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Transport Canada Flight Dispatcher Meteorology Exam study materials
Transport Canada Flight Dispatcher Meteorology Exam study resources

Transport Canada Flight Dispatcher Meteorology Exam practice questions

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Question 1The Atmosphere: Composition, Structure, and Standard Atmosphere

In the standard atmosphere, what is the approximate composition of dry air by volume?

  1. 50% nitrogen, 49% oxygen, 1% other gases
  2. 78% nitrogen, 21% oxygen, 1% other gases Correct answer.
  3. 90% nitrogen, 9% oxygen, 1% other gases
  4. 21% nitrogen, 78% oxygen, 1% other gases

Explanation

Dry air is dominated by nitrogen at roughly 78 percent by volume, with oxygen near 21 percent. The remaining one percent consists of argon, carbon dioxide, and trace gases. These proportions stay essentially constant through the lower atmosphere.

Question 2The Atmosphere: Composition, Structure, and Standard Atmosphere

Which gas in the atmosphere varies most in concentration from place to place and is a significant greenhouse gas?

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

Explanation

Water vapour is highly variable, ranging from near zero in cold dry air to several percent in warm humid air. It is also the most important greenhouse gas by amount. Nitrogen and argon are nearly constant, so they do not fit the description.

Question 3The Atmosphere: Composition, Structure, and Standard Atmosphere

Which atmospheric layer contains most of the weather and the majority of the atmosphere's mass?

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

Explanation

The troposphere is the lowest layer and holds the bulk of the atmospheric mass and water vapour. Nearly all clouds and weather occur within it. Temperature decreases with height there, which promotes vertical mixing.

Question 4The Atmosphere: Composition, Structure, and Standard Atmosphere

What is the standard sea-level pressure used in the ICAO standard atmosphere?

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

Explanation

The ICAO standard atmosphere defines sea-level pressure as 1013.25 hectopascals, equivalent to 29.92 inches of mercury. This reference value underlies altimetry and standard-atmosphere calculations. Other values are arbitrary and not part of the standard.

Question 5The Atmosphere: Composition, Structure, and Standard Atmosphere

In the troposphere, how does temperature typically change with increasing altitude?

  1. It decreases with height at about 2 degrees Celsius per 1000 feet Correct answer.
  2. It remains constant with height
  3. It increases steadily with height
  4. It decreases then sharply increases within the layer

Explanation

The troposphere is characterised by a lapse rate, with temperature falling as height increases. The standard-atmosphere average is about 2 degrees Celsius per 1000 feet, or 6.5 degrees per kilometre. This cooling supports convection and cloud formation.

Question 6The Atmosphere: Composition, Structure, and Standard Atmosphere

What marks the boundary between the troposphere and the stratosphere?

  1. The homopause, where gases begin to separate
  2. The tropopause, where the temperature lapse rate becomes nearly isothermal Correct answer.
  3. The stratopause, where temperature peaks
  4. The mesopause, where temperature reaches a minimum

Explanation

The tropopause is identified where the temperature stops falling with height and becomes roughly constant before rising in the stratosphere. It varies in height with latitude and season. It is the boundary dispatchers use when assessing tropopause height for flight planning.

Question 7The Atmosphere: Composition, Structure, and Standard Atmosphere

Why does temperature increase with height in the stratosphere?

  1. Because the surface radiates heat upward
  2. Because rising air parcels release latent heat there
  3. Because ozone absorbs ultraviolet solar radiation Correct answer.
  4. Because pressure increases with height

Explanation

Ozone in the stratosphere absorbs ultraviolet radiation from the sun, warming that layer. This creates a temperature inversion that stabilises the stratosphere and limits vertical mixing. The warming is not caused by surface heating or latent heat release.

Question 8The Atmosphere: Composition, Structure, and Standard Atmosphere

In the standard atmosphere, what is the approximate temperature at 10,000 feet pressure altitude on a standard day?

