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NCAA CPL Meteorology study materials
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NCAA CPL Meteorology practice questions

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Question 1The atmosphere, temperature, pressure, density, standard atmosphere and altimetry

Under the ICAO International Standard Atmosphere at mean sea level, what are the standard temperature and pressure?

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

Explanation

The ICAO International Standard Atmosphere sets mean sea level at 15°C and 1013.25 hPa, with a standard density of 1.225 kg/m³. These reference values let pilots compare actual conditions with a common baseline.

Question 2The atmosphere, temperature, pressure, density, standard atmosphere and altimetry

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

  1. oxygen (~21%)
  2. nitrogen (~78%) Correct answer.
  3. argon (~0.9%)
  4. carbon dioxide (~0.04%)

Explanation

Nitrogen accounts for about 78% of dry air by volume, while oxygen is about 21%. Argon and carbon dioxide are present in much smaller amounts.

Question 3The atmosphere, temperature, pressure, density, standard atmosphere and altimetry

In the troposphere of the International Standard Atmosphere, temperature decreases with height at an average lapse rate of approximately:

  1. 1.98°C per km
  2. 6.5°C per km Correct answer.
  3. 3.0°C per km
  4. 9.8°C per km

Explanation

In the ISA troposphere, temperature falls at 6.5°C per kilometre, equivalent to about 1.98°C per 1,000 ft. This lapse rate applies up to the tropopause, where the temperature stops decreasing.

Question 4The atmosphere, temperature, pressure, density, standard atmosphere and altimetry

An aircraft is parked at an airport with elevation 1,200 ft AMSL. The altimeter is set to the local QNH. What should the altimeter indicate?

  1. pressure altitude of the field
  2. height above the field
  3. 0 ft
  4. 1,200 ft Correct answer.

Explanation

With QNH set, the altimeter is referenced to mean sea level, so on the ground it should read the surveyed field elevation. QFE would read zero or height above the field, while 1013.25 hPa would give pressure altitude.

Question 5The atmosphere, temperature, pressure, density, standard atmosphere and altimetry

Which conditions produce the highest density altitude at an airport?

  1. Warm, dry air at a low-elevation airport
  2. Warm, humid air at a high-elevation airport Correct answer.
  3. Cold, humid air at a low-elevation airport during winter
  4. Cold, dry air at a low-elevation airport

Explanation

High elevation, high temperature, and high humidity all lower air density, so they increase density altitude. Warm, humid air at a high-elevation airport therefore gives the highest density altitude of the choices.

Question 6The atmosphere, temperature, pressure, density, standard atmosphere and altimetry

An aircraft maintains a constant indicated altitude while flying from an area of high sea-level pressure toward an area of lower sea-level pressure, without changing the altimeter setting. What happens to true altitude?

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

Explanation

When pressure decreases along the route and the altimeter setting is not updated, the altimeter interprets the lower pressure as a higher altitude. To hold a constant indicated altitude, the aircraft must descend, so its true altitude decreases.

Question 7The atmosphere, temperature, pressure, density, standard atmosphere and altimetry

Which statement best explains why temperature generally increases with height in the stratosphere?

  1. Ozone absorbs UV radiation and warms the layer Correct answer.
  2. Water vapor absorbs solar ultraviolet radiation
  3. Pressure increases rapidly with height
  4. Convection transports warm air upward

Explanation

Ozone in the stratosphere absorbs solar ultraviolet radiation, converting that energy into heat. This absorption produces the temperature increase with height that characterises the stratosphere.

Question 8The atmosphere, temperature, pressure, density, standard atmosphere and altimetry

A temperature inversion is best described as a layer in which:

  1. temperature decreases more rapidly than 6.5°C/km
  2. temperature remains constant with height only
  3. temperature increases with height Correct answer.
  4. temperature decreases with height at the standard lapse rate

Explanation

A temperature inversion is a layer where temperature increases with height instead of decreasing. Inversions are very stable and can trap moisture, haze, and pollutants beneath them.

Question 9The atmosphere, temperature, pressure, density, standard atmosphere and altimetry

An airport has a surveyed elevation of 2,000 ft AMSL and the current altimeter setting is 30.12 inHg. Using the approximate rule of 1,000 ft per 0.01 inHg, what is the pressure altitude?

  1. 2,000 ft
  2. 3,200 ft
  3. 2,200 ft
  4. 1,800 ft Correct answer.

