CAAS PPL Meteorology Study Guide

Of all the PPL theory subjects, meteorology is the one that most directly keeps you alive. Aircraft do not fall out of clear, stable air; they get into trouble when a pilot misreads the weather, presses on into deteriorating conditions, or fails to understand why the wind, cloud and visibility behave the way they do. The aim of this subject is not to turn you into a forecaster but to give you a working mental model of the atmosphere so that you can interpret a forecast, anticipate how the day will develop, and make sound go or no-go decisions before and during a flight.

The examiner is therefore far more interested in understanding than in rote recall. A good meteorology question rarely asks "what is the name of this cloud"; it asks "given these conditions, what will happen next, and why". You will be expected to reason from cause to effect: warm moist air plus lifting gives cloud; a steep pressure gradient gives strong wind; a temperature inversion gives trapped haze and smooth, stable air. The handful of figures you genuinely must memorise are the International Standard Atmosphere values below. Almost everything else flows from a small number of physical principles applied consistently.

Across the syllabus the recurring themes are: the structure of the atmosphere and the ISA; the relationship between temperature, pressure, density and the altimeter; humidity and the dew point; how cloud and precipitation form; atmospheric stability and instability; air masses, fronts and pressure systems; the family of winds from the large scale down to local sea and valley breezes; visibility, fog and mist; the hazards bundled into a thunderstorm; and how to decode the standard weather reports and forecasts. This guide walks through each in a sensible teaching order, and throughout it flags the tropical considerations that make flying in Singapore subtly different from the mid-latitude examples in most textbooks.

The atmosphere is a thin shell of gas held to the Earth by gravity. From the surface upward it is divided into several layers, of which the lowest two matter for PPL flight: the troposphere, which extends to about 11 km (36,000 ft) at mid-latitudes — higher over the equator, lower over the poles — and contains essentially all weather; and the stratosphere above it, where temperature increases with altitude and the air is dry and stable.

Pilots use the International Standard Atmosphere (ISA) as a reference for performance calculations and altimeter behaviour. The ISA values are worth memorising verbatim because they are used in dozens of downstream calculations:

• Sea-level pressure: 1013.25 hPa (also written 29.92 inHg).
• Sea-level temperature: +15 °C (288.15 K).
• Sea-level density: 1.225 kg/m³.
• Temperature lapse rate in the troposphere: 1.98 °C per 1,000 ft, conventionally rounded to 2 °C per 1,000 ft.
• Tropopause: ~36,000 ft, temperature ~−56.5 °C, isothermal above.

The environmental lapse rate (ELR) is the actual change of temperature with height on a given day. Compared with the dry-adiabatic lapse rate (3 °C per 1,000 ft) and the saturated adiabatic lapse rate (about 1.5 °C per 1,000 ft, varying with temperature), the ELR tells you whether the atmosphere is stable or unstable. If a rising parcel cools more slowly than its environment it keeps rising — unstable conditions, conducive to cumulus development.

It is worth being clear about why pressure, temperature and density are bound together, because a large slice of the exam tests nothing more than this triangle. Atmospheric pressure at any level is simply the weight of all the air stacked above it, which is why pressure always falls with height. Density — the mass packed into a given volume — depends on both pressure and temperature: squeezing air harder (more pressure) packs the molecules closer, while heating it makes them spread out. The practical consequence for a pilot is that high pressure and low temperature give dense air, whereas low pressure and high temperature give thin air. Thin air degrades engine power, propeller thrust and wing lift, which is exactly why a hot, humid Singapore afternoon produces poor take-off performance and long ground rolls — the phenomenon you will meet again as "density altitude".

Near the surface, pressure falls by roughly 1 hPa for every 27 ft of height in ISA conditions. This single figure links the altimeter to the atmosphere and explains the altimetry rules discussed later: a 27 ft error for every hectopascal of mis-set subscale, and a column that has dropped from 1,000 hPa at the surface to roughly 970 hPa by 1,500 ft. Commit the ISA numbers and this 27 ft per hPa relationship to memory and a surprising number of questions answer themselves.

