Top 25 PPL Meteorology Questions (with EASA-Sourced Answers)

Meteorology rewards understanding and punishes memorising. Most of the paper hangs off half a dozen numbers and one idea — whether a parcel of air ends up warmer or colder than the air around it. These 25 questions cover the material that actually appears: the standard atmosphere, lapse rates and stability, cloud, fronts, icing, thunderstorms, fog, wind, and decoding a METAR and a TAF. Where a figure exists we give it; where a figure is a rule of thumb rather than a definition we say so, because on this subject the distractors are usually plausible numbers.

The atmosphere and the ICAO Standard Atmosphere

1. What is the ICAO Standard Atmosphere, and what are its values? A fixed model agreed so that altimeters, airspeed indicators and performance charts share one reference. It is not a forecast. At mean sea level: pressure 1013.25 hPa (29.92 inHg), temperature +15°C, density 1.225 kg/m³. Temperature falls at 1.98°C per 1000 ft — universally rounded to 2°C per 1000 ft — to the standard tropopause at 11 000 m (36 090 ft) and −56.5°C, then stays constant to 20 km. So ISA temperature below the tropopause is 15 − 2 × (thousands of feet): FL100 is −5°C, FL180 is −21°C. Subtract that from the actual OAT and you have your ISA deviation — the figure that drives true airspeed, performance and, in winter, how far your altimeter over-reads.

2. How do pressure and density change with height? Pressure falls roughly exponentially, so the same height change costs fewer hectopascals the higher you go: about 1 hPa per 27 ft near sea level, roughly 1 hPa per 50 ft at 18 000 ft — where pressure is about half its sea level value and half the atmosphere's mass is below you. Density falls too, but more slowly than pressure, because the temperature is falling as well: density = pressure ÷ (R × temperature), so colder air is denser for a given pressure and partly offsets the drop. At the tropopause pressure is down to about 22% of its sea level value while density is still about 30%. Density is what the wing, propeller and engine feel, and on a hot day the temperature term stops helping you. Hence density altitude: a hot, high day degrades take-off performance long before it troubles the altimeter.

3. Where is the real tropopause? The troposphere holds roughly three-quarters of the atmosphere's mass and almost all its water vapour, so essentially all weather is in it. The tropopause is where temperature stops falling — an isothermal cap that lids convection, which is why a cumulonimbus flattens into an anvil there. Above it the stratosphere is isothermal, then warms as ozone absorbs ultraviolet. It is not at one height: highest and coldest over the equator (roughly 53 000–56 000 ft, around −75°C), lowest and comparatively warmest over the poles (roughly 25 000–28 000 ft, around −45°C), with the ISA figure representing mid-latitudes. Higher in summer than winter. The point examiners like: a higher tropopause is a colder one.

Lapse rates and atmospheric stability

Get these four figures exactly right and a third of the paper answers itself.

4. Define the ELR, the DALR and the SALR, give their values, and add the dew point lapse rate.

Of the three temperature lapse rates the SALR is the only one genuinely approximate. In warm, very moist air more vapour condenses per 1000 ft and it can fall to about 1.2°C per 1000 ft; in very cold air there is almost nothing left to condense, so it approaches the DALR.

5. How do those rates decide stability? Compare the ELR with the adiabatic rates, because that settles whether a displaced parcel ends up warmer and less dense than its surroundings and keeps going, or colder and denser and sinks back. ELR less than the SALR: a rising parcel is always colder and denser, and sinks back — absolutely stable. ELR greater than the DALR: always warmer and lighter, so it keeps going — absolutely unstable. ELR between the two: conditionally unstable — stable while the parcel is dry and cooling at 3°C per 1000 ft, unstable the moment it saturates and switches to the SALR. That third case — conditional instability, which the ATPL paper takes a good deal further — is the normal state of the atmosphere, and it is why a benign morning becomes a cumulonimbus afternoon. Inversions and isothermal layers are strongly stable and act as lids.

6. What does stability look like out of the window? Stable air: layered stratiform cloud, continuous rain or drizzle rather than showers, smooth flying, steady wind — and poor visibility, because with no vertical mixing the smoke, dust and moisture stay trapped in the lowest few thousand feet. Fog and mist are stable-air phenomena. Unstable air: heaped cumuliform cloud, showers, turbulence, a gusty variable surface wind — and good visibility between showers, because convection scours the low levels clean. Learn the visibility pairing deliberately; it is the item most students answer backwards.

