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ATPL Meteorology practice questions

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Question 1The Atmosphere: Composition, Structure and Standard Atmosphere

What is the approximate composition of the dry atmosphere by volume?

  1. 78% nitrogen, 21% oxygen, 1% other gases Correct answer.
  2. 50% nitrogen, 40% oxygen, 10% other gases
  3. 21% nitrogen, 78% oxygen, 1% other gases
  4. 90% nitrogen, 9% oxygen, 1% other gases

Explanation

Dry air is dominated by nitrogen at about 78 percent by volume and oxygen at about 21 percent, with argon, carbon dioxide and trace gases making up the remaining one percent. These proportions are essentially constant up to the homosphere. Water vapour is variable and is excluded from the dry-air figure.

Question 2The Atmosphere: Composition, Structure and Standard Atmosphere

Which gas in the atmosphere is the most important absorber of outgoing long-wave radiation and the principal natural greenhouse gas?

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

Explanation

Water vapour absorbs strongly across several infrared bands and is the dominant greenhouse gas by its radiative effect in the troposphere, followed by carbon dioxide. Its concentration is highly variable in space and time. Nitrogen and oxygen are essentially transparent to long-wave radiation.

Question 3The Atmosphere: Composition, Structure and Standard Atmosphere

In which atmospheric layer does temperature increase with height, and what is the main cause of that increase?

  1. Mesosphere, because of compression of descending air
  2. Thermosphere, because of conduction from the surface
  3. Stratosphere, because of ozone absorption of ultraviolet radiation Correct answer.
  4. Troposphere, because of latent heat release

Explanation

The stratosphere contains the ozone layer, and ozone absorbs solar ultraviolet radiation, heating the air and producing a temperature inversion with height. This warming stabilises the layer and limits vertical motion. The troposphere cools with height and the mesosphere cools strongly with height.

Question 4The Atmosphere: Composition, Structure and Standard Atmosphere

What is the standard sea-level pressure and temperature used in the ISA?

  1. 1000 hPa and 15 degrees Celsius
  2. 1013.25 hPa and 15 degrees Celsius Correct answer.
  3. 1000 hPa and 0 degrees Celsius
  4. 1013.25 hPa and 0 degrees Celsius

Explanation

The International Standard Atmosphere assumes a mean sea-level pressure of 1013.25 hPa and a temperature of 15 degrees Celsius. It also assumes a mean sea-level density of 1225 g per cubic metre and a tropospheric lapse rate of 1.98 degrees Celsius per 1000 ft. These values are the reference for pressure and temperature altimetry.

Question 5The Atmosphere: Composition, Structure and Standard Atmosphere

In the ICAO Standard Atmosphere, what is the temperature lapse rate assumed in the troposphere?

  1. 1.98 degrees Celsius per 1000 ft Correct answer.
  2. 0.65 degrees Celsius per 100 m
  3. 3 degrees Celsius per 1000 ft
  4. 6.5 degrees Celsius per 1000 ft

Explanation

The ISA assumes a constant tropospheric lapse rate of 1.98 degrees Celsius per 1000 ft, which corresponds to about 6.5 degrees Celsius per km. The lapse rate applies up to 11 km, where the temperature reaches about minus 56.5 degrees Celsius. Above that the standard assumes an isothermal layer.

Question 6The Atmosphere: Composition, Structure and Standard Atmosphere

What is the tropopause generally like in the region of the polar front?

  1. A single continuous surface at a constant pressure
  2. A break or discontinuity where tropical and polar tropopauses are separated Correct answer.
  3. A region where temperature increases rapidly with height
  4. A region where the tropopause descends to the surface

Explanation

The tropopause is not continuous everywhere; at the polar front it is broken, with the higher tropical tropopause and the lower polar tropopause separated by a discontinuity. This break is associated with the jet stream and with the frontal zone. It is one reason tropopause height varies strongly with latitude and season.

Question 7The Atmosphere: Composition, Structure and Standard Atmosphere

Why does atmospheric pressure decrease with height?

  1. Because the mass of air above a given level decreases as height increases Correct answer.
  2. Because the proportion of oxygen increases with height
  3. Because gravity increases with height
  4. Because temperature always decreases with height

Explanation

Pressure at any level is the weight of the column of air above it, so as height increases the overlying mass decreases and pressure falls. The decrease is roughly exponential with height. Temperature and composition changes affect the rate of decrease but are not the fundamental cause.

