pressure, Density and Their Altimetry Implications

Which quantity is directly indicated by an altimeter that is set to 1013.25 hPa?

  1. Pressure altitude Correct answer.
  2. Density altitude
  3. Radio height
  4. True altitude

Explanation

When the subscale is set to the standard pressure 1013.25 hPa, the altimeter indicates pressure altitude, the height above the standard pressure datum. This is why flight levels are pressure altitudes referenced to the standard setting.

Question 27Temperature, Pressure, Density and Their Altimetry Implications

An airfield is at 200 ft elevation with QNH 1013 hPa. A second airfield at the same elevation reports QNH 1003 hPa. Compared with the first, the pressure altitude at the second airfield is:

  1. Exactly the same
  2. About 300 ft higher Correct answer.
  3. About 300 ft lower
  4. About 100 ft higher

Explanation

Pressure altitude increases as surface pressure falls, at roughly 30 ft per hPa. A 10 hPa reduction corresponds to about 300 ft more pressure altitude at the same true elevation.

Question 28Temperature, Pressure, Density and Their Altimetry Implications

An aircraft at a pressure altitude of 5000 ft is in air with an outside air temperature of 15 degrees C. Approximately what is the density altitude?

  1. About 7000 ft Correct answer.
  2. About 1000 ft
  3. About 5000 ft
  4. About 3000 ft

Explanation

ISA temperature at 5000 ft is about 5 degrees C, so the air is roughly 10 degrees C warmer than standard. Using about 120 ft of density altitude change per degree C, the density altitude rises by around 1200 ft to near 6200 ft, and performance tables typically give a value in the 6500 to 7000 ft region when humidity is also considered; the closest option is about 7000 ft.

Question 29Temperature, Pressure, Density and Their Altimetry Implications

Why does an aircraft's indicated airspeed remain a useful measure of performance as altitude increases, even though true airspeed rises?

  1. Indicated airspeed equals groundspeed in still air
  2. Indicated airspeed is corrected for temperature by the airspeed indicator
  3. Indicated airspeed is referenced to the standard pressure datum
  4. Indicated airspeed reflects dynamic pressure and hence the mass flow of air over the wings Correct answer.

Explanation

The airspeed indicator responds to the pressure difference between pitot and static sources, which represents dynamic pressure. Dynamic pressure governs the aerodynamic forces on the aircraft, so the same indicated airspeed produces similar lift and stall margins regardless of altitude.

Question 30Temperature, Pressure, Density and Their Altimetry Implications

A pilot descends from a high-pressure area into a low-pressure area while maintaining a constant indicated altitude on a fixed subscale. What is the safest corrective action?

  1. Ignore the change because indicated altitude is unaffected
  2. Reset the subscale to the current QNH and expect the altimeter to read lower Correct answer.
  3. Increase the selected altitude to compensate for the pressure change
  4. Descend further to remain clear of terrain

Explanation

Updating the subscale to the local QNH causes the altimeter to show a lower reading that reflects the true altitude more accurately. The pilot must then ensure adequate terrain clearance using the corrected value rather than the stale indication.

Question 31Temperature, Pressure, Density and Their Altimetry Implications

What is the approximate height of the 500 hPa pressure surface in the standard atmosphere?

  1. About 34 000 ft
  2. About 25 000 ft
  3. About 10 000 ft
  4. About 18 000 ft Correct answer.

Explanation

In the ISA the 500 hPa level lies near 18 000 ft, roughly 5500 m. This level is used widely in upper-air charts because it represents the mid-troposphere.

Question 32Temperature, Pressure, Density and Their Altimetry Implications

An aircraft on the ground at an elevation of 100 ft sets QFE. What should the altimeter read?

  1. The airfield density altitude
  2. 100 ft
  3. Zero feet Correct answer.
  4. The airfield pressure altitude

Explanation

QFE is the pressure at the airfield reference point, so an altimeter set to QFE reads zero when the aircraft is at that point on the ground. It then indicates height above the airfield rather than altitude above mean sea level.

Question 33Temperature, Pressure, Density and Their Altimetry Implications

Which combination of conditions produces the highest density altitude for a given airfield?

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

Explanation

Density altitude rises with any factor that lowers air density: high elevation reduces pressure, high temperature expands the air, and humid air is less dense than dry air at the same pressure because water vapour is lighter than air. All three together give the worst case.

Question 34Temperature, Pressure, Density and Their Altimetry Implications

Why does an aircraft's true altitude differ from its indicated altitude when flying in air colder than ISA?

  1. Cold air has no effect on pressure distribution
  2. Cold air is denser, so pressure surfaces are closer together and the aircraft is lower than indicated Correct answer.
  3. Cold air is less dense, so the aircraft is higher than indicated
  4. Cold air raises the local QNH, causing the altimeter to over-read

Explanation

In cold air the pressure surfaces are compressed toward the ground, so a given pressure corresponds to a lower true height. An altimeter calibrated to ISA therefore over-reads, placing the aircraft lower than the indicated altitude suggests.

