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CPL Meteorology practice questions

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Question 1The Atmosphere: Composition, Structure and Standard Atmosphere

Which gas forms the largest proportion of the dry atmosphere by volume?

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

Explanation

Dry air is dominated by nitrogen, which makes up roughly 78 percent of its volume. Oxygen is the second most abundant at about 21 percent, while argon and carbon dioxide are trace constituents. Water vapour varies but is never a major fixed component of dry air.

Question 2The Atmosphere: Composition, Structure and Standard Atmosphere

In the standard atmosphere, what is the approximate sea-level pressure?

  1. 1000.00 hPa
  2. 960.00 hPa
  3. 1040.00 hPa
  4. 1013.25 hPa Correct answer.

Explanation

The International Standard Atmosphere defines mean sea-level pressure as 1013.25 hPa, equivalent to 29.92 inHg. This value is the reference from which pressure altitude and altimeter settings are derived. Other values listed are not standard references.

Question 3The Atmosphere: Composition, Structure and Standard Atmosphere

Approximately what is the standard sea-level temperature in the International Standard Atmosphere?

  1. 0 degrees Celsius
  2. -5 degrees Celsius
  3. 15 degrees Celsius Correct answer.
  4. 25 degrees Celsius

Explanation

The ISA sets mean sea-level temperature at 15 degrees Celsius with a standard lapse rate of 6.5 degrees Celsius per kilometre. These values anchor density and pressure computations used in aircraft performance. Freezing and warm tropical values do not represent the standard.

Question 4The Atmosphere: Composition, Structure and Standard Atmosphere

Which layer of the atmosphere contains most of the water vapour and virtually all weather?

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

Explanation

The troposphere holds nearly all atmospheric water vapour and is where temperature falls with height, driving convection and cloud formation. Weather phenomena such as fronts and storms are therefore concentrated there. The overlying layers are dry and stable by comparison.

Question 5The Atmosphere: Composition, Structure and Standard Atmosphere

What is the standard average lapse rate of temperature in the troposphere?

  1. 6.5 degrees Celsius per 1000 ft
  2. 3.0 degrees Celsius per 1000 m
  3. 1.98 degrees Celsius per 1000 ft Correct answer.
  4. 2.0 degrees Celsius per 1000 ft

Explanation

The standard tropospheric lapse rate is about 2 degrees Celsius per 1000 ft, more precisely 1.98 degrees Celsius per 1000 ft, equivalent to 6.5 degrees Celsius per km. This rate is used to compute temperature at altitude in the standard atmosphere. The other figures misstate the gradient.

Question 6The Atmosphere: Composition, Structure and Standard Atmosphere

Which atmospheric layer is characterised by increasing temperature with height due to ozone absorption?

  1. Mesosphere
  2. Troposphere
  3. Stratosphere Correct answer.
  4. Planetary boundary layer

Explanation

In the stratosphere, ozone absorbs ultraviolet radiation, warming the air and producing a temperature inversion with height. This stability suppresses vertical motion and limits weather there. The troposphere and mesosphere instead cool with height.

Question 7The Atmosphere: Composition, Structure and Standard Atmosphere

If the sea-level temperature is 20 degrees Celsius, what temperature would the standard lapse rate predict at 5000 ft?

  1. 10 degrees Celsius Correct answer.
  2. 0 degrees Celsius
  3. 15 degrees Celsius
  4. 5 degrees Celsius

Explanation

Applying about 2 degrees Celsius per 1000 ft over 5000 ft gives a 10-degree reduction, so 20 minus 10 equals 10 degrees Celsius. The standard lapse rate is the rule being applied here. The other values reflect incorrect multiplication of the gradient.

Question 8The Atmosphere: Composition, Structure and Standard Atmosphere

Why does atmospheric pressure decrease with increasing altitude?

  1. Because gravity weakens sharply at flight levels
  2. Because fewer air molecules lie above a given level Correct answer.
  3. Because the proportion of oxygen rises with height
  4. Because temperature always falls with height

Explanation

Pressure at any level results from the weight of the air column above it, so as altitude increases the overlying mass decreases and pressure falls. Composition stays broadly constant in the lower atmosphere, and gravity changes negligibly over normal flight levels. The relationship is essentially hydrostatic.

Question 9The Atmosphere: Composition, Structure and Standard Atmosphere

What happens to air density as altitude increases in the standard atmosphere?

