What the atmosphere is made of

The atmosphere is roughly 78% nitrogen, 21% oxygen, and 1% other gases including water vapour and CO2.

These proportions stay constant up to about 80 km, but water vapour content varies hugely and is what drives most weather.

The layers

• Troposphere: from the surface up to the tropopause, where nearly all weather happens.
• Tropopause height varies with latitude and season: around 16-17 km over the equator (warmer, higher) and around 8-9 km over the poles (colder, lower).
• Temperature in the troposphere generally decreases with height at the International Standard Atmosphere (ISA) average lapse rate of 1.98 degrees C per 1000 ft, usually rounded to 2 degrees C per 1000 ft.
• Above the tropopause is the stratosphere, where temperature is roughly constant then starts to increase with height.

ISA - the reference model

ISA gives pilots a fixed baseline to compare real conditions against, so instruments and performance charts mean the same thing everywhere.

• Sea level pressure: 1013.25 hPa (often written 1013 hPa).
• Sea level temperature: 15 degrees C (288.15 K).
• Standard lapse rate: 1.98 degrees C per 1000 ft (about 2 degrees C per 1000 ft) up to the tropopause at 36,090 ft (11 km).
• Above 36,090 ft, ISA temperature is constant at minus 56.5 degrees C up to around 65,600 ft.

Pressure with height

Air pressure falls as you climb because there is less air weighing down from above.

• Near sea level, pressure drops roughly 1 hPa per 30 ft (a rough rule of thumb, not exact at altitude).
• The rate of pressure decrease itself decreases with height, since air gets thinner - this is why pressure altimeters are less accurate at very high altitudes.

Density and the density formula

Density depends on pressure, temperature and humidity: density increases with higher pressure, and decreases with higher temperature or higher humidity (moist air is less dense than dry air because water vapour molecules are lighter than nitrogen and oxygen molecules).

• High density altitude (hot, humid, high pressure altitude airfields) means reduced engine and aerofoil performance - longer takeoff runs, poorer climb.
• Density altitude = pressure altitude corrected for temperature deviation from ISA.

Common mistakes to avoid

• Do not confuse pressure altitude (set 1013 hPa) with density altitude (also corrected for temperature) - they are not the same thing.
• Do not assume the lapse rate is constant everywhere - it is only an ISA average; real atmosphere varies daily, including inversions where temperature increases with height.
• Remember humid air is LESS dense, not more - this trips a lot of candidates up.

How clouds form

Clouds form when air is cooled to its dew point (or gains moisture) and condensation nuclei are present. The three main cooling mechanisms are orographic uplift (air forced over high ground), convective heating (surface heating causes air to rise), and frontal lifting (warm air forced up over cooler air at a front). Once air rises past the condensation level it cools at either the Dry Adiabatic Lapse Rate (DALR, 3.0 degrees C per 1000 ft) before saturation, or the Saturated Adiabatic Lapse Rate (SALR, roughly 1.5 degrees C per 1000 ft, varies with temperature) after saturation, because latent heat release slows the cooling.

Stability and instability

Compare the Environmental Lapse Rate (ELR, the actual temperature drop with height on the day) to the DALR/SALR of a rising parcel.

• If ELR is less than SALR: absolutely stable air - parcel sinks back, layered (stratiform) cloud, poor visibility, smooth flight, drizzle.
• If ELR is greater than DALR: absolutely unstable air - parcel keeps rising, cumuliform cloud, good visibility between showers, turbulence, showers and possible thunderstorms.
• If ELR sits between SALR and DALR: conditionally unstable - stable for a dry parcel but unstable once saturated.

The standard ISA average ELR is 1.98 degrees C per 1000 ft (often rounded to 2 degrees C/1000 ft).

Cloud types and the letter codes

Clouds are grouped by height of base: High (CI, CC, CS, bases above 20,000 ft), Medium (AS, AC, bases 6,500-20,000 ft), Low (ST, SC, NS, bases up to 6,500 ft), plus vertical development clouds (CU, CB) which can span all levels. CB (cumulonimbus) is the one to fear - associated with severe turbulence, icing, hail, lightning, microbursts and windshear; a CB needs a towering cumulus stage, a mature stage (precipitation begins, downdraughts) and a dissipating stage.