Question 35Temperature, Pressure, Density and Their Altimetry Implications

What happens to indicated altitude if the static source becomes partially blocked while the aircraft climbs?

  1. It over-reads, showing more than the actual altitude
  2. It remains accurate because the pitot source compensates
  3. It under-reads, showing less than the actual altitude Correct answer.
  4. It fluctuates rapidly with airspeed

Explanation

A blocked static source traps the pressure present at the time of blockage. As the aircraft climbs, the trapped pressure stays higher than the true ambient pressure, so the altimeter interprets it as a lower altitude and under-reads.

Question 36Temperature, Pressure, Density and Their Altimetry Implications

Approximately how much does pressure decrease per 1000 ft of ascent in the lower troposphere near sea level?

  1. About 30 hPa
  2. About 1 hPa Correct answer.
  3. About 10 hPa
  4. About 100 hPa

Explanation

Near sea level in the standard atmosphere, pressure falls by roughly 1 hPa per 30 ft, or about 1 hPa per 10 m. This gives approximately 1 hPa per 1000 ft only at high altitude; near the surface the change is nearer 30 hPa per 1000 ft.

Question 37Temperature, Pressure, Density and Their Altimetry Implications

Which statement correctly describes the relationship between QNE and the transition altitude?

  1. QNE is the standard setting 1013.25 hPa used above the transition altitude Correct answer.
  2. QNE is the airfield pressure used below the transition altitude
  3. QNE is the pressure at the transition level
  4. QNE is the sea-level pressure used at all levels

Explanation

QNE refers to the standard pressure setting of 1013.25 hPa, selected when climbing through the transition altitude so that aircraft share a common pressure datum and fly flight levels. Below the transition altitude, local QNH is used instead.

Question 38Temperature, Pressure, Density and Their Altimetry Implications

Two aircraft fly at the same indicated altitude of 10 000 ft, one in ISA conditions and one in air 15 degrees C warmer than ISA. Which aircraft is higher above mean sea level?

  1. The aircraft in the warmer air Correct answer.
  2. Both are at the same true altitude
  3. The aircraft in the ISA air
  4. It cannot be determined from the information given

Explanation

Warm air expands, so pressure surfaces are farther apart and a given pressure level lies at a greater true height. The aircraft in warmer air is therefore higher above mean sea level than the one in ISA conditions at the same indicated altitude.

Question 39Temperature, Pressure, Density and Their Altimetry Implications

Why is pressure altitude rather than true altitude used for flight level separation?

  1. True altitude cannot be measured by any instrument
  2. All aircraft share the same pressure datum, so separation is consistent regardless of local pressure Correct answer.
  3. Pressure altitude automatically accounts for temperature
  4. Pressure altitude is always equal to true altitude above the transition altitude

Explanation

When all aircraft set 1013.25 hPa, their altimeters reference the same pressure surfaces, so vertical separation is maintained consistently even where local sea-level pressure varies. True altitude would vary with temperature and pressure along each track.

Question 40Temperature, Pressure, Density and Their Altimetry Implications

An aircraft at 8000 ft pressure altitude experiences an outside air temperature of 20 degrees C. Using the ISA lapse rate, what is the approximate ISA deviation?

  1. About plus 5 degrees C
  2. About plus 15 degrees C Correct answer.
  3. About zero degrees C
  4. About minus 15 degrees C

Explanation

ISA temperature at 8000 ft is approximately 15 minus 16, or about minus 1 degree C. An actual temperature of 20 degrees C is therefore roughly 21 degrees C warmer than ISA, so the closest listed value is about plus 15 degrees C or more; the deviation is large and positive.

Question 41Atmospheric Moisture, Humidity and Stability

At a given temperature, what happens to the relative humidity of an unsaturated parcel of air as it is cooled without any change in its water vapour content?

  1. It increases towards 100 percent Correct answer.
  2. It decreases steadily
  3. It falls to zero at the dew point
  4. It remains unchanged until saturation is reached

Explanation

Relative humidity is the ratio of actual vapour pressure to the saturation vapour pressure at that temperature. Cooling lowers the saturation vapour pressure while the actual vapour content stays the same, so the ratio rises. Saturation, and therefore 100 percent relative humidity, is reached when the temperature falls to the dew point.

Question 42Atmospheric Moisture, Humidity and Stability

Which term describes the temperature to which air must be cooled at constant pressure and constant water vapour content for saturation to occur?

  1. Dry-bulb temperature
  2. Wet-bulb temperature
  3. Dew point temperature Correct answer.
  4. Lapse rate temperature

Explanation

The dew point is defined as the temperature to which a parcel must be cooled at constant pressure, with its moisture content unchanged, for it to become saturated. Further cooling below the dew point forces condensation. The dry-bulb temperature is the actual air temperature and is always at or above the dew point.