  1. It first increases then decreases
  2. It increases because pressure rises
  3. It decreases because pressure and temperature both fall Correct answer.
  4. It remains constant because composition is fixed

Explanation

Density depends on both pressure and temperature through the gas law. With altitude, pressure falls markedly and temperature falls in the troposphere, so density decreases steadily. Constant or increasing density contradicts the standard atmosphere.

Question 10The Atmosphere: Composition, Structure and Standard Atmosphere

What is the primary significance of the tropopause to aviation?

  1. It is the boundary where the lapse rate changes and jet streams often occur Correct answer.
  2. It is the level of constant pressure
  3. It marks the top of the ozone layer
  4. It is where most thunderstorms form

Explanation

The tropopause is the transition between the troposphere and stratosphere, where the lapse rate becomes near-isothermal. It is associated with jet streams and often marks the top of significant weather and the base of smooth, stable stratospheric air. It is not the top of the ozone layer.

Question 11The Atmosphere: Composition, Structure and Standard Atmosphere

Which statement best describes how water vapour in the atmosphere affects aircraft performance?

  1. Humid air is less dense, reducing performance Correct answer.
  2. Humid air is denser, improving performance
  3. Water vapour has no effect on density
  4. Humidity only affects pressure, not density

Explanation

Water vapour molecules are lighter than the average dry-air molecule, so moist air is less dense than dry air at the same pressure and temperature. Reduced density decreases lift and engine power, degrading performance. Humidity therefore matters for takeoff and climb calculations.

Question 12The Atmosphere: Composition, Structure and Standard Atmosphere

In the standard atmosphere, at approximately what altitude does the troposphere give way to the stratosphere in mid-latitudes?

  1. About 5000 ft
  2. About 18000 ft
  3. About 60000 ft
  4. About 36000 ft Correct answer.

Explanation

The tropopause typically lies near 36000 ft in mid-latitudes, though it varies with latitude and season. It is higher near the equator and lower near the poles. The other altitudes fall within the troposphere or well into the stratosphere.

Question 13The Atmosphere: Composition, Structure and Standard Atmosphere

How does the standard atmosphere treat the temperature in the lower stratosphere?

  1. It falls to absolute zero
  2. It decreases at 2 degrees Celsius per 1000 ft
  3. It is assumed constant up to about 20000 ft above the tropopause Correct answer.
  4. It increases at 2 degrees Celsius per 1000 ft

Explanation

In the ISA, the lower stratosphere is modelled as isothermal, with temperature held constant at about minus 56.5 degrees Celsius over a defined band. This simplifies pressure and density calculations above the tropopause. It is not a zone of continued cooling or strong warming in the model.

Question 14The Atmosphere: Composition, Structure and Standard Atmosphere

A pilot reads a pressure altimeter set to 1013.25 hPa while the actual sea-level pressure is 1003 hPa. What is the effect on the indicated altitude?

  1. The altimeter reading is unaffected by pressure
  2. The altimeter under-reads, showing lower than actual altitude
  3. The altimeter reads correctly
  4. The altimeter over-reads, showing higher than actual altitude Correct answer.

Explanation

When the subscale is set higher than the actual pressure, the altimeter assumes a lower-pressure environment and indicates an altitude greater than the true one. The rule is that setting the subscale above actual pressure causes over-reading. This is a classic altimeter error affecting terrain clearance.

Question 15The Atmosphere: Composition, Structure and Standard Atmosphere

Which of the following best explains why the troposphere is heated primarily from below?

  1. Pressure is highest at the tropopause
  2. Cosmic rays deposit heat near the ground
  3. Ozone absorbs ultraviolet radiation there
  4. The air is transparent to solar radiation but absorbs terrestrial infrared and receives heat from the surface Correct answer.

Explanation

The troposphere is largely transparent to incoming shortwave solar radiation, so the surface absorbs most of it. The warmed ground then heats the air by conduction, convection and absorption of outgoing longwave radiation. This is why temperature generally decreases with height in the layer.

Question 16The Atmosphere: Composition, Structure and Standard Atmosphere

Compared with the actual atmosphere on a hot day, the standard atmosphere at the same pressure altitude will generally be:

  1. Cooler and denser Correct answer.
  2. Identical in temperature
  3. Warmer but denser
  4. Warmer and less dense

Explanation

The standard atmosphere assumes a fixed temperature profile, so on a hot day the real air is warmer than standard at the same pressure level. Warmer air is less dense, meaning the standard model is cooler and denser than reality. This underlies the use of temperature corrections in performance work.