Question 8The Atmosphere: Composition, Structure and Standard Atmosphere

At approximately what height does the ISA place the tropopause?

  1. 20 km
  2. 11 km Correct answer.
  3. 5 km
  4. 50 km

Explanation

The ICAO Standard Atmosphere places the tropopause at 11 km, where the temperature is about minus 56.5 degrees Celsius and the pressure is roughly 226 hPa. Above this level the standard assumes an isothermal stratosphere up to 20 km. Real tropopause heights vary with latitude and weather.

Question 9The Atmosphere: Composition, Structure and Standard Atmosphere

How does the scale height of the atmosphere relate to pressure decrease with altitude?

  1. Pressure falls to about 37 percent of its value over one scale height Correct answer.
  2. Pressure halves for every scale height gained
  3. Pressure doubles over one scale height
  4. Pressure remains constant over one scale height

Explanation

In an isothermal atmosphere pressure falls exponentially, decreasing by a factor of e (about 2.718) for each scale height, so about 37 percent remains after one scale height. The scale height is roughly 8 km in the lower atmosphere. This exponential behaviour explains why pressure falls quickly at low levels and slowly at high levels.

Question 10The Atmosphere: Composition, Structure and Standard Atmosphere

In which layer do most of the weather phenomena and almost all of the water vapour occur?

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

Explanation

The troposphere contains nearly all the atmospheric water vapour and is where temperature decreases with height, allowing convection and cloud formation. Consequently almost all weather occurs there. The stratosphere is dry and stable, and the higher layers contain very little moisture.

Question 11The Atmosphere: Composition, Structure and Standard Atmosphere

What is the significance of the homosphere?

  1. It is the layer where temperature is constant with height
  2. It is the region above 80 km where gases separate by molecular weight
  3. It is the layer in which the composition of the air is essentially uniform in its main gases Correct answer.
  4. It is the layer in which ozone is concentrated

Explanation

The homosphere extends from the surface to about 80 km and is the region where turbulent mixing keeps the proportions of the main gases essentially constant. Above it, in the heterosphere, gases tend to separate according to molecular weight. The uniform composition of the homosphere justifies using a single standard atmosphere for aviation.

Question 12The Atmosphere: Composition, Structure and Standard Atmosphere

A temperature inversion is present near the surface. What effect does this have on vertical motion and visibility?

  1. It has no effect on vertical motion but always clears cloud
  2. It suppresses vertical motion and can trap pollutants and moisture, reducing visibility Correct answer.
  3. It encourages strong convection and improves visibility
  4. It causes pressure to increase rapidly with height

Explanation

In an inversion temperature increases with height, so a rising parcel becomes colder than its surroundings and is suppressed. This stable stratification limits vertical mixing and can trap moisture, haze and pollutants beneath the inversion, reducing visibility. Inversions are therefore associated with smooth air and poor low-level visibility.

Question 13The Atmosphere: Composition, Structure and Standard Atmosphere

How does the actual lapse rate compare with the standard atmosphere when an aircraft reports a temperature higher than ISA at a given level?

  1. The air is warmer than standard and true altitude is higher than pressure altitude
  2. There is no relationship between temperature and altitude
  3. The air is warmer than standard and pressure altitude is higher than true altitude Correct answer.
  4. The air is colder than standard and pressure altitude is higher than true altitude

Explanation

When air is warmer than ISA it is less dense, so the aircraft is higher above the surface than the pressure altimeter indicates; pressure altitude is greater than true altitude. The altimeter is calibrated to the standard atmosphere, so warm air produces a genuine height greater than the indicated one. This is the basis of the high-to-low and hot-to-cold altimetry rules.

Question 14The Atmosphere: Composition, Structure and Standard Atmosphere

Which statement about the stratosphere is correct?

  1. It is warmer at its base than at its top
  2. It contains most of the atmospheric ozone and is stable with little vertical motion Correct answer.
  3. It contains most of the atmospheric water vapour and is unstable
  4. It is the layer in which the tropopause break occurs

Explanation

The stratosphere contains the bulk of atmospheric ozone, which absorbs ultraviolet radiation and warms the layer with height. Because temperature increases with height, the stratosphere is stable and vertical motion is limited. This stability and dryness make it generally smooth for high-altitude flight.