7. How do you estimate convective cloud base from temperature and dew point? A rising unsaturated parcel cools at 3°C per 1000 ft while its dew point falls at only about 0.5°C per 1000 ft — the two adiabatic figures from the table above. They converge at about 2.5°C per 1000 ft, and cloud forms where they meet: base (ft above the surface) = (T − T_d) × 400. Temperature 20°C, dew point 9°C gives a spread of 11 and a base of about 4400 ft. It applies to convective cloud rising off a heated surface in a well-mixed layer, not to frontal or orographic cloud.

Cloud, air masses and fronts

8. Name the ten cloud genera and their height bands. Three high, three medium, four low. In temperate latitudes: high, roughly 16 500 to 45 000 ft, all ice crystals — cirrus, cirrocumulus, cirrostratus. Medium, roughly 6500 to 23 000 ft — altocumulus, altostratus, nimbostratus (nimbostratus is classed medium but normally extends well above and below). Low, surface to 6500 ft — stratus, stratocumulus, cumulus, cumulonimbus; the last two are classified by their base, which is low, even though they tower through all three levels. Bands are lower towards the poles, higher in the tropics. The names give the mechanism: strato- layer, from widespread gentle lift in stable air; cumulo- heap, from convection in unstable air; nimbo- or -nimbus means it is precipitating; alto- medium; cirro- high and made of ice.

9. What are the ways air is lifted to form cloud? Convection from surface heating; orographic lift over terrain; frontal lift of one air mass over another; turbulence and mechanical mixing, friction stirring the lowest layer to give stratocumulus with a flat top at the top of the mixed layer; and convergence into a low or a sea breeze front. A sixth mechanism produces fog rather than cloud: contact cooling of air already touching a cold surface. Which mechanism is at work predicts cloud type better than any other rule — gentle widespread lift in stable air gives stratiform, vigorous local lift in unstable air gives cumuliform.

10. How is an air mass classified, and what does each bring to Europe? By source region — Arctic, Polar, Tropical, Equatorial — and by the surface it formed over, maritime (m) or continental (c). What matters in flight is what happens on its track: air colder than the surface it crosses is heated from below and becomes unstable; air warmer than the surface is cooled from below and becomes stable.

11. Contrast a warm front with a cold front. A warm front is warm air rising over retreating colder air on a shallow slope, of the order of 1:150. The weather therefore arrives hundreds of nautical miles ahead of the surface front and lasts hours: cirrus, cirrostratus (the halo), altostratus, then nimbostratus with continuous rain, low stratus and falling visibility. Hazards: prolonged IMC, a low base, severe icing in the nimbostratus above the frontal surface, and in winter freezing rain below it — in the wedge of sub-zero air ahead of the surface front, where rain falling out of the overrunning warm air arrives supercooled. That wedge is exactly the level a light aeroplane flies to stay below the cloud; see question 16. A cold front is cold air undercutting warm air on a much steeper slope, of the order of 1:50, so its weather is narrow and violent: towering cumulus and cumulonimbus, heavy showers, hail, a squally wind shift, then rapid clearance into cold unstable showery air with excellent visibility. Cold fronts move faster. Learn the signature too: pressure falls ahead of the warm front, steadies in the warm sector, rises sharply behind the cold front, and in the northern hemisphere the surface wind veers at each front.

12. What is an occlusion? The cold front catches the warm front and lifts the whole warm sector clear of the surface. Which type it is depends on how the air behind the cold front compares with the air ahead of the warm front — not with the warm sector. If the air behind is the colder of the two it undercuts: a cold occlusion. If it is the less cold it rides over: a warm occlusion. Either way you get a broad band of layered cloud and precipitation with embedded convection, usually the most persistent and least flyable part of a depression.