Precipitation

For precipitation to reach the ground, cloud droplets must grow large enough to overcome updraughts - via the Bergeron-Findeisen process (ice crystals grow at the expense of supercooled water droplets in mixed clouds) or by coalescence (droplets colliding and merging, typical in warm cumuliform cloud). Drizzle falls from stratiform cloud with small water content; showers fall from cumuliform cloud; freezing rain/drizzle occurs when rain falls through a sub-zero layer near the surface and freezes on contact - a serious icing hazard.

Common exam traps

• Do not confuse DALR (3 degrees C/1000 ft) with SALR (about 1.5 degrees C/1000 ft) - SALR is always less because of latent heat release.
• Fog is not cloud with a base at the surface for exam purposes but behaves by the same cooling principles (radiation, advection, frontal).
• Stratiform does not mean no turbulence at all if embedded CB is present - always check for embedded CB in layered cloud along a front.
• Absolute instability, not just conditional instability, is what is needed for CB without any forcing mechanism.

Air masses

An air mass is a large body of air with roughly uniform temperature and humidity, taking on the characteristics of its source region.

• Source regions are areas where air sits long enough to stabilise, e.g. polar continental (cold, dry), polar maritime (cool, moist), tropical continental (hot, dry), tropical maritime (warm, moist).
• UK weather is dominated by polar maritime and tropical maritime air, which is why the UK is generally mild and damp.
• As an air mass moves it is modified by the surface it crosses - warmed from below over warm ground/sea makes it unstable, cooled from below makes it stable.

Fronts - the basics

A front is the boundary between two air masses of different temperature and density.

• Warm front: warm air overtakes and rides up over cold air on a shallow slope (about 1:150). Produces a long belt of layered cloud (cirrus, then altostratus, then nimbostratus) with steady rain well ahead of the surface position, poor visibility, and a gradual pressure fall.
• Cold front: cold air undercuts warm air on a steeper slope (about 1:50 to 1:100), forcing it up quickly. Produces cumulonimbus and cumulus, heavy showers, possible thunderstorms, a sharp wind veer, and a rapid pressure rise once it passes.
• Occluded front: forms when a faster cold front catches up with a warm front, lifting the warm air off the surface entirely. A cold occlusion (cold air behind colder than air ahead) behaves like a cold front; a warm occlusion behaves like a warm front. Weather is typically a mix of both, often with the worst of the rain.
• Stationary front: little or no movement, can bring prolonged unsettled weather in one area.

Common mistakes

• Don't mix up the slopes - warm fronts are shallow and give a long, gradual weather sequence; cold fronts are steep and give a short, sharp one.
• Wind always backs ahead of a warm front and veers behind a cold front in the northern hemisphere - know this for both interpretation and forecasting questions.
• Visibility is usually worse in warm sector/warm front conditions (drizzle, mist) than in the clearer, showery air behind a cold front.

Pressure systems

• Low pressure (depression/cyclone): air spirals inwards and rises, associated with cloud, precipitation and unsettled weather. In the northern hemisphere the circulation is anticlockwise.
• High pressure (anticyclone): air spirals outwards and descends (subsides), suppressing cloud formation, generally giving settled weather. Circulation is clockwise in the northern hemisphere.
• Isobars close together mean a strong pressure gradient and stronger winds; widely spaced isobars mean light winds.
• Buys Ballot's Law: with your back to the wind in the northern hemisphere, low pressure is on your left.
• Anticyclones can still cause problems for pilots - fog, mist and low stratus from radiation cooling under calm, clear, subsiding air, especially in winter.
• A col is a saddle-shaped area of slack pressure between two highs and two lows - winds are light and weather can be changeable, often with fog or thundery conditions in summer.

What is wind gradient?

Wind gradient (wind shear) is the change of wind speed and/or direction with height, most marked in the lowest few hundred feet above the surface.