Water vapour is an invisible gas, and the amount of it that air can hold depends almost entirely on temperature: warm air can hold a great deal of vapour, cold air very little. This is the key to almost all cloud and fog. Relative humidity is the amount of vapour present expressed as a percentage of the maximum the air could hold at that temperature. Crucially, relative humidity can reach 100% in two ways — by adding moisture, or simply by cooling the air until its capacity shrinks to match what is already there.

The temperature at which a parcel of air becomes saturated, given its current moisture content, is the dew point. The gap between the air temperature and the dew point — the temperature/dew-point spread — is one of the most useful numbers a pilot can read off a METAR. A small spread means the air is close to saturation, so cloud, mist or fog is likely; a large spread means dry air and good visibility. When the spread closes to within about 2 °C, watch for fog forming. In the Singapore METAR earlier in this guide, 30/24 shows a 6 °C spread: humid, but not on the edge of fog.

Cloud forms when moist air is cooled to its dew point so that vapour condenses onto tiny particles (condensation nuclei) as visible water droplets. There are four broad ways air gets cooled enough to condense, and the exam expects you to recognise each:

• Orographic lifting — air forced to rise over high ground, cooling adiabatically as it climbs; cloud and rain pile up on the windward side.
• Convection — the sun heats the surface, the surface heats the air above it, and bubbles of warm air (thermals) rise and cool. This is the dominant process in the tropics and the engine behind Singapore's afternoon cumulus and thunderstorms.
• Frontal and convergence lifting — air forced upward at the boundary between two air masses, or where surface winds flow together and must rise.
• Turbulent / radiative cooling — mechanical mixing or overnight radiation cooling the air to its dew point, producing low stratus or fog rather than convective cloud.

Notice the common thread: every cloud-forming process is some way of lifting or cooling moist air to its dew point. If you can identify the lifting mechanism, you can usually predict the cloud type, and from the cloud type the likely flight hazards.

Stability describes whether a parcel of air, once nudged upward, tends to keep rising or to sink back. It is decided by comparing the environmental lapse rate (how fast the surrounding air actually cools with height) against the rate at which a rising parcel cools itself. A rising parcel cools by expansion at the dry adiabatic lapse rate (about 3 °C/1,000 ft) while it remains unsaturated, and at the slower saturated adiabatic lapse rate (about 1.5 °C/1,000 ft) once condensation begins, because the latent heat released by condensing vapour partly offsets the cooling.

• Stable air — the environment cools slowly with height (shallow ELR). A displaced parcel quickly becomes colder and denser than its surroundings and sinks back. Result: layered (stratiform) cloud, smooth flight, steady drizzle, poor visibility and a tendency to trap haze.
• Unstable air — the environment cools rapidly with height (steep ELR). A displaced parcel stays warmer and less dense than its surroundings and keeps rising. Result: heaped (cumuliform) cloud, gusty winds, showers, good visibility between showers, turbulence, and in the extreme, thunderstorms.
• Conditional instability — the ELR lies between the dry and saturated adiabatic rates. The air is stable while a parcel stays dry but becomes unstable once it is lifted to saturation. This is the everyday tropical situation: stable-looking morning air that erupts into towering cumulus by mid-afternoon.

An inversion is a layer in which temperature increases with height, the opposite of the normal pattern. Inversions are intensely stable: a parcel rising into one immediately finds itself colder than its surroundings and is suppressed. They form by overnight radiation cooling of the surface (a radiation inversion), by sinking air in a high-pressure system (a subsidence inversion), or along fronts. Practically, an inversion acts like a lid — it caps convection, traps moisture, smoke and pollutants beneath it, and is the reason haze and shallow fog sit in a defined layer with a sharp top. Smooth air, a sudden deterioration in visibility at a fixed height, and a layer of stratus or haze are all signatures of an inversion aloft.

Precipitation is what happens when cloud droplets grow heavy enough to fall. The type of precipitation tells you about the cloud that produced it, which in turn tells you about the stability of the atmosphere — a neat chain of reasoning the exam likes to test.