Question 15The Atmosphere: Composition, Structure and Standard Atmosphere

What happens to the temperature of a rising parcel of unsaturated air as it ascends, and why?

  1. It warms because pressure increases
  2. It cools only because of mixing with surrounding air
  3. It remains constant because no heat is exchanged
  4. It cools mainly because of adiabatic expansion as pressure decreases Correct answer.

Explanation

A rising parcel expands as ambient pressure falls, doing work on its surroundings, so its internal energy and temperature decrease. For unsaturated air this dry adiabatic cooling is about 3 degrees Celsius per 1000 ft. The process is essentially adiabatic because little heat is exchanged with the environment.

Question 16The Atmosphere: Composition, Structure and Standard Atmosphere

Which statement best describes the temperature profile of the standard atmosphere above 20 km?

  1. Temperature increases with height throughout the stratosphere and mesosphere
  2. Temperature decreases steadily to the top of the atmosphere
  3. Temperature is constant to 20 km, then increases through the stratosphere to the stratopause Correct answer.
  4. Temperature is assumed constant to 32 km, then increases in the stratosphere

Explanation

The ICAO Standard Atmosphere assumes an isothermal layer from 11 km to 20 km, after which temperature increases with height through the stratosphere up to the stratopause at about 47 km. This warming reflects ozone absorption of ultraviolet radiation. Above the stratopause the standard assumes temperature decreases again through the mesosphere.

Question 17Temperature, Pressure, Density and Their Altimetry Implications

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

  1. 980.00 hPa and 20 °C
  2. 1013.25 hPa and 0 °C
  3. 1000.00 hPa and 15 °C
  4. 1013.25 hPa and 15 °C Correct answer.

Explanation

The ISA defines mean sea level as 1013.25 hPa with a temperature of 15 °C, and these values anchor all subsequent lapse-rate calculations. Every pressure altitude and density altitude computation starts from this datum pair.

Question 18Temperature, Pressure, Density and Their Altimetry Implications

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

  1. 0.65 °C per 100 m
  2. 3.00 °C per 1000 ft
  3. 2.00 °C per 1000 ft
  4. 1.98 °C per 1000 ft Correct answer.

Explanation

The ISA troposphere cools at 6.5 °C per km, which converts to approximately 1.98 °C per 1000 ft. This gradient is applied up to the tropopause at 11 km, where the temperature becomes constant at about −56.5 °C.

Question 19Temperature, Pressure, Density and Their Altimetry Implications

An aircraft flies at a constant indicated altitude from a warm air mass into a cold air mass, with the altimeter sub-scale unchanged. What happens to the true altitude?

  1. It remains equal to the indicated altitude
  2. It oscillates around the indicated altitude
  3. It decreases below the indicated altitude Correct answer.
  4. It increases above the indicated altitude

Explanation

The altimeter assumes ISA temperatures, so over cold air the pressure surfaces are compressed and the aircraft is actually lower than indicated. This is the classic 'high to low, look out below' hazard for terrain clearance.

Question 20Temperature, Pressure, Density and Their Altimetry Implications

How does an increase in humidity affect the density of a parcel of air at constant pressure and temperature?

  1. It has no effect on the density
  2. It decreases the density Correct answer.
  3. It first increases then decreases the density
  4. It increases the density

Explanation

Water vapour is lighter than the dry air it displaces, so moist air is less dense than dry air at the same pressure and temperature. This is why humid air gives a higher density altitude and longer take-off runs.

Question 21Temperature, Pressure, Density and Their Altimetry Implications

What does the QNH setting represent when it is dialled into an altimeter on the ground at an aerodrome?

  1. The pressure altitude of the aerodrome
  2. The height above the aerodrome
  3. The flight level of the aerodrome
  4. The aerodrome elevation Correct answer.

Explanation

QNH is the sea-level pressure derived from the station pressure, so an altimeter set to QNH reads aerodrome elevation when the aircraft is on the ground. It therefore provides true altitude above mean sea level in the vicinity.

Question 22Temperature, Pressure, Density and Their Altimetry Implications

An aerodrome with an elevation of 2000 ft reports QNH 1020 hPa. Using 1 hPa ≈ 30 ft, what is the approximate QFE?