• Friction with the ground slows the wind and backs its direction (in the northern hemisphere) compared with the gradient wind above.
• A typical rule of thumb: surface wind is roughly 2/3 the speed of the wind at 2000 ft, and backed by about 30 degrees over land (less over sea, maybe 10-15 degrees, due to lower friction).
• Above about 2000 ft the wind blows close to the geostrophic/gradient wind, following the isobars, backed only slightly from the gradient value due to reduced friction aloft.
• On approach to land, expect a headwind component to decrease and the wind to veer as you descend through the friction layer - this can catch pilots out on final approach, giving a lower airspeed than expected unless corrected.
• Negative wind shear (loss of headwind, or a tailwind gust) on approach causes a sudden drop in airspeed and lift - a classic cause of undershoot; pilots should carry extra speed in gusty or shear-prone conditions.

Local winds

• Sea breeze: by day, land heats faster than sea, air rises over land, cooler air flows in from the sea - typically a light-to-moderate onshore wind developing by early-to-mid afternoon, strongest with strong sun and light gradient wind.
• Land breeze: at night, land cools faster than sea, so the flow reverses and a light offshore breeze forms, usually weaker than the sea breeze.
• Anabatic wind: daytime upslope wind, warm air rising up sun-heated valley sides.
• Katabatic wind: night-time downslope wind, cold dense air draining down valley sides and pooling in valley bottoms - can bring fog and frost risk there.
• Valley/funnelling winds: wind speed increases where terrain channels airflow through a gap or valley (venturi effect).

Turbulence

• Mechanical turbulence: caused by wind flowing over rough or obstructed terrain (buildings, trees, hills) - worse with stronger wind and rougher surface; expect it downwind of hangars, trees and ridgelines.
• Thermal turbulence: caused by convection currents (thermals) on warm sunny days over unevenly heated ground - most active in the afternoon.
• Turbulence increases with wind speed and surface roughness, and can extend downwind of an obstacle for a distance of roughly 10-20 times the obstacle's height.
• Wave (mountain wave/lee wave) turbulence can occur downwind of hills/mountains when a stable airflow crosses a ridge at a good angle and speed - can extend well above the terrain and far downwind, marked by lenticular clouds.
• Rotor turbulence sits beneath mountain waves, close to the surface, and can be severe - avoid low flight in the lee of high ground in strong wind.

Common mistakes

• Forgetting to add a margin to approach speed in gusty/shear conditions.
• Assuming surface wind equals the forecast wind aloft - always apply the backing/friction rule.
• Confusing anabatic (up, day) with katabatic (down, night) - remember 'a' for 'ascending, afternoon'.
• Underestimating turbulence risk on the lee side of high ground even in apparently calm surface conditions.

Visibility - the basics

Visibility is reported in metres or kilometres and drives whether you can legally fly VFR.

In a METAR, visibility is given as the lowest value observed. Below 5000 m it's reported in metres (steps of 50 m below 800 m, 100 m steps 800-5000 m); at or above 10 km it's reported as 9999.

CAVOK means visibility 10 km or more, no cloud below 5000 ft (or below the highest MSA if higher), and no significant weather.

Fog - how it forms

Fog is cloud (water droplets) sitting at the surface, reducing visibility below 1000 m.

Mist is the same thing but visibility stays 1000 m or more; haze is dry particles, not water droplets.

• Radiation fog: forms overnight over land with clear skies, light wind (ideally 2-8 kt to gently mix the cooling air - dead calm gives dew instead) and moist air. Classic autumn/winter valley and airfield fog. Burns off from the edges as the sun heats the ground, usually by mid-morning.
• Advection fog: forms when warm, moist air moves horizontally over a colder surface (sea or cold land), cooling it below its dew point. Common on coasts and can persist day and night, and even in wind - it does not need calm conditions to form or clear.
• Frontal fog: forms near a warm front as rain falls through cooler air below, saturating it (also called precipitation-induced fog).
• Upslope fog: moist air forced up rising ground cools adiabatically until it saturates.