Question 17The Atmosphere: Composition, Structure and Standard Atmosphere

Which combination of gases is chiefly responsible for the greenhouse effect in the troposphere?

  1. Argon and neon
  2. Water vapour and carbon dioxide Correct answer.
  3. Nitrogen and oxygen
  4. Ozone and helium

Explanation

Water vapour and carbon dioxide absorb and re-emit longwave infrared radiation, trapping heat in the troposphere. Nitrogen and oxygen are essentially transparent to infrared and do not contribute significantly. This selective absorption is the basis of the greenhouse effect.

Question 18The Atmosphere: Composition, Structure and Standard Atmosphere

Why does the standard atmosphere define pressure altitude rather than true altitude for performance calculations?

  1. To provide a common reference independent of local pressure variations Correct answer.
  2. Because pressure altitude always equals true altitude
  3. Because true altitude is impossible to measure
  4. To eliminate the effect of temperature on performance

Explanation

Pressure altitude is referenced to the standard 1013.25 hPa datum, giving a consistent basis for comparing aircraft performance across different days and locations. It removes the variability of local sea-level pressure. Temperature effects are handled separately through density altitude.

Question 19The Atmosphere: Composition, Structure and Standard Atmosphere

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

  1. About three quarters Correct answer.
  2. About ninety-nine percent
  3. About one quarter
  4. About half

Explanation

Because air is compressed near the surface, the troposphere, despite being the lowest layer, contains roughly three quarters of the atmosphere's mass. The overlying layers are increasingly rarefied. This concentration explains why weather and most aviation activity occur there.

Question 20The Atmosphere: Composition, Structure and Standard Atmosphere

An aircraft climbs from sea level to 10000 ft in the standard atmosphere. What approximate temperature does the standard lapse rate predict at that level, given a sea-level value of 15 degrees Celsius?

  1. 15 degrees Celsius
  2. Minus 5 degrees Celsius Correct answer.
  3. 10 degrees Celsius
  4. 5 degrees Celsius

Explanation

Using about 2 degrees Celsius per 1000 ft, a 10000 ft climb gives a 20-degree reduction from 15 degrees Celsius, yielding about minus 5 degrees Celsius. The standard lapse rate is applied directly. Values that ignore the full gradient are incorrect.

Question 21Temperature, Pressure, Density and Their Altimetry Implications

In the International Standard Atmosphere, what is the mean sea-level pressure and temperature?

  1. 1000 hPa and 15 °C
  2. 1013.25 hPa and 15 °C Correct answer.
  3. 980 hPa and 20 °C
  4. 1013.25 hPa and 0 °C

Explanation

The ISA sets mean sea-level pressure at 1013.25 hPa and temperature at 15 °C, with a lapse rate of 6.5 °C per km in the lower layer. These reference values underpin altimeter and performance calculations.

Question 22Temperature, Pressure, Density and Their Altimetry Implications

Approximately how does air density change with height in the lower atmosphere?

  1. It increases steadily with height
  2. It decreases with height Correct answer.
  3. It stays constant up to the tropopause
  4. It decreases then rises sharply

Explanation

Both pressure and temperature fall with height, so density falls as well; roughly half the mass of the atmosphere lies below about 5500 m. Density halving occurs near 5500 m in the standard atmosphere.

Question 23Temperature, Pressure, Density and Their Altimetry Implications

What is the standard ISA temperature lapse rate in the troposphere?

  1. 3 °C per 1000 ft
  2. 6.5 °C per km Correct answer.
  3. 1.98 °C per 1000 ft
  4. 2 °C per 1000 ft

Explanation

The ISA tropospheric lapse rate is 6.5 °C per kilometre, equivalent to about 1.98 °C per 1000 ft. This rate is used to compute ISA temperature at any level for altimetry.

Question 24Temperature, Pressure, Density and Their Altimetry Implications

An aircraft flies from a cold air mass into a warm air mass at constant indicated altitude and constant true altitude. What happens to the true altitude if indicated altitude is held constant?

  1. True altitude becomes zero
  2. True altitude decreases
  3. True altitude increases Correct answer.
  4. True altitude stays the same

Explanation

In warm air the pressure levels are farther apart, so a given pressure level lies higher. Holding indicated altitude constant while flying into warmer air means the aircraft is actually higher above the surface than indicated.

Question 25Temperature, Pressure, Density and Their Altimetry Implications

What does QNH represent?