The defining distinction is continuous versus showery: continuous precipitation comes from layered, stable cloud, whereas showers (by definition starting and stopping) come from convective, unstable cloud. In Singapore the daily pattern is overwhelmingly showery — heavy convective downpours from cumulonimbus that build through the afternoon — rather than the steady frontal rain typical of temperate latitudes. Freezing rain, where supercooled raindrops freeze on impact, is a serious icing hazard in colder climates but is not a surface phenomenon in tropical Singapore.

A high-pressure system (anticyclone) is a region where surface pressure exceeds that of its surroundings. Air converges aloft and sinks (subsidence), which warms it and inhibits cloud formation, so highs are typically associated with fair weather, light winds and good visibility — but subsidence inversions can trap moisture or pollutants, producing haze and shallow fog.

A low-pressure system (depression, cyclone) is a region of lower pressure where surface air converges and rises. Rising air cools, water vapour condenses, and cloud and precipitation follow. Lows are associated with weather you would rather avoid: cloud, rain, strong winds, wind shear, and reduced visibility.

Buys-Ballot's law is a useful shorthand: in the Northern Hemisphere, if you stand with your back to the wind, low pressure is on your left. In the Southern Hemisphere (and at Singapore latitudes the law is weaker because Coriolis force is small), low pressure is on your right when you face into the wind. The deflection that creates this rule is the Coriolis force, which is zero at the equator and increases toward the poles — one reason large rotating depressions are rare in equatorial Singapore but common at higher latitudes.

Wind is simply air flowing from higher to lower pressure, and four forces between them decide its direction and strength. Understanding this force balance lets you read wind straight off a chart of isobars, which is a perennial exam favourite.

1. Pressure gradient force drives air directly from high to low pressure. The closer together the isobars, the steeper the gradient and the stronger the wind. This is the prime mover; everything else only deflects or slows it.
2. Coriolis force, an apparent force arising from the Earth's rotation, deflects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. It is zero at the equator and strongest at the poles, and it grows with wind speed.
3. Centrifugal force acts outward around curved isobars and modifies the balance around the centres of highs and lows.
4. Friction acts only near the surface, slowing the wind and reducing the Coriolis deflection.

High above the friction layer, pressure gradient and Coriolis come into balance and the air flows parallel to straight isobars — the geostrophic wind. Around curved isobars, adding centrifugal force gives the gradient wind, which still follows the isobars but with a speed adjusted by the curvature. Near the ground, friction enters the balance: it slows the wind (so the Coriolis deflection weakens) and the net effect is that the surface wind crosses the isobars at an angle, blowing slightly out of high pressure and into low pressure, and is weaker than the wind just above. Over the sea, where friction is low, the surface wind is faster and crosses the isobars at a smaller angle than over rough land.

The friction layer also explains the familiar diurnal wind variation: by day, surface heating mixes faster air down from aloft, so the surface wind tends to back and ease at night and veer and strengthen, often becoming gusty, by mid-afternoon. On top of this large-scale flow sit several local winds driven purely by uneven heating, and these dominate light-wind days in Singapore:

• Sea breeze — by day the land heats faster than the sea, the warm air over the land rises, and cooler air flows in from the sea to replace it. Sea breezes set in during late morning, strengthen through the afternoon, and frequently trigger the convergence that fires off Singapore's afternoon thunderstorms where breezes from different coasts meet.
• Land breeze — the reverse, and weaker. At night the land cools faster than the sea, so the air flows gently from the land out to sea. Land breezes can carry early-morning showers offshore.
• Anabatic (valley) wind — sun-warmed valley slopes heat the adjacent air, which flows up the slope during the day. Generally light.
• Katabatic (drainage) wind — at night, slopes cool by radiation, chilling the air against them so that dense cold air drains down into the valley floor. Katabatic flow can be locally strong and pools cold air in hollows, feeding radiation fog.

A quick reminder on naming convention: wind is always reported by the direction it blows from. A "090" wind comes from the east; a sea breeze on a west-facing coast is a westerly. Getting this round the wrong way is a classic and entirely avoidable exam error.