Explanation

Pressure altitude is found by correcting field elevation for the difference between the current altimeter setting and 29.92 inHg. Here, 30.12 is 0.20 inHg above standard, so subtract about 200 ft from 2,000 ft to get 1,800 ft.

Question 10The atmosphere, temperature, pressure, density, standard atmosphere and altimetry

At an airport with pressure altitude 5,000 ft, the outside air temperature is 25°C. Using the standard atmosphere and the approximation that density altitude increases about 120 ft per 1°C above ISA temperature, what is the approximate density altitude?

  1. 7,400 ft Correct answer.
  2. 2,600 ft
  3. 9,800 ft
  4. 5,000 ft

Explanation

At 5,000 ft the ISA temperature is about 5°C, so 25°C is 20°C above standard. Using the 120 ft per °C rule, add 2,400 ft to the 5,000 ft pressure altitude to get about 7,400 ft.

Question 11The atmosphere, temperature, pressure, density, standard atmosphere and altimetry

Which atmospheric constituent is highly variable in concentration and can significantly lower air density when present in large amounts?

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

Explanation

Water vapor is the most variable major constituent of the lower atmosphere. Because a water molecule is lighter than the average dry-air molecule, replacing dry air with water vapor lowers density at the same temperature and pressure. Its concentration changes greatly with temperature and weather.

Question 12The atmosphere, temperature, pressure, density, standard atmosphere and altimetry

An altimeter is set to 29.92 inHg (1013.25 hPa). The altitude it displays is called:

  1. Pressure altitude Correct answer.
  2. Density altitude
  3. Absolute altitude
  4. True altitude

Explanation

Setting the altimeter to 29.92 inHg or 1013.25 hPa references all readings to the standard sea-level pressure. The resulting height above that standard pressure datum is pressure altitude. It is used for flight levels and performance calculations.

Question 13The atmosphere, temperature, pressure, density, standard atmosphere and altimetry

If the temperature of a sample of air increases while its pressure remains constant, what happens to its density?

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

Explanation

For a fixed mass of air, the ideal gas law relates pressure, density and temperature. At constant pressure, raising temperature makes the gas expand, so the same mass occupies more volume. Therefore density decreases.

Question 14The atmosphere, temperature, pressure, density, standard atmosphere and altimetry

The tropopause is best described as:

  1. The level at which atmospheric pressure is exactly half of its sea-level value
  2. The boundary between the troposphere and the stratosphere, where the lapse rate changes Correct answer.
  3. The boundary between the stratosphere and the mesosphere
  4. The altitude at which the ozone concentration reaches its maximum

Explanation

The tropopause is the boundary between the troposphere and the stratosphere. In the troposphere temperature generally falls with height, while above the tropopause it becomes nearly constant or begins to rise. This change in lapse rate marks the tropopause.

Question 15The atmosphere, temperature, pressure, density, standard atmosphere and altimetry

Why does atmospheric pressure decrease as altitude increases?

  1. The air temperature always falls with height.
  2. There is less mass of overlying air above the point. Correct answer.
  3. Gravity increases with height, compressing the air.
  4. The proportion of oxygen in the air decreases with height.

Explanation

Atmospheric pressure is the weight per unit area of the air above a point. As altitude increases, there is less overlying mass, so pressure decreases. The decrease is rapid near the surface and becomes more gradual aloft.

Question 16The atmosphere, temperature, pressure, density, standard atmosphere and altimetry

In the mesosphere, temperature generally decreases with height primarily because:

  1. Convection is stronger there than in the troposphere.
  2. There is little ozone to absorb ultraviolet radiation, and carbon dioxide radiates heat to space. Correct answer.
  3. The mesosphere contains most of the atmosphere's water vapor.
  4. The mesosphere is heated mainly by conduction from the thermosphere above.

Explanation

The mesosphere contains very little ozone, so it absorbs little direct solar ultraviolet radiation. Carbon dioxide in the mesosphere radiates infrared energy to space, producing a net cooling with height. Hence temperature decreases with altitude through this layer.

Question 17The atmosphere, temperature, pressure, density, standard atmosphere and altimetry

An altimeter set to QFE at an airport will indicate what when the aircraft is on the ground at that airport?

  1. Pressure altitude
  2. Zero altitude Correct answer.
  3. Field elevation above mean sea level
  4. Density altitude

Explanation

QFE is the atmospheric pressure at the airport elevation. When the altimeter is set to QFE, it reads height above the airport reference point, so it indicates zero on the ground. QNH, by contrast, makes it read field elevation.