  1. 5 degrees Celsius
  2. -20 degrees Celsius
  3. -5 degrees Celsius Correct answer.
  4. 15 degrees Celsius

Explanation

Starting from 15 degrees Celsius at sea level and applying the standard lapse rate of about 2 degrees per 1000 feet, 10,000 feet gives roughly -5 degrees Celsius. This standard value supports performance and altimetry calculations. It reflects the average, not the actual temperature on a given day.

Question 9The Atmosphere: Composition, Structure, and Standard Atmosphere

A dispatcher notes the tropopause is higher over the tropics than over the poles. What is the primary reason?

  1. The tropical troposphere is warmer and convectively deeper Correct answer.
  2. The poles have more ozone in the troposphere
  3. The tropics have lower surface pressure
  4. The tropics have stronger surface winds

Explanation

Warmer tropical air and vigorous convection extend the troposphere to greater heights, so the tropopause sits higher there. Colder polar air produces a shallower troposphere. This latitude variation affects where the tropopause is found on a given route.

Question 10The Atmosphere: Composition, Structure, and Standard Atmosphere

Which statement best describes the temperature profile of the mesosphere?

  1. Temperature is constant throughout the layer
  2. Temperature increases sharply due to solar radiation
  3. Temperature decreases with height, reaching the coldest part of the atmosphere Correct answer.
  4. Temperature increases with height due to ozone absorption

Explanation

Above the stratopause, temperature falls again with height through the mesosphere, reaching the coldest temperatures in the atmosphere near the mesopause. This occurs because ozone absorption weakens and there is little heating. Warming with height resumes only in the thermosphere.

Question 11The Atmosphere: Composition, Structure, and Standard Atmosphere

In the standard atmosphere, what is the approximate height of the 500 hPa pressure level?

  1. About 12000 metres
  2. About 3000 metres
  3. About 9000 metres
  4. About 5500 metres Correct answer.

Explanation

The 500 hPa level sits near 5500 metres, roughly 18,000 feet, in the standard atmosphere. This level is a key reference for upper-air charts and analyses. The other heights correspond to different pressure levels.

Question 12The Atmosphere: Composition, Structure, and Standard Atmosphere

A parcel of air rises and expands without exchanging heat with its surroundings. What happens to its temperature?

  1. It decreases because the parcel does work expanding Correct answer.
  2. It stays the same because no heat is exchanged
  3. It increases because pressure rises
  4. It first rises then falls

Explanation

In an adiabatic process no heat is exchanged, but the expanding parcel does work against the surrounding pressure. That work consumes internal energy, so the parcel cools. This is why rising air cools at the dry adiabatic lapse rate.

Question 13The Atmosphere: Composition, Structure, and Standard Atmosphere

What is the standard sea-level temperature in the ICAO standard atmosphere?

  1. 0 degrees Celsius
  2. 20 degrees Celsius
  3. 10 degrees Celsius
  4. 15 degrees Celsius Correct answer.

Explanation

The ICAO standard atmosphere sets sea-level temperature at 15 degrees Celsius. Combined with the standard lapse rate, this defines the reference temperature profile used in altimetry and performance. Other values do not match the defined standard.

Question 14The Atmosphere: Composition, Structure, and Standard Atmosphere

A flight is planned at a pressure altitude where the actual temperature is much colder than standard. Compared with the standard atmosphere, how does the true altitude relate to the indicated altitude?

  1. True altitude is lower than indicated altitude Correct answer.
  2. True altitude equals indicated altitude
  3. True altitude is higher than indicated altitude
  4. True altitude cannot be determined from temperature

Explanation

Cold air is denser, so pressure levels are compressed toward the surface. For a given indicated altitude, the aircraft is actually lower than it would be in standard conditions. This is why cold-temperature altimetry errors require correction on approach.

Question 15The Atmosphere: Composition, Structure, and Standard Atmosphere

Which of the following best explains why the tropopause acts as a lid on most vertical motion?

