PPL(A) 050 MeteorologyRevision notes and practice questions

PPL 050 MeteorologyFree sample, 10 questions

1. 1Which statement about the composition of the atmosphere is correct?

   1. Dry air is about 78 % nitrogen and 21 % oxygen, and water vapour varies from 0 to about 4 % by volume.
   2. Dry air is about 78 % oxygen and 21 % nitrogen, and water vapour is a constant 1 % by volume.
   3. The percentage of oxygen in dry air falls with height, which is the cause of hypoxia at altitude.
   4. Water vapour is a fixed 4 % of the air at all heights, so it plays no part in the weather.
   Show the answer

   Answer A. Dry air contains about 78 % nitrogen, 21 % oxygen and 1 % other gases, in proportions that stay the same to very high altitude. Water vapour is the variable part, from almost 0 to about 4 % by volume, and it makes the weather. Hypoxia comes from the fall in pressure, and therefore in the partial pressure of oxygen, not from a lower percentage of oxygen.

2. 2Which statements about altimeter settings are correct?
   1. With QFE set, the altimeter reads height above the aerodrome.
   2. With QNH set, the altimeter reads the aerodrome elevation when the aeroplane is on the ground.
   3. With 1013 hPa set, the altimeter reads a flight level, that is a pressure altitude.
   4. QNH is the pressure measured at the aerodrome reference level.

   1. 1 and 2 only.
   2. 2, 3 and 4.
   3. 1, 3 and 4.
   4. 1, 2 and 3.
   Show the answer

   Answer D. QFE is the pressure at aerodrome level, so with QFE set the altimeter reads zero on the ground and height above the aerodrome in flight. QNH is the QFE reduced to mean sea level with the standard atmosphere, so with QNH set the altimeter reads altitude, and elevation on the ground. With 1013 hPa set, it reads pressure altitude, expressed as a flight level. Statement 4 describes QFE, not QNH, which is the classic confusion.

3. 3Which statement about the geostrophic wind is correct?

   1. It results from the Coriolis force alone, which creates the wind from the rotation of the Earth.
   2. It results from a balance between the pressure gradient force and the Coriolis force, and blows parallel to straight isobars.
   3. It blows across the isobars from high to low pressure, because friction is at its strongest above 2 000 ft.
   4. It results from a balance between the pressure gradient force and the Coriolis force, and is deflected to the left in the northern hemisphere.
   Show the answer

   Answer B. The pressure gradient force is the only driving force: it pushes air from high to low pressure, perpendicular to the isobars. The Coriolis force, due to the rotation of the Earth, only deflects the moving air, to the right in the northern hemisphere and to the left in the southern hemisphere. Above the friction layer, with straight isobars, the two forces balance and the wind blows parallel to the isobars: this is the geostrophic wind. Coriolis never creates wind on its own.

4. 4On a summer afternoon at an aerodrome, the surface temperature is +22 °C and the dew point +14 °C. Cumulus is starting to form. Using the usual rule of thumb, at about what height above the aerodrome is the cumulus base?

   1. 2 000 ft
   2. 2 700 ft
   3. 3 200 ft
   4. 4 000 ft
   Show the answer

   Answer C. A rising unsaturated parcel cools at about 3 °C per 1 000 ft while its dew point falls at about 0.5 °C per 1 000 ft, so the spread closes at 2.5 °C per 1 000 ft, that is 400 ft per °C. The spread is 22 − 14 = 8 °C, so the base is 8 × 400 = 3 200 ft above the ground. 2 700 ft comes from dividing the spread by the dry adiabatic rate alone, which forgets that the dew point also falls as the parcel rises.

5. 5Precipitation at an aerodrome starts suddenly, is heavy for about 10 minutes, then stops abruptly and the sky clears between the cells. Which cloud is most likely producing it?

   1. Nimbostratus
   2. Altostratus
   3. Stratus
   4. Cumulonimbus
   Show the answer

   Answer D. Precipitation that starts and stops abruptly and changes quickly in intensity is a shower, which falls from cumuliform cloud: cumulus congestus or cumulonimbus. Nimbostratus gives continuous rain or snow over a wide area and altostratus lighter continuous precipitation. Stratus produces at most drizzle, very fine drops that reduce the visibility.

6. 6In winter, an air mass that has travelled from the subtropical Atlantic near the Azores reaches western France. What weather does it typically bring?