Question 18The atmosphere, temperature, pressure, density, standard atmosphere and altimetry

Density altitude is best defined as:

  1. The altitude in the standard atmosphere at which the air density equals the actual density Correct answer.
  2. The true height above mean sea level
  3. The altitude at which the air pressure equals 1013.25 hPa
  4. The indicated altitude when the altimeter is set to the local QNH

Explanation

Density altitude is the height in the International Standard Atmosphere where the air density equals the density at the point in question. It combines the effects of pressure altitude and temperature on aircraft performance. High density altitude means degraded performance.

Question 19The atmosphere, temperature, pressure, density, standard atmosphere and altimetry

An aircraft maintains a constant indicated altitude while flying from a warm air mass into a colder air mass at the same altimeter setting. What happens to the aircraft's true altitude?

  1. It remains equal to the indicated altitude.
  2. It becomes equal to the pressure altitude.
  3. It becomes higher than the indicated altitude.
  4. It becomes lower than the indicated altitude. Correct answer.

Explanation

Altimeters are calibrated to the standard atmosphere, in which pressure decreases at a fixed rate with height. In colder air, pressure decreases more rapidly with height, so a given pressure level lies closer to the ground. Maintaining the same indicated altitude therefore places the aircraft at a lower true altitude.

Question 20The atmosphere, temperature, pressure, density, standard atmosphere and altimetry

An airport has elevation 1,000 ft and current QNH 1005 hPa. An aircraft on the ground has its altimeter set to 1013 hPa. Approximately what does the altimeter indicate?

  1. About 1,000 ft
  2. About 1,240 ft Correct answer.
  3. About 1,300 ft
  4. About 760 ft

Explanation

The altimeter is set 8 hPa higher than the current QNH (1013 minus 1005). In the lower atmosphere, 1 hPa corresponds to roughly 30 ft. The altimeter therefore overreads by about 8 × 30 = 240 ft, indicating approximately 1,240 ft.

Question 21The atmosphere, temperature, pressure, density, standard atmosphere and altimetry

Which altimeter subscale setting causes the altimeter to indicate the surveyed elevation of an airport when the aircraft is parked on that airport's ramp?

  1. QNH Correct answer.
  2. QFE
  3. QFF
  4. QNE

Explanation

QNH is the aerodrome pressure reduced to mean sea level using the International Standard Atmosphere, so an altimeter set to QNH reads field elevation when the aircraft is on the ground. QFE is the actual station pressure at the aerodrome and reads zero on the ground, while QNE is the standard setting of 1013.25 hPa used for flight levels. QFF is mean sea-level pressure reduced using actual temperature and is not normally used as an aviation altimeter setting.

Question 22The atmosphere, temperature, pressure, density, standard atmosphere and altimetry

Using the International Standard Atmosphere tropospheric lapse rate, what is the standard temperature at a pressure altitude of 18,000 ft?

  1. −21°C Correct answer.
  2. +15°C
  3. −57°C
  4. −6°C

Explanation

Standard sea-level temperature is 15°C and the ISA tropospheric lapse rate is about 1.98°C per 1,000 ft, so the temperature at 18,000 ft is approximately 15 − (1.98 × 18) = −20.6°C, rounded to −21°C. The value −57°C is close to the standard tropopause temperature near 36,000 ft, not at 18,000 ft.

Question 23The atmosphere, temperature, pressure, density, standard atmosphere and altimetry

Which statement correctly distinguishes true altitude from absolute altitude?

  1. True altitude is indicated altitude corrected for instrument error; absolute altitude is indicated altitude corrected for temperature.
  2. True altitude is height above the terrain directly below; absolute altitude is height above mean sea level.
  3. True altitude is height above the 1013.25 hPa datum; absolute altitude is height above sea level.
  4. True altitude is height above mean sea level; absolute altitude is height above the terrain directly below. Correct answer.

Explanation

True altitude is the actual vertical distance of the aircraft above mean sea level, the datum used for terrain clearance and charted elevations. Absolute altitude is the height above the surface immediately beneath the aircraft, often called height above ground level. Pressure altitude, by contrast, is referenced to the 1013.25 hPa datum, and indicated altitude is what the altimeter shows with the current subscale setting.