Airframe icing: types, temperatures and escape

13. What two conditions must both be met before airframe ice forms? Visible supercooled liquid water, and an airframe surface at or below 0°C. Both, together. Cloud at −30°C made of ice crystals gives negligible icing because the crystals bounce off; clear air at −10°C gives none because there is no water. Most airframe icing therefore occurs between 0 and −20°C. Between −20 and −40°C the liquid water content is small, and below about −40°C water cannot stay liquid at all. It is the airframe temperature that counts, not the OAT: a cold-soaked aeroplane descending into damp air can pick up ice with the OAT above zero.

14. Distinguish rime ice from clear (glaze) ice. The driver is droplet size and how fast the droplet can shed its latent heat of fusion. Rime: small droplets freezing almost on impact; trapped air makes it white, opaque and brittle, and it builds forward along the leading edge in roughly the aerofoil's shape. Typical of stratiform cloud. Low density and relatively easy to shed, but it blocks intakes, vents and pitot heads. Clear or glaze: large droplets that do not freeze instantly, so part of the water runs back before freezing. It spreads over a wide area of the wing and can form horns behind a protected surface — transparent, dense, hard, firmly attached, and far worse for weight, drag and shape. Typical of cumuliform cloud, and produced in its worst form by freezing rain. The conventional exam bands are clear ice roughly 0 to −10°C, mixed −10 to −20°C and rime −20 to −40°C — but read those as consequences of droplet size, not hard boundaries. Rime forms perfectly happily just below 0°C in fine stratiform cloud.

15. What is hoar frost, and why does a thin layer matter? Hoar frost is not frozen dew. It is deposition — water vapour turning directly into ice crystals, skipping the liquid phase entirely, on a surface below 0°C and below the frost point of the air touching it. You meet it twice: on an aeroplane parked out under a clear night sky, and in flight when a cold-soaked aeroplane descends quickly into warmer, moister air, where it forms on the outside of the windscreen. It barely changes the aerofoil's shape, which is exactly why it is underestimated. What it changes is surface roughness, and roughness on the upper surface disrupts the boundary layer: lift falls, drag rises, and the wing stalls at a lower angle of attack and a higher speed, with less warning. NCO.OP.180 (Part-NCO, Ice and other contaminants — ground procedures) allows the pilot-in-command to commence take-off only when the aeroplane is clear of any deposit — frost, ice, snow or slush — that might adversely affect its performance or controllability, except as permitted in the aircraft flight manual. Remove all of it, including the tailplane — which you cannot see from the cockpit and which is more sensitive to contamination than the wing. See principles of flight for why.

16. Why is freezing rain the worst of the lot? Freezing rain is supercooled rain: liquid drops below 0°C that freeze on contact with your airframe. It needs a layer above 0°C sitting on top of a layer below 0°C — classically, rain falling out of warm air overrunning colder air ahead of a warm front. Raindrops are enormous next to cloud droplets, so accretion is fast, clear and over a huge area, including places no ice protection reaches. Ice pellets (PL) are the same structure with the drops frozen through on the way down, and are a direct warning that freezing rain exists somewhere in that column. The escape logic follows from the structure. The warm air is above you, but a light aeroplane already loaded with clear ice usually has nothing left with which to climb into it. The first choice is a 180° turn back into the air you have just flown through, because that is the only air you have any evidence about. Descending helps only in the less common structure where the sub-zero layer is shallow and aloft with above-freezing air beneath it; in the classic warm-front case that cold layer reaches the ground — which is why the drops freeze on contact with the ground as well — so descending takes you deeper into the accretion, not out of it. Check the surface temperature and the freezing level before you treat descent as an option at all. For a non-de-iced aeroplane the real answer is not to be there, and that decision is made on the ground, from the freezing level and the forecast, not in the ice.

Thunderstorms and the cumulonimbus life cycle

17. What does a thunderstorm need, and what are the three stages of a cumulonimbus? Three ingredients: sufficient moisture through a deep layer, an unstable — at least conditionally unstable — lapse rate, and a trigger to start the lift (surface heating, terrain, a front, or convergence). Remove any one and there is no thunderstorm. The life cycle:

1. Cumulus, or developing, stage. Updraught throughout the cell, cloud growing fast, no precipitation yet reaching the ground.
2. Mature stage. Precipitation begins to fall and drags air down with it, so updraught and downdraught sit side by side in the same cloud, and the top spreads into the anvil at the tropopause. Every hazard is present: severe turbulence, severe clear icing in the updraught, hail — which can be thrown out of the anvil into apparently clear air — lightning, torrential rain, and at the surface a gust front and the risk of a microburst.
3. Dissipating stage. The cold outflow spreads out at the surface and cuts the cell off from its own warm inflow. Downdraughts dominate and the cloud spreads into layer.