  1. About 953 hPa Correct answer.
  2. About 987 hPa
  3. About 1000 hPa
  4. About 947 hPa

Explanation

The station is 2000 ft above sea level, so the pressure there is lower than QNH by about 2000/30 ≈ 67 hPa. Subtracting gives a QFE of roughly 953 hPa, consistent with the altimeter reading zero on the ground.

Question 23Temperature, Pressure, Density and Their Altimetry Implications

A pressure altimeter is set to 1013.25 hPa and reads 8000 ft. If the actual sea-level pressure is 1030 hPa, what is the approximate true altitude?

  1. About 8500 ft Correct answer.
  2. About 8000 ft
  3. About 7500 ft
  4. About 8900 ft

Explanation

The real sea-level pressure is higher than standard by 17 hPa, so the pressure surface at 8000 ft pressure altitude lies higher than the ISA assumes. Multiplying 17 hPa by about 30 ft gives roughly 500 ft, so true altitude is near 8500 ft.

Question 24Temperature, Pressure, Density and Their Altimetry Implications

Which combination of conditions produces the highest density altitude on a given runway?

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

Explanation

Density altitude rises with any factor that thins the air: high field elevation, high temperature and high humidity all reduce density. The worst combination is therefore the hot, humid, high-elevation aerodrome.

Question 25Temperature, Pressure, Density and Their Altimetry Implications

What is the tropopause temperature and pressure in the International Standard Atmosphere?

  1. −45.0 °C and 300.00 hPa
  2. −56.5 °C and 226.32 hPa Correct answer.
  3. −70.0 °C and 150.00 hPa
  4. −56.5 °C and 1013.25 hPa

Explanation

Applying the 6.5 °C/km lapse rate from 15 °C over 11 km gives about −56.5 °C, and the corresponding pressure is 226.32 hPa. Above this level the ISA assumes a constant temperature in the lower stratosphere.

Question 26Temperature, Pressure, Density and Their Altimetry Implications

An aircraft at a constant true altitude flies from a region of high pressure into a region of low pressure without resetting the sub-scale. What is the effect on the altimeter reading?

  1. It over-reads, showing higher than the true altitude Correct answer.
  2. It reads the true altitude exactly
  3. It under-reads, showing lower than the true altitude
  4. It reads zero until reset

Explanation

Flying towards lower pressure at constant height means the aircraft is entering air that is less compressed, so the pressure sensed corresponds to a higher level in the standard atmosphere. The altimeter therefore indicates more than the true altitude.

Question 27Temperature, Pressure, Density and Their Altimetry Implications

Why does the altimeter over-read when flying at high true altitude in very cold air?

  1. The cold column compresses the pressure levels, so a given pressure occurs at a lower true height Correct answer.
  2. The cold air raises the pressure at the aircraft, so the capsule expands more
  3. The sub-scale setting becomes inaccurate at low temperature
  4. The capsule stiffens in the cold and reads high

Explanation

In cold air the pressure surfaces are closer together, so the pressure corresponding to a given indicated altitude is found at a lower true height. The aircraft is therefore lower than the altimeter suggests, which is the over-reading error.

Question 28Temperature, Pressure, Density and Their Altimetry Implications

Approximately how much does pressure decrease per 1000 ft of ascent in the lower atmosphere under ISA conditions?

  1. About 36 hPa Correct answer.
  2. About 1 hPa
  3. About 12 hPa
  4. About 100 hPa

Explanation

Near sea level the ISA pressure gradient is roughly 36 hPa per 1000 ft, which is why the 1 hPa ≈ 30 ft rule is used for altimetry. The gradient decreases with height as the air thins.

Question 29Temperature, Pressure, Density and Their Altimetry Implications

What is the significance of the transition altitude in relation to altimeter settings?

  1. It is the level at which the altimeter is switched off
  2. Below it, altimeters use 1013.25 hPa; above it, they use QNH
  3. Below it, altimeters use QNH and indicate altitude; above it, they use 1013.25 hPa Correct answer.
  4. It marks the level at which QFE is set to zero

Explanation

The transition altitude separates the use of QNH for terrain-referenced altitude from the standard pressure setting used for flight levels. This ensures all aircraft above the transition altitude share a common pressure datum for vertical separation.