The key exam trap

Radiation fog needs LIGHT wind to form (some turbulence helps deepen it) but STRONG wind destroys it by mixing too much dry air in.

Advection fog is different - it can form and persist in moderate wind because it's driven by the air mass moving over a cold surface, not by radiational cooling.

Icing

Airframe icing needs two things together: visible moisture (cloud, rain, drizzle) and a temperature at or below 0°C (icing can occur down to about -40°C).

• Clear ice: forms in large supercooled water droplets (freezing rain, cumuliform cloud) - freezes slowly, spreads over the surface, hardest to remove, most hazardous, most disrupts lift.
• Rime ice: forms in small supercooled droplets (stratiform cloud) - freezes instantly, rough and opaque, more brittle, easier to shed.
• Mixed ice: a combination, common in cumulonimbus.

Carburettor icing can occur even in clear air with no visible moisture, at temperatures up to about +30°C, whenever humidity is high - this is the classic trick question. It's worst at low power settings (closed/partly-closed throttle) because of the extra cooling from fuel vaporisation and pressure drop.

Structural icing is most severe in cumulonimbus and towering cumulus due to large supercooled droplets, and freezing rain/drizzle produces the fastest, most dangerous clear ice buildup, sometimes ahead of a warm front.

Common mistakes

• Confusing mist (1000 m+) with fog (below 1000 m).
• Thinking fog always needs calm air - true for radiation fog, false for advection fog.
• Forgetting carb icing needs no visible moisture and can happen on a warm, humid day.
• Assuming clear ice is 'safer' because it's smooth - it's actually the worst for performance and hardest to remove.

What TAFs and METARs actually are

A METAR is an actual observed weather report, issued for a specific aerodrome, normally every 30 or 60 minutes. A TAF (Terminal Aerodrome Forecast) is a forecast of expected conditions at an aerodrome, covering a set validity period - usually 9, 24 or 30 hours depending on the airfield.

METAR structure

A METAR gives, in order: station identifier, date/time (as a six-figure group, day then hour and minute UTC, ending in Z), wind direction and speed, visibility, present weather, cloud, temperature and dew point, and QNH. CAVOK replaces visibility, weather and cloud groups when visibility is 10km or more, no cloud below 5000ft (or below the highest minimum sector altitude if higher) and no cumulonimbus or towering cumulus, and no significant weather present.

Wind and visibility coding

Wind is given as a three-figure true direction plus a two or three-figure speed in knots, for example 24012KT. Gusts are shown with a G, for example 24012G22KT. Visibility is given in metres up to 9999 (meaning 10km or more) then in whole kilometres. A variable wind direction is shown when the direction varies by 60 degrees or more and speed is above 3kt, coded as VRB.

Cloud groups

Cloud is reported in oktas using FEW (1-2 oktas), SCT (3-4), BKN (5-7) and OVC (8), followed by height in hundreds of feet above the aerodrome. NSC means no significant cloud below 5000ft (or MSA) and no CB/TCU. NCD means no cloud detected by an automated station.

TAF-specific coding

TAFs use change groups to show expected variation: BECMG (becoming - a gradual permanent change over a stated period), TEMPO (temporary - fluctuations lasting under an hour at a time, and in total less than half the period), and PROB30 or PROB40 (a probability of 30% or 40% that stated conditions will occur, often combined with TEMPO). FM (from) marks a rapid, significant change and starts a completely new set of conditions from that time.

Common mistakes to avoid

Don't confuse BECMG (gradual, permanent) with TEMPO (temporary, fluctuating) - BECMG conditions are expected to persist once reached, TEMPO ones come and go. Don't forget CAVOK has strict criteria - all three conditions (visibility, cloud, weather) must be met, not just one. Remember METAR times are actual observation times, TAF times define a validity period, not certainty. Always read wind, visibility and cloud direction of change (e.g. improving vs deteriorating) from the sequence of groups, not just the first one given.

Amendments

A TAF can be amended (TAF AMD) when actual or expected conditions differ significantly from the original forecast, and a corrected METAR is issued as METAR COR if an error is found in the original.