Question 24The atmosphere, temperature, pressure, density, standard atmosphere and altimetry

An airport at elevation 3,000 ft AMSL reports QNH 1008 hPa. Using the approximate relationship of 1 hPa to 27 ft in the lower atmosphere, what is the QFE at the airport?

  1. About 940 hPa
  2. About 897 hPa Correct answer.
  3. About 970 hPa
  4. About 1008 hPa

Explanation

QFE is the station pressure at the aerodrome, so it is lower than QNH by the pressure equivalent of the field elevation. With 1 hPa approximating 27 ft, 3,000 ft corresponds to about 111 hPa, giving QFE ≈ 1008 − 111 = 897 hPa. Setting QFE on the ground makes the altimeter read zero at the aerodrome reference point.

Question 25Moisture, thermodynamics, stability and instability, clouds and precipitation

An air parcel with a constant water-vapor mixing ratio is warmed isobarically. How does its relative humidity change?

  1. It first rises, then falls after the dew point is reached.
  2. It increases because the parcel holds more moisture.
  3. It decreases because saturation vapor pressure rises with temperature. Correct answer.
  4. It stays the same because the mixing ratio is unchanged.

Explanation

Relative humidity compares actual vapor content with the saturation vapor pressure at the same temperature. Warming raises saturation vapor pressure while mixing ratio stays fixed, so relative humidity falls. No condensation occurs unless the parcel is cooled to its dew point.

Question 26Moisture, thermodynamics, stability and instability, clouds and precipitation

Which statement defines the dew point temperature of an air parcel?

  1. The temperature to which the parcel must cool at constant pressure to become saturated. Correct answer.
  2. The temperature at which every bit of water vapor in the parcel has condensed out.
  3. The temperature at which the parcel begins to follow the dry adiabatic lapse rate during lifting.
  4. The temperature at which relative humidity reaches 100 percent only if pressure rises.

Explanation

The dew point is defined by cooling at constant pressure with no change in moisture content. At that temperature the parcel reaches saturation and relative humidity is 100 percent. Condensation begins there, but not all vapor condenses.

Question 27Moisture, thermodynamics, stability and instability, clouds and precipitation

Condensation of water vapor in a rising cloud parcel affects the parcel mainly by doing what?

  1. Warming it through release of latent heat. Correct answer.
  2. Increasing its salinity and thereby raising its density.
  3. Removing all buoyancy by eliminating water vapor.
  4. Cooling it through absorption of latent heat.

Explanation

Condensation releases latent heat into the parcel, which offsets some of the adiabatic cooling caused by expansion. This latent heating is why the saturated adiabatic lapse rate is smaller than the dry adiabatic lapse rate. The parcel is not made warmer than its environment by condensation alone.

Question 28Moisture, thermodynamics, stability and instability, clouds and precipitation

A layer has an environmental lapse rate of 8°C per kilometer. The dry adiabatic lapse rate is 10°C per kilometer and the saturated adiabatic lapse rate is 6°C per kilometer. How is a saturated parcel displaced upward through this layer best classified?

  1. Conditionally unstable, since the environmental rate lies between the adiabatic rates. Correct answer.
  2. Absolutely unstable, since the environmental rate exceeds the saturated adiabatic rate.
  3. Absolutely stable, since the environmental rate is below the dry adiabatic rate.
  4. Neutral, since the environmental rate lies between the two adiabatic rates.

Explanation

For a saturated parcel, compare the environmental lapse rate with the saturated adiabatic lapse rate. Here 8°C/km exceeds 6°C/km, so the parcel remains warmer than its surroundings and is unstable. Because 8°C/km is below the 10°C/km dry rate, the instability is conditional rather than absolute.

Question 29Moisture, thermodynamics, stability and instability, clouds and precipitation

Surface air temperature is 25°C and the dew point is 15°C. Using the approximation that the lifted condensation level rises about 125 meters for each 1°C of dew-point depression, what is the approximate LCL height above the surface?

  1. 1,250 meters Correct answer.
  2. 1,000 meters
  3. 2,500 meters
  4. 500 meters

Explanation

The dew-point depression is the temperature minus the dew point, or 25°C minus 15°C, which equals 10°C. Multiplying 10°C by the 125-meter approximation gives about 1,250 meters. This is the height at which the lifted parcel cools to its dew point.