   1. Cumulus with showers and very good visibility between them.
   2. Low stratus, drizzle and poor visibility, with a risk of advection fog.
   3. Clear skies, very low temperatures and dry air.
   4. Hot, dry air with dust reducing the visibility.
   Show the answer

   Answer B. This is tropical maritime air: mild and very moist. Over the colder winter surface it is cooled from below, which makes it stable in the lower layers, so it brings stratus, drizzle, poor visibility and advection fog. Showers with good visibility are typical of polar maritime air, heated from below on its way south. Clear, very cold and dry weather comes with polar continental air from the east, and hot dusty air with tropical continental air from North Africa.

7. 7How does the air circulate around a depression at the surface in the northern hemisphere?

   1. Anticlockwise, crossing the isobars inwards towards the centre, where it rises.
   2. Clockwise, crossing the isobars inwards towards the centre, where it rises.
   3. Anticlockwise, crossing the isobars outwards from the centre, where it sinks.
   4. Clockwise, crossing the isobars outwards from the centre, where it sinks.
   Show the answer

   Answer A. In the northern hemisphere the wind blows anticlockwise around a low, with low pressure on the left when the wind is behind you (Buys-Ballot's law). Friction near the surface makes the wind cross the isobars towards the low pressure, so the air converges at the centre and is forced to rise, which gives cloud and precipitation. Around an anticyclone the circulation is clockwise and the surface air diverges outwards, fed by sinking air.

8. 8Moist air at sea level, temperature 15 °C, is forced over a mountain range by the wind. It becomes saturated at 3 000 ft, stays saturated and loses its water as precipitation up to the crest at 9 000 ft, then descends to sea level on the lee side. Using 3 °C per 1 000 ft for dry air and 1.5 °C per 1 000 ft for saturated air, what is the temperature at sea level on the lee side?

   1. 15 °C
   2. 24 °C
   3. 28.5 °C
   4. 10.5 °C
   Show the answer

   Answer B. Up to 3 000 ft the air cools at the dry rate: 15 − 3 × 3 = 6 °C. From 3 000 to 9 000 ft it cools at the saturated rate: 6 − 6 × 1.5 = −3 °C at the crest. Having lost its water, it descends at the dry rate: −3 + 9 × 3 = 24 °C. This is the föhn effect: the lee side is warmer and drier than the windward side at the same level, and 15 °C would only be found if no condensation took place.

9. 9In which of the following conditions is the risk of airframe icing greatest for a light aeroplane?

   1. In clear air below the cloud base, at an outside air temperature of −15 °C.
   2. In cirrus cloud, at an outside air temperature of −45 °C.
   3. In cumulus cloud, at an outside air temperature of −5 °C.
   4. In light rain below the cloud, at an outside air temperature of +5 °C.
   Show the answer

   Answer C. Structural icing needs two conditions at the same time: supercooled liquid water droplets and an airframe at or below 0 °C. Cumulus cloud just below 0 °C, roughly 0 to −10 °C, holds large and plentiful supercooled droplets, so the risk is high. Clear air holds no liquid water, cirrus at −45 °C is made of ice crystals that do not stick, and rain at +5 °C does not freeze on a normal airframe.

10. 10Read the following METAR:
    LFMN 151330Z 22012G24KT 180V250 8000 -SHRA FEW020 SCT035CB BKN080 22/16 Q1012 NOSIG
    What are the reported wind and visibility?

    1. Wind 220° true, 12 kt gusting 24 kt, direction varying between 180° and 250°; visibility 8 km.
    2. Wind 220° magnetic, 12 kt gusting 24 kt, direction varying between 180° and 250°; visibility 8 km.
    3. Wind 220° true, 12 kt gusting 24 kt, direction varying between 180° and 250°; visibility 800 m.
    4. Wind 220° true, 12 kt with gusts of 36 kt, direction varying between 180° and 250°; visibility 8 km.
    Show the answer

    Answer A. In a METAR the wind direction is given in degrees true, followed by the mean speed and, after G, the maximum gust speed: 22012G24KT means 220° at 12 kt with gusts to 24 kt, not 12 + 24 kt. The group 180V250 shows the direction varying between these two values. Visibility is in metres, so 8000 is 8 km; the magnetic wind is the one given by ATIS and ATC for take-off and landing.