  1. Water vapour increases sharply above the tropopause
  2. Wind speed drops to zero at the tropopause
  3. The temperature inversion there makes the layer stable and limits rising parcels Correct answer.
  4. Pressure increases rapidly above the tropopause

Explanation

At the tropopause the lapse rate becomes isothermal and then reverses, producing a stable layer. Rising parcels lose buoyancy and are generally capped there. This stability confines most weather to the troposphere.

Question 16The Atmosphere: Composition, Structure, and Standard Atmosphere

Using the standard atmosphere, estimate the pressure altitude of an aircraft flying where the outside air temperature is -30 degrees Celsius and the tropopause has not been reached, assuming standard lapse rate applies.

  1. About 15,000 feet
  2. About 5000 feet
  3. About 22,500 feet Correct answer.
  4. About 30,000 feet

Explanation

Subtracting the sea-level value of 15 degrees Celsius from -30 gives a change of 45 degrees. Dividing by the standard lapse rate of about 2 degrees per 1000 feet yields roughly 22,500 feet. This illustrates how temperature can be inverted to estimate pressure altitude in the troposphere.

Question 17Temperature, Pressure, Density, and Their Altimeter Implications

In the ICAO Standard Atmosphere, what are the sea-level pressure and temperature values on which pressure altitude and density altitude are based?

  1. 1000.00 hPa and 15 °C
  2. 1013.25 hPa and 15 °C Correct answer.
  3. 1013.25 hPa and 0 °C
  4. 1023.25 hPa and 10 °C

Explanation

The ICAO Standard Atmosphere defines mean sea-level pressure as 1013.25 hPa (29.92 inHg) and sea-level temperature as 15 °C, with a standard lapse rate of about 6.5 °C per 1000 m. These reference values are the datum for pressure altitude and for altimeter subscale settings.

Question 18Temperature, Pressure, Density, and Their Altimeter Implications

An aircraft flies at a constant indicated altitude from a warm air mass into a cold air mass at the same altimeter setting. What happens to the true altitude of the aircraft?

  1. It remains equal to the indicated altitude
  2. It decreases below the indicated altitude Correct answer.
  3. It fluctuates with no net change
  4. It increases above the indicated altitude

Explanation

In cold air the pressure surfaces are compressed toward the ground, so a given pressure level lies at a lower true altitude. When the aircraft maintains the same indicated altitude, it is actually flying lower than the altimeter suggests, which is the basis of the "high to low, look out below" rule.

Question 19Temperature, Pressure, Density, and Their Altimeter Implications

What does density altitude represent?

  1. True altitude corrected for terrain elevation
  2. Height above the standard datum plane
  3. Pressure altitude corrected for non-standard temperature Correct answer.
  4. Indicated altitude corrected for instrument error

Explanation

Density altitude is the altitude in the standard atmosphere at which the prevailing air density would occur. It is obtained by correcting pressure altitude for the difference between actual temperature and standard temperature at that level.

Question 20Temperature, Pressure, Density, and Their Altimeter Implications

An aerodrome at elevation 500 ft reports a QNH of 1013 hPa. What is the approximate pressure altitude of the aerodrome?

  1. 1000 ft
  2. 100 ft
  3. 500 ft Correct answer.
  4. 0 ft

Explanation

Pressure altitude is the elevation above the 1013.25 hPa datum. When QNH equals 1013 hPa, the aerodrome elevation and its pressure altitude coincide, so the pressure altitude is about 500 ft.

Question 21Temperature, Pressure, Density, and Their Altimeter Implications

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

  1. About 1 hPa
  2. About 50 hPa
  3. About 10 hPa Correct answer.
  4. About 30 hPa

Explanation

Near sea level the pressure lapse rate is roughly 1 hPa per 30 ft, which corresponds to about 10 hPa per 1000 ft. This rate is used for quick estimates of pressure altitude differences and altimeter error.