Question 30Moisture, thermodynamics, stability and instability, clouds and precipitation

In a mixed-phase cloud at temperatures between about -10°C and -20°C, ice crystals often grow at the expense of supercooled liquid droplets. What is this process called?

  1. The Bergeron-Findeisen process Correct answer.
  2. Aggregation of ice crystals into snowflakes
  3. Riming of ice particles by supercooled droplets
  4. Collision-coalescence between liquid droplets

Explanation

Because saturation vapor pressure is lower over ice than over supercooled water, vapor diffuses toward the ice crystals and away from the droplets. This Bergeron-Findeisen process allows ice crystals to grow rapidly in mixed-phase clouds. It is distinct from droplet collision and ice-particle collection.

Question 31Moisture, thermodynamics, stability and instability, clouds and precipitation

A sounding shows a large positive value of convective available potential energy (CAPE). What does this most directly imply?

  1. The environment strongly inhibits parcels from rising.
  2. Precipitation must be falling as snow rather than rain.
  3. A parcel lifted to its level of free convection can gain substantial upward buoyancy. Correct answer.
  4. The cloud layer is absolutely stable at all levels.

Explanation

CAPE integrates the positive buoyancy available to a parcel between its level of free convection and equilibrium level. A large positive CAPE therefore favors strong updrafts once the parcel reaches that layer. It does not by itself determine precipitation type or guarantee that convection will initiate.

Question 32Moisture, thermodynamics, stability and instability, clouds and precipitation

How does the saturated adiabatic lapse rate typically change as a saturated parcel rises into colder air?

  1. It becomes larger, approaching the dry adiabatic rate. Correct answer.
  2. It stays exactly equal to the dry adiabatic lapse rate.
  3. It becomes smaller because colder air holds more water vapor.
  4. It becomes negative because latent heating stops.

Explanation

Colder air has a lower saturation vapor pressure, so less water vapor condenses for each degree of cooling. Less latent heat is released, so the saturated adiabatic lapse rate increases and approaches the dry adiabatic lapse rate. It remains positive while saturation is maintained.

Question 33Moisture, thermodynamics, stability and instability, clouds and precipitation

Which form of potential temperature is approximately conserved when a saturated air parcel ascends pseudoadiabatically with condensate falling out?

  1. Mixing ratio
  2. Potential temperature
  3. Relative humidity
  4. Equivalent potential temperature Correct answer.

Explanation

Equivalent potential temperature combines temperature and moisture into a single thermodynamic variable. It is approximately conserved in moist adiabatic processes, including pseudoadiabatic ascent with precipitation fallout. It is useful for comparing air masses and tracking moist processes.

Question 34Moisture, thermodynamics, stability and instability, clouds and precipitation

A subsidence inversion forms aloft as air sinks and warms. How does this layer most directly affect convective development?

  1. It increases the environmental lapse rate within the inversion to the dry adiabatic value.
  2. It removes the level of free convection by cooling the boundary layer.
  3. It guarantees that any surface-based parcel will become absolutely unstable.
  4. It caps deep updrafts unless parcels can overcome the stable layer. Correct answer.

Explanation

Sinking air warms and creates a stable layer, often with temperature increasing with height through the inversion. This inversion can act as a cap that suppresses deep updrafts unless surface heating or moisture allows parcels to break through. It does not by itself make the boundary layer cooler or more unstable.

Question 35Moisture, thermodynamics, stability and instability, clouds and precipitation

Which temperature is defined as the lowest temperature to which air can be cooled by evaporating water into it at constant pressure?

  1. The wet-bulb temperature Correct answer.
  2. The frost point temperature
  3. The dry-bulb temperature
  4. The dew point temperature

Explanation

The wet-bulb temperature is the temperature air would reach if water evaporated into it until saturation, with latent heat drawn from the air. It is lower than the dry-bulb temperature unless the air is already saturated. Dew point is reached by cooling without adding moisture, and frost point is the subfreezing equivalent.

Question 36Moisture, thermodynamics, stability and instability, clouds and precipitation

Snowflakes fall through a layer where the temperature is above freezing, completely melt, and then pass through a subfreezing layer thick enough to refreeze before reaching the ground. Which precipitation type reaches the surface?

  1. Drizzle
  2. Hail
  3. Sleet Correct answer.
  4. Freezing rain

Explanation

Sleet consists of ice pellets that form when snowflakes melt in a warm layer aloft and then refreeze while falling through a subfreezing layer. If the drops do not refreeze before reaching the surface, they produce freezing rain instead.