An air mass is a large body of air that has taken on roughly uniform temperature and humidity from the surface over which it formed, called its source region. Air that sits for days over warm tropical ocean becomes warm and moist; air that stagnates over a cold continental interior becomes cold and dry. Air masses are classified by both: maritime (moist) or continental (dry), combined with tropical (warm) or polar/arctic (cold).

What matters for the pilot is what happens when an air mass moves away from its source and is warmed or cooled from below. A warm air mass moving over a cooler surface is cooled from beneath, which makes it more stable — expect stratus, mist, fog and poor visibility but smooth air. A cold air mass moving over a warmer surface is heated from beneath, which makes it unstable — expect cumulus, showers, gusty winds, turbulence but good visibility between showers. This one idea — heated from below means unstable, cooled from below means stable — ties air masses, stability and cloud together and is worth far more than memorising the four-letter classifications. Singapore's air is almost permanently warm and maritime, which is why its weather is dominated by convective instability and high humidity rather than by the air-mass contrasts of higher latitudes.

A front is the boundary between two air masses of differing temperature and humidity. There are three types you must distinguish:

• Cold front — cold air advances and undercuts warm air, lifting it sharply. Slope is steep (typically 1:50 to 1:100), passage is rapid, cloud is convective (cumulonimbus), precipitation is intense but short, and winds back to a new direction with a sharp pressure rise. A line of thunderstorms may precede the front.
• Warm front — warm air advances over cold air, sliding up a shallow slope (typically 1:150 to 1:300). Cloud is stratiform, precipitation is steady and prolonged, visibility is poor, and the warm sector behind the front is often muggy with overcast skies.
• Occluded front — a cold front catches up to a slower-moving warm front, lifting the warm air aloft. Weather combines features of both, with cloud and precipitation extending across a wide band.

Fronts feature heavily in mid-latitude meteorology but are less prominent in equatorial Singapore, where convergence zones and convective activity (especially the Inter-Tropical Convergence Zone, the ITCZ) dominate. Exam questions on fronts are still likely because the syllabus is internationally aligned.

Clouds are classified by appearance (cumuliform, stratiform, cirriform) and by altitude (low, medium, high). The PPL exam expects you to know the main types and what they imply about the atmosphere.

Nimbus means "rain" — so nimbostratus is a steady-rain stratiform cloud, and cumulonimbus is the towering thunderstorm cloud. A "TCU" on a chart is a warning that the cell may develop into a Cb. The ICAO METAR uses three-letter codes (SCT for scattered, BKN for broken, OVC for overcast) with a base in hundreds of feet AGL.

Cumulonimbus cloud combines almost every flight hazard in one package. Singapore lies in a region of high convective activity year-round, with thunderstorm activity peaking in the inter-monsoon months. A PPL exam will expect you to enumerate the hazards:

• Severe turbulence — vertical gusts strong enough to exceed structural limits.
• Wind shear and microbursts — a microburst is a concentrated descending column of air that hits the ground and spreads radially. On approach, an aircraft entering a microburst first experiences a strong headwind (lift increase), then a downdraft, then a tailwind — a rapid loss of airspeed and altitude precisely when neither can be spared. Microbursts have caused fatal approach accidents worldwide.
• Hail — large hail can shatter windshields and dent leading edges even outside the cloud, sometimes thrown several miles from the parent cell.
• Lightning — direct strike rarely brings down a metal aircraft but can damage avionics and produce temporary night-vision blindness.
• Icing — within the cloud, especially in the −10 °C to 0 °C layer.
• Heavy rain — can reduce visibility on approach, mask runway markings, and (in extreme cases) affect engine breathing.

The standard guidance is to avoid thunderstorms by at least 10 nautical miles laterally where possible and to never attempt to fly under a Cb, where the strongest downdrafts and the worst microburst risk live. As a VFR pilot, deviate around or land short of the cell. Singapore-based students will see plenty of textbook Cb development in practice and should plan for go-arounds and divert fuel accordingly.