Question 22Temperature, Pressure, Density, and Their Altimeter Implications

An altimeter is set to 1013 hPa while the local QNH is 1003 hPa. What does the altimeter indicate relative to the aerodrome elevation?

  1. About 300 ft lower than the true elevation
  2. About 300 ft higher than the true elevation Correct answer.
  3. The true elevation exactly
  4. About 30 ft higher than the true elevation

Explanation

The altimeter subscale is set 10 hPa higher than the actual QNH. Since 1 hPa corresponds to roughly 30 ft, the instrument over-reads by about 300 ft, showing an altitude higher than the true elevation.

Question 23Temperature, Pressure, Density, and Their Altimeter Implications

Why does an increase in humidity cause density altitude to rise?

  1. Water vapour is lighter than dry air at the same pressure and temperature Correct answer.
  2. Humidity raises the local surface pressure
  3. Water vapour increases the total molecular weight of the air
  4. Humidity reduces the temperature lapse rate

Explanation

Water vapour molecules have a lower molecular mass than the nitrogen and oxygen they displace. Humid air is therefore less dense than dry air at the same pressure and temperature, which raises density altitude.

Question 24Temperature, Pressure, Density, and Their Altimeter Implications

A station reports QFE of 990 hPa at an aerodrome with elevation 600 ft. Using 1 hPa ≈ 30 ft, what is the approximate QNH?

  1. 1000 hPa
  2. 1020 hPa Correct answer.
  3. 980 hPa
  4. 1010 hPa

Explanation

QNH exceeds QFE by the pressure equivalent of the aerodrome elevation. Elevation 600 ft divided by 30 ft per hPa gives about 20 hPa, so QNH is approximately 990 + 20 = 1010 hPa.

Question 25Temperature, Pressure, Density, and Their Altimeter Implications

Which statement correctly distinguishes true altitude from absolute altitude?

  1. Both are measured above the aerodrome reference point
  2. True altitude is height above the surface; absolute altitude is height above mean sea level
  3. Both are measured above the standard pressure datum
  4. True altitude is height above mean sea level; absolute altitude is height above the surface below Correct answer.

Explanation

True altitude is the vertical distance above mean sea level, while absolute altitude (height) is the vertical distance above the underlying surface. The two coincide only when the terrain is at sea level.

Question 26Temperature, Pressure, Density, and Their Altimeter Implications

During a very cold winter, an aircraft on final approach at an aerodrome with high terrain nearby uses an uncorrected QNH-based altimeter. What is the principal hazard?

  1. The altimeter over-reads and shows excessive terrain clearance
  2. The aircraft is higher than indicated and may overshoot the runway
  3. The altimeter becomes unreliable because pressure is too low
  4. The aircraft is lower than indicated and terrain clearance is reduced Correct answer.

Explanation

Cold temperatures compress pressure surfaces, so the altimeter over-reads and the aircraft is lower than indicated. Near high terrain this reduces actual clearance, which is why cold-temperature altitude corrections are applied at affected aerodromes.

Question 27Temperature, Pressure, Density, and Their Altimeter Implications

What is the definition of QFE?

  1. The pressure at the standard 1013.25 hPa datum
  2. The pressure at aerodrome elevation, so the altimeter reads height above the aerodrome Correct answer.
  3. The pressure at the transition level
  4. The sea-level pressure reduced to mean sea level

Explanation

QFE is the atmospheric pressure at the aerodrome elevation. When it is set on the subscale, the altimeter reads zero on the ground and indicates height above the aerodrome.

Question 28Temperature, Pressure, Density, and Their Altimeter Implications

Two aircraft fly at the same indicated altitude of 10 000 ft. Aircraft A is in air at −20 °C and aircraft B is in air at +10 °C. How do their true altitudes compare?

  1. A and B have identical true altitudes
  2. A is lower than B in true altitude Correct answer.
  3. The comparison depends only on the altimeter settings used
  4. A is higher than B in true altitude

Explanation

Colder air is denser and compresses the pressure levels, so the 10 000 ft pressure surface lies at a lower true altitude. Aircraft A in the colder air is therefore lower in true altitude than aircraft B.