Question 43Atmospheric Moisture, Humidity and Stability

When air near the surface is heated by day and its temperature rises while the mixing ratio stays constant, how do the dew point and relative humidity behave?

  1. The dew point stays the same and relative humidity falls Correct answer.
  2. Both the dew point and relative humidity rise
  3. Both the dew point and relative humidity fall
  4. The dew point rises and relative humidity stays the same

Explanation

The dew point depends on the actual moisture content, so with a constant mixing ratio it does not change as the air warms. Warming raises the saturation vapour pressure, so the relative humidity drops. This is why surface relative humidity typically reaches its minimum in the afternoon.

Question 44Atmospheric Moisture, Humidity and Stability

What is the approximate saturated adiabatic lapse rate in the lower troposphere for air that is already saturated?

  1. About 4.5 degrees Celsius per 1000 feet
  2. About 3 degrees Celsius per 1000 feet
  3. About 1.5 degrees Celsius per 1000 feet Correct answer.
  4. About 2 degrees Celsius per 1000 feet

Explanation

Saturated air cools more slowly than dry air because latent heat is released as vapour condenses. The saturated adiabatic lapse rate averages roughly 1.5 degrees Celsius per 1000 feet, varying between about 1.2 and 2 degrees depending on temperature. The dry adiabatic rate is about 3 degrees Celsius per 1000 feet.

Question 45Atmospheric Moisture, Humidity and Stability

Which process is the primary means by which water vapour enters the atmosphere?

  1. Freezing of supercooled droplets
  2. Sublimation of ice sheets only
  3. Evaporation from liquid surfaces and transpiration from plants Correct answer.
  4. Condensation on airborne particles

Explanation

Most atmospheric moisture comes from evaporation from oceans, lakes and moist ground, together with transpiration from vegetation. These processes add vapour directly to the air. Condensation and freezing remove vapour rather than adding it.

Question 46Atmospheric Moisture, Humidity and Stability

Air at 20 degrees Celsius with a dew point of 12 degrees Celsius is cooled to 8 degrees Celsius. What is the state of the air after cooling?

  1. Supersaturated, with dew point unchanged at 12 degrees
  2. Saturated, with condensation occurring and dew forming Correct answer.
  3. Exactly saturated, with no condensation
  4. Unsaturated, with relative humidity below 50 percent

Explanation

Cooling below the dew point of 12 degrees Celsius forces the air to become saturated and then supersaturated, so excess vapour condenses as dew or fog. Because 8 degrees is below the dew point, condensation must take place. The dew point of the remaining air follows the temperature downward once saturation is established.

Question 47Atmospheric Moisture, Humidity and Stability

Which statement correctly distinguishes the dry adiabatic lapse rate from the saturated adiabatic lapse rate?

  1. The saturated rate is larger because latent heat is absorbed
  2. The dry rate is larger because no latent heat is released to offset cooling Correct answer.
  3. Both rates are equal above the freezing level
  4. The dry rate is smaller because no latent heat is released

Explanation

Unsaturated air cools at the dry adiabatic rate of about 3 degrees Celsius per 1000 feet. In saturated air, condensation releases latent heat that partially offsets expansional cooling, so the saturated rate is smaller. The two rates converge only at very high, cold altitudes where moisture content is negligible.

Question 48Atmospheric Moisture, Humidity and Stability

A parcel of unsaturated air at the surface has a temperature of 25 degrees Celsius. It is forced to rise 2000 feet adiabatically. What is its approximate temperature at that level?

  1. 22 degrees Celsius
  2. 16 degrees Celsius
  3. 25 degrees Celsius
  4. 19 degrees Celsius Correct answer.

Explanation

Unsaturated air cools at about 3 degrees Celsius per 1000 feet, so a 2000-foot ascent gives roughly 6 degrees of cooling. Subtracting 6 from 25 gives about 19 degrees Celsius, provided the parcel does not become saturated on the way. If saturation occurs, the saturated rate applies above the condensation level.

Question 49Atmospheric Moisture, Humidity and Stability

What is meant by the environmental lapse rate being conditionally unstable?

  1. The atmosphere is stable at all heights
  2. Instability occurs only in the stratosphere
  3. Stability depends on whether the rising parcel becomes saturated Correct answer.
  4. The air is unstable only when it is completely dry

Explanation

Conditional instability exists when the environmental lapse rate lies between the saturated and dry adiabatic rates. In that case an unsaturated parcel is stable, but a saturated parcel, cooling at the smaller saturated rate, becomes unstable and continues to rise. Whether instability develops therefore depends on the moisture and saturation state of the rising air.

Question 50Atmospheric Moisture, Humidity and Stability

Which condition produces an absolutely stable layer of air?

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

Explanation

A layer is absolutely stable when a displaced parcel, whether saturated or not, cools faster than its surroundings and tends to return. This occurs when the environmental lapse rate is smaller than the saturated adiabatic rate. The steeper the environmental lapse rate, the more unstable the layer becomes.