Question 30Temperature, Pressure, Density and Their Altimetry Implications

An aircraft is at flight level 100 in an area where the QNH is 990 hPa. What is the approximate true altitude?

  1. About 10 000 ft
  2. About 8700 ft
  3. About 9300 ft Correct answer.
  4. About 10 700 ft

Explanation

The sub-scale is set 23 hPa above the actual sea-level pressure, so the altimeter over-reads. Converting 23 hPa at about 30 ft per hPa gives roughly 700 ft, so true altitude is near 9300 ft.

Question 31Temperature, Pressure, Density and Their Altimetry Implications

What effect does a high-density-altitude day have on aircraft performance?

  1. Reduced engine power, reduced lift and a longer take-off run Correct answer.
  2. No measurable effect on performance
  3. Increased engine power and a shorter take-off run
  4. Increased lift with unchanged take-off run

Explanation

Thin air reduces both the mass flow to a piston or turbine engine and the dynamic pressure generated by the wing at a given true airspeed. The result is less thrust, less lift and a longer ground roll.

Question 32Temperature, Pressure, Density and Their Altimetry Implications

If the actual sea-level pressure is 1013 hPa but the temperature is 15 °C higher than ISA at all levels, how does the true height of the 500 hPa surface compare with the standard atmosphere?

  1. It is unchanged
  2. It cannot be determined from the information given
  3. It lies higher than standard Correct answer.
  4. It lies lower than standard

Explanation

A warmer column expands, so a given pressure level is found at a greater height above the surface. With the same sea-level pressure but warmer air throughout, the 500 hPa surface is therefore higher than the ISA value.

Question 33Temperature, Pressure, Density and Their Altimetry Implications

Which statement correctly describes the relationship between pressure altitude and indicated altitude when the sub-scale is set to the local QNH?

  1. Indicated altitude equals height above the aerodrome
  2. Indicated altitude approximates true altitude above mean sea level Correct answer.
  3. Indicated altitude equals pressure altitude
  4. Indicated altitude equals density altitude

Explanation

With QNH set, the altimeter is referenced to sea level and, in the vicinity of the station, reads approximately true altitude. Pressure altitude would require the standard 1013.25 hPa setting.

Question 34Temperature, Pressure, Density and Their Altimetry Implications

A pilot descends at a constant indicated altitude on the QNH of a distant station while the true QNH along the route is lower. What risk arises?

  1. The aircraft is higher than the altimeter indicates
  2. The altimeter reading becomes unreliable
  3. The altimeter will show zero
  4. The aircraft is lower than the altimeter indicates Correct answer.

Explanation

If the real pressure along the route is lower than the set value, the altimeter over-reads and the aircraft is closer to the ground than shown. This is the basis of the 'high to low' warning for terrain clearance.

Question 35Atmospheric Moisture, Humidity and Phase Changes

In the troposphere, how does the saturated adiabatic lapse rate compare with the dry adiabatic lapse rate as an unsaturated parcel is forced to ascend?

  1. The two rates are exactly equal at every temperature.
  2. The saturated rate is larger in magnitude than the dry rate.
  3. The saturated rate equals the environmental lapse rate.
  4. The saturated rate is smaller in magnitude than the dry rate. Correct answer.

Explanation

As a parcel rises and cools, it eventually reaches saturation; the latent heat released by condensing water vapour partly offsets further cooling. This makes the saturated adiabatic lapse rate smaller in magnitude than the dry adiabatic rate, typically near 0.5 degrees Celsius per 100 m against about 1 degree per 100 m.

Question 36Atmospheric Moisture, Humidity and Phase Changes

A parcel of air at 20 degrees Celsius contains 10 g/kg of water vapour. Given a saturation mixing ratio of about 15 g/kg at that temperature, what is the relative humidity?

  1. 67 percent Correct answer.
  2. 150 percent
  3. 75 percent
  4. 50 percent

Explanation

Relative humidity is the ratio of actual vapour content to the saturation value at the same temperature, expressed as a percentage. Dividing 10 by 15 gives 0.67, so the relative humidity is about 67 percent.

Question 37Atmospheric Moisture, Humidity and Phase Changes

Which quantity describes the temperature to which air must be cooled at constant pressure and constant water-vapour content to reach saturation?