Question 29Temperature, Pressure, Density, and Their Altimeter Implications

An aerodrome at sea level experiences a heat wave with temperature 20 °C above standard. How does density altitude compare with pressure altitude?

  1. Density altitude equals pressure altitude
  2. Density altitude is unaffected by temperature
  3. Density altitude is higher than pressure altitude Correct answer.
  4. Density altitude is lower than pressure altitude

Explanation

Density altitude increases above pressure altitude when the air is warmer than standard because warm air is less dense. The magnitude of the difference grows with the temperature departure from standard.

Question 30Temperature, Pressure, Density, and Their Altimeter Implications

An aircraft at flight level 180 experiences an unusually high-pressure system with QNH 1035 hPa while the altimeter is set to 1013 hPa. What is the approximate true altitude relative to the flight level?

  1. About 660 ft lower than the flight level
  2. About 220 ft higher than the flight level
  3. Exactly equal to the flight level
  4. About 660 ft higher than the flight level Correct answer.

Explanation

The QNH is 22 hPa above the standard setting. At about 30 ft per hPa this gives roughly 660 ft, and because the pressure surfaces are raised in a high-pressure system, the true altitude is higher than the nominal flight level.

Question 31Temperature, Pressure, Density, and Their Altimeter Implications

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

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

Explanation

Density altitude rises when air density falls. Warm temperature, high humidity and low pressure each reduce density, so combining all three gives the highest density altitude and the poorest performance.

Question 32Temperature, Pressure, Density, and Their Altimeter Implications

An aircraft maintains a constant indicated altitude while flying from a region of high pressure toward a region of low pressure without updating the subscale. What is the resulting flight profile?

  1. The aircraft climbs in true altitude
  2. The aircraft maintains constant true altitude
  3. The aircraft descends in true altitude Correct answer.
  4. The aircraft climbs and then descends symmetrically

Explanation

In lower pressure the pressure surfaces descend, so maintaining the same indicated altitude means descending in true altitude. Pilots must update the subscale to avoid drifting below the intended profile, particularly near terrain.

Question 33Moisture, Humidity, Condensation, and Stability

In the standard atmosphere, what happens to the air temperature in the troposphere as altitude increases?

  1. It stays constant up to the tropopause.
  2. It decreases at a nearly constant lapse rate. Correct answer.
  3. It decreases only above the tropopause.
  4. It increases steadily with height.

Explanation

The troposphere is characterized by a general decrease of temperature with height, averaging about 2 degrees Celsius per 300 metres (roughly 6.5 degrees Celsius per kilometre). This average lapse rate reflects the typical vertical structure of the lower atmosphere in which weather occurs.

Question 34Moisture, Humidity, Condensation, and Stability

A parcel of air is described as saturated. What does this mean?

  1. It holds the maximum water vapour possible at its temperature. Correct answer.
  2. It contains no water vapour at all.
  3. Its temperature equals the surrounding air temperature.
  4. It has begun to precipitate as rain.

Explanation

Saturation means the air contains the maximum water vapour it can hold at a given temperature and pressure; relative humidity is then 100 percent. Any further cooling or added moisture leads to condensation.

Question 35Moisture, Humidity, Condensation, and Stability

What is the dew point temperature?

  1. The temperature of the air at the surface only.
  2. The temperature at which water freezes in cloud.
  3. The temperature to which air must be cooled to become saturated. Correct answer.
  4. The temperature at which relative humidity equals 50 percent.

Explanation

The dew point is the temperature to which a parcel must be cooled, at constant pressure and moisture content, for saturation to occur. When the dew point and temperature converge, relative humidity approaches 100 percent.

Question 36Moisture, Humidity, Condensation, and Stability

Which process directly produces condensation in the atmosphere?