Question 37Moisture, thermodynamics, stability and instability, clouds and precipitation

Which high-level cloud genus consists mostly of ice crystals and appears as thin, wispy streaks or hooks?

  1. Cirrus Correct answer.
  2. Stratocumulus
  3. Cumulus congestus
  4. Altostratus

Explanation

Cirrus clouds form at high altitudes where temperatures are cold enough for ice crystals, and they appear thin and wispy because they contain little water and are often sheared by strong winds. Altostratus and stratocumulus are mid- or low-level cloud types.

Question 38Moisture, thermodynamics, stability and instability, clouds and precipitation

Rain falls through a deep layer of unsaturated air. What is the primary reason the air near the ground cools during this process?

  1. Evaporation of raindrops absorbs latent heat from the air. Correct answer.
  2. Condensation releases latent heat into the air.
  3. The rain carries cold air down from the cloud base.
  4. The raindrops compress the air as they fall.

Explanation

Evaporation requires latent heat, which is supplied by the surrounding air, so rain falling through unsaturated air cools that air. This evaporative cooling can produce a strong downdraft and gust front at the surface.

Question 39Moisture, thermodynamics, stability and instability, clouds and precipitation

Which quantity is conserved when an unsaturated air parcel is lifted or lowered adiabatically and is used to compare parcels at a standard pressure?

  1. Equivalent potential temperature
  2. Relative humidity
  3. Potential temperature Correct answer.
  4. Dew point temperature

Explanation

Potential temperature is the temperature an air parcel would have if brought adiabatically to a reference pressure of 1000 hPa. It is conserved during dry adiabatic ascent or descent, unlike relative humidity and dew point, which change as pressure and temperature change.

Question 40Moisture, thermodynamics, stability and instability, clouds and precipitation

A growing cumulus cloud entrains dry environmental air through its sides. What is the most direct effect on the cloud's updraft and liquid water content?

  1. The liquid water content increases because of added moisture.
  2. The updraft strengthens because the entrained air is denser.
  3. The cloud becomes absolutely stable and stops growing.
  4. The updraft weakens and some cloud droplets evaporate. Correct answer.

Explanation

Entrainment mixes cooler, drier environmental air into the cloud. This evaporates some droplets, cools the parcel, reduces buoyancy, and can weaken the updraft.

Question 41Moisture, thermodynamics, stability and instability, clouds and precipitation

A sounding shows a parcel that must be lifted through a layer of negative buoyancy before reaching its level of free convection. What does this layer represent?

  1. Convective inhibition (CIN) Correct answer.
  2. The lifting condensation level (LCL)
  3. The equilibrium level (EL)
  4. Convective available potential energy (CAPE)

Explanation

The layer of negative buoyancy below the level of free convection is the convective inhibition (CIN). It represents the energy a parcel must overcome, often through surface heating or forced lifting, before it can become freely buoyant.

Question 42Moisture, thermodynamics, stability and instability, clouds and precipitation

Why is condensational growth alone usually insufficient to produce precipitation-sized droplets in a warm cloud?

  1. Condensation cannot occur on cloud condensation nuclei.
  2. Growth slows as the vapor-pressure difference driving condensation falls. Correct answer.
  3. Condensation quickly removes essentially all water vapor from the cloud.
  4. Droplets stop growing once they reach a radius of 1 micrometer.

Explanation

Condensational growth slows greatly as droplets enlarge because the difference between the ambient vapor pressure and the droplet's equilibrium vapor pressure shrinks. Collision-coalescence or ice processes are therefore needed to produce precipitation-sized particles efficiently.

Question 43Moisture, thermodynamics, stability and instability, clouds and precipitation

In a well-mixed unsaturated boundary layer, the dew point temperature typically decreases with height at approximately what rate?

  1. About 6°C per kilometer
  2. It remains constant with height
  3. About 1.8°C per kilometer Correct answer.
  4. About 10°C per kilometer

Explanation

In a well-mixed unsaturated layer, the mixing ratio is nearly constant with height. Because saturation vapor pressure decreases as air cools, the dew point falls much more slowly than temperature, typically at about 1.8°C per kilometer.

Question 44Moisture, thermodynamics, stability and instability, clouds and precipitation

Which set of conditions is most favorable for hail to grow to large size in a thunderstorm?