Two types of structural icing are commonly distinguished:

• Clear (glaze) ice — forms from large supercooled water droplets that spread before freezing. Glassy, dense, hard to remove, and forms in temperatures near 0 °C, especially in cumulus cloud and freezing rain. The most dangerous form of icing for general aviation.
• Rime ice — forms from small supercooled droplets that freeze on contact. Opaque, brittle, and accumulates on leading edges. Typical of stratiform cloud and lower water content. Less dangerous than clear ice but still degrades performance.
• Mixed ice — a combination, often found in cumulus that contains both droplet sizes.

PPL aircraft are typically not certified for flight in known icing. Even in Singapore, where surface icing is impossible, a climb into a Cb at altitude can quickly accumulate ice. The PIC's decision is to descend, divert, or avoid entry in the first place.

Fog is cloud at ground level, reducing visibility below 1,000 m. The main types:

• Radiation fog — forms on clear, calm nights as the ground radiates heat away, cooling the air to its dew point. Common over land, dissipates with morning sun or any wind above about 5 knots.
• Advection fog — forms when warm, moist air moves over a colder surface (typically warm air over cool sea). Common at coastal airfields. May persist all day.
• Steam fog — cold air over warm water; localised, often over rivers and lakes on cold mornings.
• Frontal fog — precipitation through cold air saturates the air below the front.

For Singapore, radiation fog can form in the early-morning hours over the airfield in calm conditions and clear away rapidly after sunrise — flight planning should consider this in the early morning, but fog is generally less of an operational issue here than in mid-latitudes.

The standard visibility ladder is worth memorising because the codes used in reports hinge on it. Fog reduces visibility below 1,000 m; mist (BR) is the same process but with visibility of 1,000 m or more, generally up to about 5,000 m, and occurs at a relative humidity near but below 100%; haze (HZ) is a dry-particle obscuration — smoke, dust or pollution trapped under an inversion — and is a recurring seasonal issue in Singapore when regional smoke is carried in on the wind. Other common visibility-reducers are heavy rain, blowing dust, and low cloud sitting on high ground. A key operational point for the VFR pilot is that visibility can collapse far faster than cloud builds, especially into a shower or a fog bank, so a deteriorating temperature/dew-point spread deserves your full attention.

The METAR (Meteorological Aerodrome Report) is an observation of current weather. The TAF (Terminal Aerodrome Forecast) is a forecast over a defined period (typically 9, 24 or 30 hours). Both are written in a standard ICAO format. Decoding speed is examinable.

A typical METAR looks like:

METAR WSSS 110800Z 09008KT 9999 FEW018 SCT025 30/24 Q1010 NOSIG=

Decoded element by element:

• WSSS — Singapore Changi (ICAO code).
• 110800Z — issued on the 11th of the month at 0800 UTC (Z = Zulu time).
• 09008KT — wind from 090° true at 8 knots.
• 9999 — visibility 10 km or more.
• FEW018 SCT025 — few clouds at 1,800 ft AGL, scattered at 2,500 ft AGL.
• 30/24 — temperature 30 °C, dew point 24 °C.
• Q1010 — QNH 1010 hPa.
• NOSIG — no significant change expected in the next two hours.

Other elements to recognise: VRB (variable wind direction), G for gusts (e.g., 09015G25KT), CAVOK (Ceiling And Visibility OK — visibility 10 km or more, no significant cloud below 5,000 ft or below the highest minimum sector altitude, no significant weather), BKN/OVC for broken/overcast, and weather codes like RA (rain), TS (thunderstorm), SH (showers), BR (mist), FG (fog).

A TAF reads similarly but adds change groups that describe how the weather is expected to evolve. The ones to recognise are FM (from a stated time, a rapid and lasting change), BECMG (a gradual change over a stated window), TEMPO(temporary fluctuations lasting less than an hour each and, in total, under half the period), and PROB30/PROB40 (a 30% or 40% probability of the stated conditions). When you read a TAF, work through it as a story: this is the prevailing forecast, then from this time it changes to that, with a temporary chance of showers in between. Always reconcile the TAF against the most recent METAR before flight — if the actual weather is already worse than the forecast, treat the forecast with caution. The golden rule for a VFR pilot is to plan against the worst conditions the forecast admits, including any TEMPO or PROB groups, not the rosy prevailing line.