  1. Pressure at the aerodrome reference point reduced to sea level Correct answer.
  2. Pressure at the standard datum of 1013.25 hPa
  3. Pressure at the highest point of the aerodrome
  4. Pressure at the aircraft's current level

Explanation

QNH is the aerodrome pressure reduced to mean sea level using the ISA, so that an altimeter set to QNH reads elevation on the ground. It is the setting used for terrain clearance near the surface.

Question 26Temperature, Pressure, Density and Their Altimetry Implications

Which pressure setting makes an altimeter read zero on the aerodrome when the aircraft is on the ground?

  1. QNE
  2. Standard 1013.25 hPa
  3. QNH
  4. QFE Correct answer.

Explanation

QFE is the actual pressure at the aerodrome reference point, so an altimeter set to QFE reads zero when the aircraft is on that point. QNH instead reads aerodrome elevation.

Question 27Temperature, Pressure, Density and Their Altimetry Implications

An aircraft is at an indicated altitude of 5000 ft with QNH 1000 hPa. If actual sea-level pressure is lower than 1013.25 hPa, how does true altitude compare with indicated altitude?

  1. True altitude equals indicated
  2. True altitude is lower than indicated Correct answer.
  3. True altitude is higher than indicated
  4. True altitude is exactly 5000 ft

Explanation

When sea-level pressure is below standard, the pressure levels are lower than the standard atmosphere predicts. The aircraft is therefore at a lower true altitude than the altimeter indicates.

Question 28Temperature, Pressure, Density and Their Altimetry Implications

What is pressure altitude?

  1. Altitude above the highest terrain
  2. Altitude corrected for temperature
  3. Altitude indicated when the subscale is set to 1013.25 hPa Correct answer.
  4. Altitude above the aerodrome

Explanation

Pressure altitude is the altitude shown when the altimeter subscale is set to the standard datum of 1013.25 hPa. It is the basis for flight levels and for performance charts.

Question 29Temperature, Pressure, Density and Their Altimetry Implications

Density altitude is best described as which of the following?

  1. The altitude shown on the altimeter with QNH set
  2. The geometric height above mean sea level
  3. The altitude at which the air temperature equals 15 °C
  4. The altitude in the standard atmosphere at which a given density occurs Correct answer.

Explanation

Density altitude expresses the actual air density as the ISA altitude that would produce that density. High temperature, high humidity, or low pressure all raise density altitude and degrade performance.

Question 30Temperature, Pressure, Density and Their Altimetry Implications

On a hot day at a high-elevation aerodrome, what is the effect on take-off performance compared with a standard day?

  1. Take-off distance increases and climb rate decreases Correct answer.
  2. Take-off distance decreases and climb rate improves
  3. Take-off distance is unchanged
  4. Climb rate improves but take-off distance increases

Explanation

High temperature and high elevation both reduce air density, which lowers engine power and thrust while increasing true airspeed for a given indicated airspeed. The result is a longer take-off run and reduced climb performance.

Question 31Temperature, Pressure, Density and Their Altimetry Implications

An aircraft maintains a constant indicated altitude while flying from a region of high pressure to a region of low pressure. What happens to its true altitude?

  1. It decreases Correct answer.
  2. It becomes negative
  3. It remains constant
  4. It increases

Explanation

Flying toward lower pressure without resetting the altimeter means the aircraft descends relative to the pressure surfaces. True altitude decreases even though indicated altitude is unchanged.

Question 32Temperature, Pressure, Density and Their Altimetry Implications

The altimeter subscale is set to 1013.25 hPa and the aircraft is above the transition level. What does the altimeter display?

  1. True altitude
  2. Elevation
  3. Height above the aerodrome
  4. Flight level Correct answer.

Explanation

With the standard pressure setting selected, the altimeter reads flight level, which is pressure altitude in hundreds of feet. This provides a common datum for all aircraft above the transition level.

Question 33Temperature, Pressure, Density and Their Altimetry Implications

Which factor causes density altitude to be higher than pressure altitude?

  1. Temperature above ISA at that pressure altitude Correct answer.
  2. Relative humidity of zero
  3. Temperature below ISA at that pressure altitude
  4. Pressure equal to 1013.25 hPa

Explanation

Density altitude exceeds pressure altitude when the air is warmer than ISA, because warm air is less dense. Humidity also raises density altitude, while dry air keeps it lower.

Question 34Temperature, Pressure, Density and Their Altimetry Implications

Why does true airspeed exceed indicated airspeed at high altitude?