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

Explanation

The dew point is defined as the temperature to which a parcel must be cooled at constant pressure, without adding or removing water vapour, for it to become saturated. When air cools to its dew point, condensation begins and cloud or fog can form.

Question 38Atmospheric Moisture, Humidity and Phase Changes

Why does the dew point depression generally decrease when an unsaturated parcel is lifted adiabatically?

  1. The dew point falls faster than the parcel temperature.
  2. The saturation vapour pressure decreases with decreasing temperature.
  3. The parcel temperature falls toward a nearly constant dew point. Correct answer.
  4. The vapour content of the parcel increases during ascent.

Explanation

Lifting cools the parcel while its mixing ratio stays essentially constant, so the dew point changes only slowly with pressure. The temperature therefore approaches the dew point, reducing the depression until saturation and condensation occur.

Question 39Atmospheric Moisture, Humidity and Phase Changes

Saturated air at 15 degrees Celsius flows over a surface at 10 degrees Celsius. What is the most likely result at the surface?

  1. No exchange of water vapour
  2. Condensation forming on the surface Correct answer.
  3. Evaporation from the surface
  4. The air becoming unsaturated

Explanation

When saturated air is cooled below its dew point, its vapour content exceeds the saturation value at the new temperature and excess vapour condenses. If the surface is below the dew point, condensation such as dew or fog deposits on it.

Question 40Atmospheric Moisture, Humidity and Phase Changes

In the formation of precipitation, why is the Bergeron process efficient in mixed-phase clouds containing both ice crystals and supercooled droplets?

  1. Supercooled droplets freeze instantly and shatter into many fragments.
  2. Ice crystals absorb liquid water directly through their surfaces.
  3. Ice crystals grow by deposition because saturation vapour pressure over ice is lower than over water. Correct answer.
  4. Vapour pressure over ice exceeds that over water at the same temperature.

Explanation

At the same subfreezing temperature the saturation vapour pressure over ice is lower than over water, so vapour migrates from droplets toward ice crystals. The crystals grow rapidly by deposition at the expense of the droplets and can later fall and melt.

Question 41Atmospheric Moisture, Humidity and Phase Changes

An aircraft flies through a cloud in which the temperature is minus 10 degrees Celsius. Which description of the cloud particles is most consistent with this temperature?

  1. Only supercooled water droplets can exist at this temperature.
  2. Only ice crystals can exist at this temperature.
  3. No liquid water can remain liquid below zero degrees Celsius.
  4. A mixture of supercooled droplets and ice crystals is possible. Correct answer.

Explanation

Between zero and roughly minus 40 degrees Celsius, water droplets can remain liquid in a supercooled state, and ice crystals can also be present. Such mixed-phase clouds are common at moderate subfreezing temperatures and present an icing hazard.

Question 42Atmospheric Moisture, Humidity and Phase Changes

What happens to the relative humidity of an isolated unsaturated parcel of air when it is warmed at constant pressure with no change in water-vapour content?

  1. It remains unchanged.
  2. It increases.
  3. It decreases. Correct answer.
  4. It first rises, then falls.

Explanation

Warming raises the saturation vapour pressure, so the same mass of vapour represents a smaller fraction of the saturation value. The relative humidity therefore decreases even though the actual vapour content is unchanged.

Question 43Atmospheric Moisture, Humidity and Phase Changes

Which process transfers latent heat from the Earth's surface to the atmosphere and later releases it aloft?

  1. Conduction followed by radiation
  2. Evaporation followed by condensation Correct answer.
  3. Subsidence followed by compression
  4. Advection followed by mixing

Explanation

Evaporation absorbs latent heat from the surface, storing it in the vapour. When that vapour later condenses aloft, the latent heat is released into the surrounding air, warming it and helping to drive convection.

Question 44Atmospheric Moisture, Humidity and Phase Changes

A parcel with a temperature of 25 degrees Celsius and a dew point of 15 degrees Celsius is lifted. Assuming a dry adiabatic lapse rate of about 1 degree Celsius per 100 m and a dew point lapse of about 0.2 degrees Celsius per 100 m, at roughly what height does condensation begin?