A single air-mass cell runs the whole cycle in roughly half an hour to an hour. A squall line or supercell — organised convection, worked through in detail at ATPL level — lasts far longer because its updraught is tilted by wind shear and so is never cut off by its own downdraught — which is why strong shear makes storms more dangerous, not less.

18. How much room do you give a cumulonimbus? Airline practice is to avoid one by 20 nm; 10 nm is the minimum commonly taught in light aviation. Neither is a regulation — both are the distance at which hail and turbulence outside the visible cloud stop being likely, and the visible cloud is not the hazard boundary: hail is thrown downwind from the anvil into clear air, the gust front runs out ahead of the storm at the surface, and severe turbulence extends well past the cloud edge. Do not try to overfly one in a light aeroplane — you cannot get above it. Underneath is worse still, because the entire downdraught, gust front and precipitation core sits between you and the ground at the lowest terrain clearance you will have all flight. A microburst, a concentrated downdraught typically under about 2 nm across lasting perhaps 5 to 15 minutes, reverses a headwind into a tailwind in seconds, and a light aeroplane on approach has no performance margin for that.

Visibility and the fog types

19. Define fog, mist and haze, and give the figures. Fog is obscuration by suspended water droplets or ice crystals reducing visibility to less than 1000 m. Mist (BR in a METAR) is the same droplets with visibility of 1000 m or more, up to 5000 m, at a relative humidity generally 95% or above. Haze (HZ) is reduction by dry particles — dust, smoke, salt, pollution — at any humidity, which is why it does not clear when the air warms and why it is worst under a subsidence inversion in a high. Two extras: freezing fog (FZFG) is fog below 0°C made of supercooled droplets, which deposits rime on your parked aeroplane and your airframe; and the visibility you get looking down the slope on approach is not the horizontal visibility measured at eye level — in shallow fog you can have the runway in sight from the descent and lose it in the flare.

20. What are the main fog types, and how does each form?

21. Why does a clear, calm autumn evening deserve more respect than a cloudy one? Because every driver of radiation fog is present at once. A clear sky means unrestricted cooling; the long night gives it time; a warm damp day has loaded the air with moisture; and the anticyclone supplying the clear sky also supplies the light wind and the subsidence inversion that caps the layer and stops anything mixing out. Watch three things through the afternoon: the temperature/dew point spread closing, the forecast overnight minimum against the dew point, and the wind forecast. If the minimum is forecast at or below the dew point with 2 to 8 kt of wind, expect fog. Note that it forms behind you as readily as ahead: a departure aerodrome can go below minima in twenty minutes while the destination stays clear, so a diversion plan that assumes you can turn round is worth nothing. If you do end up in cloud, the danger is not the aeroplane but your vestibular system — see spatial disorientation and visual illusions.

Wind: from the pressure gradient to the runway

22. Build the surface wind from first principles. The pressure gradient force starts everything: it acts from high pressure towards low, at right angles to the isobars, and its strength depends on the spacing — close isobars, strong wind. Coriolis is the apparent deflection from the earth's rotation, acting at 90° to the direction of motion, to the right in the northern hemisphere and the left in the southern. It is proportional to wind speed and to the sine of the latitude, so it is zero at the equator and greatest at the poles, and it changes direction only, never speed.

The geostrophic wind is those two in balance, for straight isobars above the friction layer (roughly 2000 ft agl). Equal and opposite, they leave the wind blowing parallel to the isobars — which gives Buys Ballot's law: in the northern hemisphere, back to the wind, low pressure on your left. The gradient wind is the same balance with curved isobars, where a centripetal component is also needed; for the same spacing it is weaker around a low and stronger around a high than the geostrophic value.