  1. Because the altimeter is set to QFE
  2. Because pressure increases with height
  3. Because temperature decreases with height
  4. Because air density decreases while the dynamic pressure needed for lift stays similar Correct answer.

Explanation

Indicated airspeed reflects dynamic pressure, which depends on density. As density falls with height, the aircraft must move faster through the air to generate the same dynamic pressure, so true airspeed is higher.

Question 35Temperature, Pressure, Density and Their Altimetry Implications

An aerodrome elevation is 2000 ft and QNH is 1013.25 hPa. What approximate indicated altitude does an altimeter show on the ground?

  1. 2000 ft Correct answer.
  2. 0 ft
  3. 1013 ft
  4. 4000 ft

Explanation

With QNH set to the standard value and the aerodrome at 2000 ft, the altimeter reads the aerodrome elevation of about 2000 ft. QNH is designed so the altimeter indicates elevation on the ground.

Question 36Temperature, Pressure, Density and Their Altimetry Implications

What happens to QFE relative to QNH at an aerodrome that is above sea level?

  1. QFE is unrelated to QNH
  2. QFE equals QNH
  3. QFE is lower than QNH Correct answer.
  4. QFE is higher than QNH

Explanation

QFE is the pressure at the aerodrome, which is higher up in the atmosphere than sea level, so it is lower than the sea-level-reduced QNH. The difference corresponds roughly to the aerodrome elevation.

Question 37Temperature, Pressure, Density and Their Altimetry Implications

In cold air, an aircraft flying at a constant indicated altitude over high terrain faces which hazard?

  1. The altimeter overreads by a fixed 100 ft
  2. The aircraft is lower than indicated, reducing terrain clearance Correct answer.
  3. The aircraft is higher than indicated, increasing clearance
  4. The indicated altitude is exactly true altitude

Explanation

Cold air compresses the pressure levels, so a given indicated altitude corresponds to a lower true altitude. Over high terrain this creates a serious risk of insufficient clearance, which is why cold-temperature corrections are applied.

Question 38Temperature, Pressure, Density and Their Altimetry Implications

An aircraft at 10 000 ft pressure altitude in air 10 °C colder than ISA has a true altitude approximately how far from indicated altitude?

  1. About 40 ft lower
  2. About 400 ft higher
  3. About 400 ft lower Correct answer.
  4. Exactly equal

Explanation

A useful approximation is 4 ft of error per 1000 ft per degree Celsius of ISA deviation. With 10 °C deviation at 10 000 ft the error is about 400 ft, and cold air makes true altitude lower than indicated.

Question 39Temperature, Pressure, Density and Their Altimetry Implications

Two aerodromes have the same elevation and QNH, but aerodrome A is much hotter than aerodrome B. How do their density altitudes compare?

  1. They are equal
  2. A has the higher density altitude Correct answer.
  3. Density altitude is independent of temperature
  4. B has the higher density altitude

Explanation

Density altitude rises with temperature because warm air is less dense. With equal elevation and pressure, the hotter aerodrome has the higher density altitude and worse performance.

Question 40Temperature, Pressure, Density and Their Altimetry Implications

An aircraft descends along a constant pressure surface from a cold region into a warm region. What happens to its true altitude?

  1. It stays constant
  2. It rises Correct answer.
  3. It first falls then rises
  4. It falls

Explanation

A constant pressure surface is higher over warm air because the column expands. Flying along it into warmer air means the aircraft's true altitude increases even though pressure altitude is unchanged.

Question 41Moisture, Humidity and Atmospheric Stability

Which process changes water vapour directly into liquid water without passing through an intermediate stage?

  1. Sublimation
  2. Evaporation
  3. Condensation Correct answer.
  4. Deposition

Explanation

Condensation is the phase change from vapour to liquid, occurring when air is cooled to its dew point or becomes saturated. It releases latent heat and is the process forming cloud droplets and dew.

Question 42Moisture, Humidity and Atmospheric Stability

What is the name of the temperature to which air must be cooled at constant pressure and moisture content to become saturated?

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

Explanation

The dew point is the temperature at which the air, cooled at constant pressure and water vapour content, reaches saturation. Further cooling below the dew point forces condensation. The smaller the spread between temperature and dew point, the closer the air is to saturation.

Question 43Moisture, Humidity and Atmospheric Stability

Relative humidity is best described as which of the following?