The altimeter is a barometer calibrated to read altitude using a standard pressure-height relationship. Because surface pressure varies day to day, the pilot has to set a reference pressure into the subscale. Three settings are universally used:

• QNH — the pressure setting that makes the altimeter read elevation above mean sea level (AMSL) when on the ground. On the runway at Seletar, with QNH set, the altimeter will read the surveyed elevation of the airfield. Used for VFR and low-level flight.
• QFE — the pressure setting that makes the altimeter read zero when on the airfield. Used occasionally for circuit work but largely retired in commercial aviation.
• QNE — the standard pressure setting (1013.25 hPa / 29.92 inHg). With QNE set the altimeter reads pressure altitude, which is then quoted as a Flight Level (e.g., FL080 = 8,000 ft on the standard datum). Used above the transition altitude.

The transition altitude is the altitude at or below which vertical position is expressed as altitude (QNH); the transition level is the lowest available Flight Level above the transition altitude. The intermediate transition layer is normally avoided in cruise. Check the Singapore AIP for the current transition altitude in Singapore airspace — do not guess at a number.

Two memory aids worth committing to heart: "From high to low, look out below" (flying from an area of high pressure into an area of low pressure with QNH not updated, your altimeter over-reads — you are lower than indicated). The same applies to temperature: cold air is denser, so your true altitude is lower than the indicated altitude in cold conditions. In tropical Singapore temperature errors are small, but pressure changes around squall lines can shift QNH significantly in minutes.

A handful of errors trip up candidates again and again. Knowing them in advance is the cheapest way to pick up marks.

• Confusing wind direction sense. Wind is named for where it blows from, not where it goes to. A 270 wind is a westerly. Read the question twice.
• Mixing up the lapse rates. The dry adiabatic rate (3 °C/1,000 ft) and saturated adiabatic rate (about 1.5 °C/1,000 ft) describe a rising parcel; the environmental lapse rate is the surrounding air on the day. Stability comes from comparing the two, not from any single number.
• The altimetry sign error. "From high to low (or hot to cold), look out below" — when you fly toward lower pressure or colder air without resetting, the altimeter over-reads and you are lower than it indicates. Many candidates get the direction of the error backwards under exam pressure.
• Assuming high pressure always means good weather. Usually yes, but a subsidence inversion under a high traps haze, moisture and pollution, giving poor visibility and stubborn low cloud or fog.
• Treating relative humidity as moisture content. Relative humidity can hit 100% just by cooling the air, with no new moisture added. The dew point, not the relative humidity, tells you the actual moisture present.
• Forgetting Coriolis is zero at the equator. Several classic mid-latitude rules (Buys-Ballot, geostrophic balance, rotating depressions) weaken dramatically at Singapore's latitude, where convection and convergence, not Coriolis-driven systems, run the show.
• Confusing showers with continuous rain. Showers come from unstable, convective (cumuliform) cloud; continuous rain comes from stable, layered (stratiform) cloud. The precipitation type reveals the stability, and vice versa.
• Reading only the prevailing TAF line and ignoring TEMPO and PROB groups — exactly the conditions a question (or a real diversion) hinges on.

Meteorology rewards understanding the physics once rather than memorising endless isolated facts. Build your revision around a small number of core models and let the detail hang off them:

1. Lock down the ISA values first. Sea-level 15 °C, 1013.25 hPa, 1.225 kg/m³, lapse rate about 2 °C/1,000 ft to the tropopause near 36,000 ft, isothermal at roughly −56.5 °C above. Plus 1 hPa per 27 ft near the surface. These are pure recall and feed dozens of other questions.
2. Master the pressure-temperature-density triangle. If you can explain why hot, low-pressure, humid air is thin and degrades performance, a whole cluster of questions opens up.
3. Practise the stability decision out loud. Given an ELR, decide stable, unstable or conditionally unstable, then state the resulting cloud, precipitation, turbulence and visibility. Make it a reflex.
4. Drill the wind force balance. Pressure gradient, Coriolis, centrifugal and friction; geostrophic, gradient and surface wind; and why the surface wind backs and weakens relative to the wind aloft.
5. Decode real reports against the clock. Pull a few live METARs and TAFs and translate every group, including change groups, until it is automatic.