The surface wind is the gradient wind degraded by friction. Friction slows the air; slower air means weaker Coriolis; the pressure gradient then wins slightly and turns the wind towards the low. In the northern hemisphere the surface wind is therefore backed and reduced relative to the wind at 2000 ft — typically backed about 30° and cut to roughly half over land, and about 10° and cut to roughly two-thirds over the smoother sea. Turn that round for the case you fly: climbing out of the friction layer the wind veers and increases; descending, it backs and decreases. That is why the wind on final is not the wind you had at 2000 ft, and why gusts — faster air brought down from above — tend to veer as well as strengthen.

23. Explain sea and land breezes, and anabatic and katabatic winds. A sea breeze is a daytime effect: land heats far faster than water, air rises over the land, surface pressure there falls slightly, and air flows in from sea to land. It typically starts late morning, peaks mid-afternoon, reaches perhaps 10 to 15 kt and pushes some tens of nautical miles inland; in the northern hemisphere Coriolis makes it veer through the day. The sea breeze front is a convergence line that grows its own row of cumulus and can swing a coastal aerodrome's surface wind through 180° in minutes. The land breeze is the overnight reverse, land to sea, and markedly weaker — a few knots — because the overnight temperature difference is smaller and the cooled layer shallower. An anabatic wind is an upslope wind: sun warms a slope, the air touching it becomes warmer and less dense than air at the same level over the valley, and flows up the hillside. Usually gentle. A katabatic wind is the downslope one — "katabatic comes down". At night the slope radiates its heat away, the air in contact cools and becomes denser than air at the same level over the valley, and drains downhill under gravity. It is generally the stronger of the two, it pools cold air, frost and fog on the valley floor, and where cold air drains off an ice cap or funnels through a gap it can become genuinely violent.

Decoding a METAR and a TAF

Two questions worked group by group. Decoding is pure method rather than judgement, which makes these the most reliable marks anywhere in the nine-subject PPL theory set.

24. Decode this METAR.

METAR EGKA 121350Z 24012G25KT 210V270 3500 -RA BR SCT008 BKN012 OVC020 09/08 Q1003 TEMPO 2000 RADZ BKN006=

CAVOK replaces the visibility, weather and cloud groups entirely, and requires all of: visibility 10 km or more; no cloud below 5000 ft or below the highest minimum sector altitude, whichever is greater, and no CB or TCU at any level; and no significant weather. A stricter statement than most students assume.

25. Decode this TAF, and say what BECMG, TEMPO, PROB30 and FM mean.

TAF EGKA 121100Z 1212/1312 24012KT 9999 SCT025 TEMPO 1214/1220 4000 SHRA BKN012 BECMG 1301/1303 20008KT PROB30 1303/1308 0800 FG BKN002=

• 121100Z 1212/1312 — issued the 12th at 1100 UTC, valid 1200 UTC on the 12th to 1200 UTC on the 13th. Routine TAFs are issued for 9, 24 or 30 hours.
• 24012KT 9999 SCT025 — the prevailing forecast: 240° at 12 kt, 10 km or more, scattered 2500 ft.
• TEMPO 1214/1220 4000 SHRA BKN012 — temporary fluctuations between 1400 and 2000 on the 12th: 4000 m in rain showers, broken 1200 ft. TEMPO means each occurrence lasts less than one hour and that together they cover less than half the stated period. It does not mean the condition holds throughout.
• BECMG 1301/1303 20008KT — a permanent change reaching the new value at some unspecified point between 0100 and 0300 on the 13th, and holding after. The change period should not normally exceed two hours, which is why the forecaster has written a two-hour window here rather than running it back to midnight.
• PROB30 1303/1308 0800 FG BKN002 — a 30% probability, between 0300 and 0800 on the 13th, of 800 m in fog with broken cloud at 200 ft. Only PROB30 and PROB40 are used, and PROB is never combined with BECMG.
• FM — not in this example, but FM131000 marks a rapid and complete change from 1000 UTC on the 13th: everything after it replaces everything before it.

Now read that TAF as one situation rather than five facts. A showery afternoon wets the ground; the TEMPO ends at 2000, leaving only the prevailing SCT025; overnight the wind drops from 12 kt to 8 kt and backs, and 8 kt is the top of the 2 to 8 kt radiation fog window from question 20. Saturated surface, scattered cloud, light wind, long night — that is the recipe from question 21. The cloud that does remain is precisely why the forecaster has hedged the result as PROB30 at dawn rather than committing to TEMPO.