  1. The mass of water vapour in a cubic metre of air
  2. The temperature at which the air becomes saturated
  3. The total mass of water vapour and dry air in a parcel
  4. The ratio of actual vapour pressure to saturation vapour pressure at the same temperature Correct answer.

Explanation

Relative humidity expresses how close the air is to saturation, as a percentage ratio of actual vapour pressure to the saturation vapour pressure at the current temperature. Because saturation vapour pressure depends on temperature, relative humidity changes with temperature even when the vapour content is constant.

Question 44Moisture, Humidity and Atmospheric Stability

An air parcel is lifted and cools at the dry adiabatic lapse rate. Which statement about its moisture is correct?

  1. Its relative humidity remains constant because no vapour is added
  2. Its relative humidity decreases because saturation vapour pressure falls
  3. Its relative humidity increases because saturation vapour pressure falls Correct answer.
  4. Its dew point rises rapidly as the parcel expands

Explanation

In an unsaturated parcel the mixing ratio stays essentially constant during lifting, but cooling lowers the saturation vapour pressure. The ratio of actual to saturation vapour pressure therefore rises, so relative humidity increases until saturation is reached at the lifting condensation level.

Question 45Moisture, Humidity and Atmospheric Stability

Air at 20 degrees Celsius with a dew point of 12 degrees Celsius is cooled overnight. Approximately at what temperature would saturation first occur?

  1. 16 degrees Celsius
  2. 20 degrees Celsius
  3. 12 degrees Celsius Correct answer.
  4. 8 degrees Celsius

Explanation

Saturation occurs when the air temperature falls to the dew point, provided pressure and moisture content remain constant. With a dew point of 12 degrees Celsius, condensation begins at about that temperature.

Question 46Moisture, Humidity and Atmospheric Stability

Which condition is required for water vapour to condense into visible cloud droplets in the atmosphere?

  1. The air must be compressed rapidly without cooling
  2. The air must reach saturation and condensation nuclei must be present Correct answer.
  3. The air must be warmed above its dew point
  4. The air must contain no suspended particles

Explanation

Condensation requires the air to become saturated, normally by cooling, and also requires surfaces or particles on which droplets can form. These condensation nuclei include dust, sea salt and combustion products. Without nuclei, substantial supersaturation would be needed before droplets appear.

Question 47Moisture, Humidity and Atmospheric Stability

A parcel of unsaturated air is forced to ascend a mountain slope. What happens to its temperature and relative humidity?

  1. Temperature falls and relative humidity rises Correct answer.
  2. Temperature rises and relative humidity rises
  3. Temperature rises and relative humidity falls
  4. Temperature falls and relative humidity falls

Explanation

Expansion in lower pressure aloft causes adiabatic cooling of the rising parcel, so temperature falls. Since saturation vapour pressure drops with temperature while vapour content stays nearly constant, relative humidity rises toward saturation.

Question 48Moisture, Humidity and Atmospheric Stability

What is the environmental lapse rate?

  1. The actual rate of temperature decrease with height measured in the surrounding atmosphere Correct answer.
  2. The rate at which a rising parcel cools
  3. The rate at which temperature increases with height in an inversion
  4. The rate at which saturated air cools when lifted

Explanation

The environmental lapse rate describes the real temperature profile of the surrounding atmosphere, obtained from measurements such as radiosondes. It must be distinguished from the dry and saturated adiabatic rates that apply to a moving parcel. Stability depends on how these rates compare.

Question 49Moisture, Humidity and Atmospheric Stability

Which combination indicates an absolutely unstable atmosphere?

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

Explanation

Absolute instability exists when the environmental lapse rate exceeds the dry adiabatic lapse rate of about 3 degrees Celsius per 1000 feet. A saturated or unsaturated parcel then remains warmer than its surroundings as it rises, so it accelerates upward.

Question 50Moisture, Humidity and Atmospheric Stability

What does conditional instability mean?

  1. The atmosphere is stable for unsaturated parcels but unstable for saturated parcels Correct answer.
  2. The atmosphere is unstable only when winds are strong
  3. The atmosphere is stable regardless of parcel moisture
  4. The atmosphere is unstable only for dry parcels

Explanation

Conditional instability occurs when the environmental lapse rate lies between the saturated and dry adiabatic rates. An unsaturated parcel resists lifting, but once it becomes saturated and cools at the smaller saturated rate it can become warmer than the environment and rise freely. Deep conditional instability favours showers and thunderstorms.