  1. Increasing wind speed at the surface.
  2. Lowering atmospheric pressure without cooling.
  3. Cooling air to its dew point. Correct answer.
  4. Heating air above its dew point.

Explanation

Condensation occurs when air is cooled to saturation, allowing water vapour to change to liquid on condensation nuclei. Heating or drying the air moves it away from saturation.

Question 37Moisture, Humidity, Condensation, and Stability

What is the dry adiabatic lapse rate used for an unsaturated rising parcel?

  1. About 1 degree Celsius per 300 metres.
  2. About 3 degrees Celsius per 300 metres. Correct answer.
  3. About 2 degrees Celsius per 300 metres.
  4. About 6 degrees Celsius per 300 metres.

Explanation

An unsaturated parcel cools at the dry adiabatic rate of approximately 3 degrees Celsius per 300 metres (about 10 degrees Celsius per kilometre) as it rises. This rate applies until the parcel reaches saturation.

Question 38Moisture, Humidity, Condensation, and Stability

When a parcel becomes saturated and continues to rise, which lapse rate applies?

  1. The saturated adiabatic lapse rate. Correct answer.
  2. The dry adiabatic lapse rate.
  3. The environmental lapse rate exactly.
  4. A rate of zero.

Explanation

Once saturated, latent heat released by condensation slows the cooling, so the parcel cools at the saturated adiabatic lapse rate, which is smaller than the dry rate. The exact value varies with temperature and pressure.

Question 39Moisture, Humidity, Condensation, and Stability

A parcel of air at 20 degrees Celsius has a dew point of 12 degrees Celsius. Using roughly 2 degrees Celsius per 300 metres for the dew point lapse, at approximately what height will the lifting condensation level occur?

  1. About 2400 metres
  2. About 1200 metres Correct answer.
  3. About 300 metres
  4. About 6000 metres

Explanation

The temperature-dew point spread is 8 degrees Celsius. The parcel cools near 3 degrees per 300 metres while the dew point falls near 0.5 degrees per 300 metres, so the gap closes at roughly 1200 metres.

Question 40Moisture, Humidity, Condensation, and Stability

What is the relative humidity of air whose temperature is 20 degrees Celsius and whose dew point is 20 degrees Celsius?

  1. 20 percent
  2. 50 percent
  3. 100 percent Correct answer.
  4. 0 percent

Explanation

When the air temperature equals the dew point, the air is saturated and relative humidity is 100 percent. Any cooling from that state produces condensation.

Question 41Moisture, Humidity, Condensation, and Stability

Air at 25 degrees Celsius with a dew point of 5 degrees Celsius is warmed to 35 degrees Celsius with no change in moisture content. What happens to its relative humidity?

  1. It decreases. Correct answer.
  2. It becomes 100 percent.
  3. It increases.
  4. It stays the same.

Explanation

Warming increases the air's capacity to hold water vapour while the actual moisture content is unchanged, so relative humidity falls. The dew point, which depends on moisture content, stays at 5 degrees Celsius.

Question 42Moisture, Humidity, Condensation, and Stability

What is the primary source of atmospheric water vapour?

  1. Combustion of aviation fuel at cruise altitude
  2. Photodissociation of ozone in the stratosphere
  3. Evaporation from oceans and moist land surfaces Correct answer.
  4. Melting of high-altitude snow and ice

Explanation

Most atmospheric moisture enters through evaporation from the oceans and from moist land surfaces, then is redistributed by vertical mixing and wind. This supply drives cloud formation and precipitation.

Question 43Moisture, Humidity, Condensation, and Stability

What is the environmental lapse rate?

  1. The rate at which a rising parcel cools when unsaturated
  2. The actual rate at which temperature changes with height in the surrounding atmosphere Correct answer.
  3. The rate at which a saturated parcel cools as it rises
  4. The rate at which the dew point falls with height

Explanation

The environmental lapse rate is the measured change of temperature with height in the ambient atmosphere, obtained from soundings. It is compared with parcel lapse rates when assessing stability.