  1. About 1250 m Correct answer.
  2. About 1000 m
  3. About 800 m
  4. About 1600 m

Explanation

The dew point depression is 10 degrees Celsius and closes at a rate of about 0.8 degrees Celsius per 100 m during ascent. Dividing 10 by 0.8 gives about 12.5 hundred metres, so saturation is reached near 1250 m.

Question 45Atmospheric Moisture, Humidity and Phase Changes

Fog forms over land on a clear, calm night with a high relative humidity. Which process is mainly responsible?

  1. Mixing of two unsaturated air masses
  2. Advection of warm moist air over a cold sea
  3. Adiabatic expansion of rising air
  4. Radiational cooling of the ground chilling the air above it Correct answer.

Explanation

On clear, calm nights the ground loses heat rapidly by radiation, cooling the air in contact with it. When that air is cooled below its dew point, condensation produces radiation fog.

Question 46Atmospheric Moisture, Humidity and Phase Changes

Air at 20 degrees Celsius with a mixing ratio of 12 g/kg is cooled to 10 degrees Celsius, where the saturation mixing ratio is about 7.6 g/kg. Approximately how much water vapour condenses per kilogram of air?

  1. 12 g
  2. 19.6 g
  3. 4.4 g Correct answer.
  4. 7.6 g

Explanation

Cooling reduces the saturation mixing ratio to about 7.6 g/kg, so only that amount can remain as vapour. Subtracting the retained value from the original 12 g/kg leaves about 4.4 g of vapour condensing per kilogram of air.

Question 47Atmospheric Moisture, Humidity and Phase Changes

Two unsaturated parcels with different temperatures and humidities are mixed. Under what condition can mixing alone produce saturation?

  1. When the warmer parcel contains less vapour than the colder parcel
  2. When the mixing occurs isobarically without any temperature change
  3. When the parcels have identical temperatures and humidities
  4. When the vapour pressure of the mixture exceeds the saturation value at the mixed temperature Correct answer.

Explanation

Mixing produces a temperature and vapour content that are averages of the two parcels, but the saturation vapour pressure varies non-linearly with temperature. If the weighted vapour pressure of the mixture exceeds the saturation value at the mixed temperature, condensation occurs and mixing fog can form.

Question 48Atmospheric Moisture, Humidity and Phase Changes

A parcel is lifted to the condensation level and continues to rise. Which statement best describes the energy released and its effect on buoyancy?

  1. No energy is exchanged once saturation is reached.
  2. The parcel cools at the dry adiabatic rate throughout its ascent.
  3. Latent heat absorption cools the parcel and reduces its buoyancy.
  4. Latent heat release warms the parcel and increases its buoyancy relative to the environment. Correct answer.

Explanation

Above the condensation level, condensing vapour releases latent heat that partly offsets adiabatic cooling. The parcel therefore cools more slowly than dry air, and if this slower cooling keeps it warmer than the environment, it gains buoyancy and can accelerate upward.

Question 49Stability, Instability and Lapse Rates

In a conditionally unstable atmosphere, which statement describes the stability of unsaturated air?

  1. It is neutral because the environmental lapse rate equals the dry adiabatic lapse rate
  2. It is unstable because the environmental lapse rate exceeds the saturated adiabatic lapse rate
  3. It is stable because the environmental lapse rate is less than the dry adiabatic lapse rate Correct answer.
  4. It is unstable because the environmental lapse rate exceeds the dry adiabatic lapse rate

Explanation

Conditional instability means the environmental lapse rate lies between the saturated and dry adiabatic lapse rates. Unsaturated air cools at the dry adiabatic rate, which is steeper than the environment, so displaced unsaturated air returns to its level and is stable. Once saturated, the parcel cools at the shallower saturated rate and becomes unstable.

Question 50Stability, Instability and Lapse Rates

What is the standard dry adiabatic lapse rate used in aviation meteorology?

  1. 1.98 degrees Celsius per 1000 ft Correct answer.
  2. 0.65 degrees Celsius per 100 m
  3. 3.0 degrees Celsius per 1000 ft
  4. 2.0 degrees Celsius per 1000 m

Explanation

The dry adiabatic lapse rate is about 3 degrees Celsius per 1000 ft, which equals 1.98 degrees Celsius per 1000 ft in precise terms. It applies to unsaturated air that is lifted or lowered without exchanging heat with its surroundings.