A few tactical habits pay off in the exam itself. Sketch a quick diagram for any wind, frontal or altimetry question — drawing the isobars or the temperature profile usually makes the answer obvious. Watch for the words increases, decreases, over-reads and under-reads, which flip an answer entirely. Treat tropical context as a feature, not an afterthought: many questions on this site are framed around Singapore conditions, so connect each principle to what you actually see — afternoon convective build-up, sea-breeze convergence, monsoon rainfall, density-altitude effects on a hot humid day, and the seasonal haze. If you can reason from cause to effect rather than reciting facts, meteorology becomes one of the most intuitive and reliably scoring subjects on the syllabus.

Across the 43 Meteorology questions in this bank, the quiz leans heavily on the physics of the atmosphere — pressure, temperature, density, lapse rates and wind generation — rather than on forecasting or weather-product interpretation. There is comparatively little METAR/TAF decoding in this particular bank (those questions tend to migrate to the Principles of Flight bank for performance-planning reasons), so do not be surprised if you see four ways of asking the same relationship between pressure and altitude before you see a cloud type. The clusters below cover essentially the entire bank.

• ISA values: a near-certain question. Sea-level temperature 15 °C, sea-level pressure 1013.25 hPa, sea-level density 1.225 kg/m³, troposphere lapse rate 1.98 °C per 1,000 ft up to about 36,000 ft, and the stratosphere isothermal at roughly −56.5 °C.
• Diurnal temperature variation: the coldest part of the day is just before dawn and the hottest is mid- to late-afternoon. Diurnal range is greatest under clear skies with light winds (think desert or inland valley), and smallest under cloud cover, which both reflects incoming solar radiation by day and traps outgoing radiation by night.
• Inversions and stability: an inversion is a layer where temperature increases with height; radiation inversions form after sunset over land with light mixing wind. Conditional stability is the regime where the environmental lapse rate lies between the saturated and dry adiabatic lapse rates.
• Pressure-temperature-density relationships: high pressure plus cold air gives high density; pressure decreases with height (and a 1,500 ft column over a 1,000 hPa surface drops to roughly 970 hPa). A low-pressure system with a cold core intensifies with height because the cold air is denser, increasing the pressure-lapse rate.
• Atmospheric composition and structure: oxygen and nitrogen dominate, leaving roughly 1% as "other gases"; the layer above the troposphere is the stratosphere; barometers measure the weight of the column of air above a point.
• Wind generation and Coriolis: wind is the horizontal movement of air driven by pressure differences, measured in knots by an anemometer. The earth's rotation deflects flowing air — Coriolis force — and the strongest winds occur where the pressure gradient is steepest (closely packed isobars).
• Geostrophic, gradient and surface winds: gradient wind is the balance of pressure gradient, centrifugal and Coriolis forces and follows curved isobars; surface wind crosses isobars at an angle (and is reduced in strength) because of surface friction. Summer lows are typically associated with low pressure at the surface and divergence aloft.
• QNH, FL and altimetry: QNH is calculated by reducing airfield pressure to mean sea level using ISA assumptions; an aircraft at airfield elevation with the altimeter set to that elevation will display QNH on the subscale. "FL340" means a pressure altitude of 34,000 ft, not 34,000 ft AMSL.
• Low-level convergence and frontal cues: an area of low-level convergence is recognised by increasing turbulence, falling pressure, and increasing cloud with possible precipitation. The relationship between surface pressure centres and pressure centres aloft is governed by the temperature of the column of air.

Because this bank is heavily front-loaded on atmospheric physics, spend the bulk of your revision on the ISA values, the diurnal cycle, the pressure-temperature-density triangle, and the wind force balance. Once those four are second nature, the cloud-type and METAR questions in the wider syllabus become easy add-ons. A useful exercise: set the altimeter subscale on a parked aircraft to your airfield elevation, read the QNH off the subscale, and confirm that you understand exactly what the instrument is doing — many of the quiz questions are simply asking you to describe that mental model in words.