  1. A shallow warm cloud in which only collision-coalescence occurs
  2. A strong, persistent updraft with abundant supercooled water Correct answer.
  3. A stable layer with no vertical motion and very high relative humidity
  4. A weak updraft and a cloud composed entirely of small ice crystals

Explanation

Large hail requires a strong, persistent updraft that can suspend growing ice particles, abundant supercooled water for riming, and often repeated cycling through the updraft so the stone accumulates layers before falling.

Question 45Moisture, thermodynamics, stability and instability, clouds and precipitation

Which fog type forms when warm, moist air moves horizontally over a colder surface and is cooled to saturation mainly by that underlying surface?

  1. Radiation fog
  2. Upslope fog
  3. Advection fog Correct answer.
  4. Steam fog

Explanation

Advection fog forms when moist air is transported over a colder surface and turbulent exchange cools the air from below until it reaches saturation. Radiation fog instead requires radiative cooling of the ground on a clear, calm night, steam fog requires evaporation from warm water into cold air, and upslope fog requires adiabatic cooling during forced ascent.

Question 46Moisture, thermodynamics, stability and instability, clouds and precipitation

A snow crystal is growing by vapor deposition in a cloud where the temperature is about -15°C. Which crystal habit is most strongly favored at that temperature?

  1. Needles
  2. Thin hexagonal plates
  3. Dendritic crystals Correct answer.
  4. Solid columns

Explanation

Snow crystal habit varies with temperature because the surface kinetics of ice growth change, and branching dendrites are favored near -15°C where depositional growth is rapid. Needles tend to form near -5°C, while plates and columns dominate at other temperatures such as just below freezing or below about -20°C.

Question 47Moisture, thermodynamics, stability and instability, clouds and precipitation

Why is virtual temperature used instead of actual temperature when computing the buoyancy of a moist air parcel?

  1. It includes the weight of condensed water droplets and ice crystals in the parcel.
  2. It accounts for the lower density of moist air relative to dry air at the same temperature and pressure. Correct answer.
  3. It converts potential temperature into the actual temperature at the parcel's pressure.
  4. It adjusts the temperature for latent heat released when water vapor condenses.

Explanation

At a given temperature and pressure, water vapor is less dense than dry air, so moist air has a slightly lower density than dry air. Virtual temperature is the temperature that dry air would need to have the same density as the moist sample, allowing buoyancy calculations to use the ideal gas law directly. Condensate loading is treated separately, and latent heating changes the actual temperature rather than defining virtual temperature.

Question 48Moisture, thermodynamics, stability and instability, clouds and precipitation

A sounding shows equivalent potential temperature decreasing rapidly with height through a moist low-level layer. What does this pattern most directly indicate about that layer?

  1. The layer is absolutely stable for both saturated and unsaturated parcels.
  2. The layer is already saturated and has reached its equilibrium level.
  3. The layer has no conditional instability, so its CAPE must be zero.
  4. It is potentially unstable: lifting the layer can release convective instability. Correct answer.

Explanation

Potential, or convective, instability exists when equivalent potential temperature decreases with height, meaning the upper part of the layer has lower moist static energy than the lower part. If the whole layer is lifted until it saturates, the lower, higher-theta-e air cools at the moist adiabatic rate while the drier upper air cools at the dry adiabatic rate, steepening the lapse rate and allowing instability to develop. It does not by itself mean the layer is stable or already at equilibrium.

Question 49Wind: global and local circulation, geostrophic and gradient wind, wind shear

In the Northern Hemisphere surface circulation, what is the prevailing direction of the trade winds?

  1. From southeast toward northwest
  2. From northwest toward southeast
  3. From northeast toward southwest Correct answer.
  4. From southwest toward northeast

Explanation

In the Northern Hemisphere, surface trade winds are deflected to the right of the equatorward pressure-gradient force, giving a northeast-to-southwest flow. The Southern Hemisphere trades mirror this as southeast-to-northwest winds.

Question 50Wind: global and local circulation, geostrophic and gradient wind, wind shear

On a sunny afternoon along a coast, which surface wind direction is produced by a sea breeze?

  1. Along the coastline from north to south
  2. Downslope from the coastal hills
  3. From the land toward the sea
  4. From the sea toward the land Correct answer.

Explanation

During daytime solar heating raises land temperature faster than water temperature, so lower pressure forms over land. Air flows from the higher-pressure sea surface toward the land, creating the sea breeze.