Question 44Moisture, Humidity, Condensation, and Stability

Which condition indicates absolute instability for a rising parcel?

  1. The environmental lapse rate is zero with height.
  2. The environmental lapse rate is less than the saturated adiabatic lapse rate.
  3. The environmental lapse rate equals the saturated adiabatic lapse rate.
  4. The environmental lapse rate exceeds the dry adiabatic lapse rate. Correct answer.

Explanation

If the surrounding air cools faster with height than the dry adiabatic rate, a lifted parcel stays warmer than its environment whether saturated or not, giving absolute instability. Such steep lapse rates favor strong vertical development.

Question 45Moisture, Humidity, Condensation, and Stability

What characterizes conditionally unstable air?

  1. The environmental lapse rate exceeds the dry adiabatic rate.
  2. The parcel is always warmer than its surroundings.
  3. The environmental lapse rate lies between the dry and saturated adiabatic rates. Correct answer.
  4. The environmental lapse rate is below the saturated adiabatic rate.

Explanation

Conditional instability exists when the environmental lapse rate is between the saturated and dry adiabatic rates. In that case an unsaturated parcel is stable, but once it becomes saturated it can become unstable and rise freely.

Question 46Moisture, Humidity, Condensation, and Stability

During the daytime, surface heating typically does what to the stability of the lower atmosphere?

  1. It destabilizes the lower layers. Correct answer.
  2. It produces an isothermal layer near the ground.
  3. It has no effect on stability.
  4. It stabilizes the lower layers.

Explanation

Strong surface heating steepens the environmental lapse rate in the lower layers, making the air more unstable and favoring convective cloud and turbulence. Cooling of the surface at night has the opposite effect.

Question 47Moisture, Humidity, Condensation, and Stability

What does a small temperature-dew point spread near the surface usually indicate?

  1. A high cloud base and very dry air
  2. Strong upper-level winds
  3. A steep dry adiabatic lapse rate
  4. A low cloud base and high relative humidity Correct answer.

Explanation

When the temperature and dew point are close, the air is near saturation and only slight lifting or cooling is needed to form cloud, giving a low condensation level. This also implies high relative humidity.

Question 48Moisture, Humidity, Condensation, and Stability

Why does the saturated adiabatic lapse rate vary with temperature and pressure?

  1. The environmental lapse rate changes the parcel's cooling.
  2. The amount of latent heat released by condensation depends on temperature and pressure. Correct answer.
  3. The dew point lapse rate is constant at all levels.
  4. The dry adiabatic rate also varies in the same way.

Explanation

Warmer air holds more vapour, so condensing a given amount of moisture releases more latent heat, which offsets cooling more strongly. As a result the saturated adiabatic rate is smaller in warm air and approaches the dry rate in very cold air.

Question 49Moisture, Humidity, Condensation, and Stability

A parcel at 15 degrees Celsius is lifted from the surface where the dew point is 10 degrees Celsius. Assuming a dry adiabatic rate near 3 degrees per 300 metres and a dew point lapse near 0.5 degrees per 300 metres, approximately how high is the condensation level?

  1. About 600 metres Correct answer.
  2. About 300 metres
  3. About 3000 metres
  4. About 1500 metres

Explanation

The 5-degree spread closes at a combined rate of about 2.5 degrees per 300 metres, so saturation is reached near 600 metres. Above that level the parcel cools at the saturated adiabatic rate.

Question 50Moisture, Humidity, Condensation, and Stability

Which combination best favors deep convective cloud development?

  1. Isothermal layers throughout the troposphere and light winds
  2. Cold surface temperatures and a large dew point spread
  3. Steep lapse rate, abundant low-level moisture, and a trigger for lifting Correct answer.
  4. Strong low-level inversion, dry air, and sinking motion

Explanation

Deep convection requires instability to sustain rising motion, sufficient moisture to supply latent heat, and a mechanism such as heating or frontal lifting to start the ascent. All three must be present for strong vertical development.