Meteorology for Pilots
Atmosphere, pressure systems, fronts, clouds, icing, and weather hazards — for PPL and IR exam preparation.
Aviation meteorology is a core PPL and IR written exam subject. Understanding weather patterns, hazards, and how to interpret aviation weather products (METAR, TAF, SIGMET) is essential for safe flight operations.
The Atmosphere
| Atmospheric Layer | Altitude | Characteristics |
|---|---|---|
| Troposphere | Surface to ~36,000 ft / FL360 | All weather occurs here; temperature decreases with altitude (~2°C/1000 ft ISA) |
| Tropopause | ~36,000 ft (varies) | Temperature inversion; jet streams found here |
| Stratosphere | Above tropopause | Temperature constant then increases; calm, very dry |
| Ozone layer | Within stratosphere | UV absorption |
ISA — International Standard Atmosphere
| Parameter | ISA Value |
|---|---|
| Sea level temperature | 15°C |
| Sea level pressure | 1013.25 hPa (29.92 inHg) |
| Temperature lapse rate | 1.98°C per 1000 ft (≈2°C/1000 ft) |
| Sea level density | 1.225 kg/m³ |
| Tropopause altitude | 36,089 ft (11 km) |
| Tropopause temperature | −56.5°C |
Density altitude = Pressure altitude + [120 × (OAT − ISA temp)]
High density altitude (hot, high, humid) significantly reduces aircraft performance — longer take-off roll, reduced climb rate, higher true airspeed for same IAS.
Pressure Systems
| Feature | Characteristics | Weather |
|---|---|---|
| High pressure (anticyclone) | Outflow, clockwise NH / anticlockwise SH | Generally settled; radiation fog possible; clear skies |
| Low pressure (depression) | Inflow, anticlockwise NH / clockwise SH | Unsettled; clouds, rain, wind |
| Trough | Elongated low | Enhanced precipitation, wind changes |
| Ridge | Elongated high | Brief improvement between systems |
| Col | Neutral between high and low | Thunderstorm risk in summer |
Pressure Gradient and Wind
- Wind flows from high to low pressure
- Geostrophic wind: above ~2000 ft — parallel to isobars (Coriolis + pressure gradient balance)
- Surface wind: crosses isobars at ~25° toward lower pressure (friction effect)
- Buys Ballot's Law: with wind on your back in NH, low pressure is to your left
Fronts
| Front Type | Characteristics | Weather |
|---|---|---|
| Warm front | Warm air replacing cold; gentle slope | Extensive cloud (Ci→Cs→As→Ns), prolonged rain, poor vis after passage |
| Cold front | Cold air undercutting warm; steep slope | CB, thunderstorms, heavy showers, rapid vis improvement after passage |
| Occluded front | Cold overtakes warm | Complex, both warm and cold characteristics; deteriorating then improving |
| Stationary front | Neither air mass advancing | Prolonged cloud, drizzle, fog |
Cloud Types and Formation
| Cloud | Type | Approx Altitude (temperate) | Formation / Notes |
|---|---|---|---|
| Cirrus (Ci) | High | >20,000 ft | Ice crystals; thin wispy; approaching warm front |
| Cirrostratus (Cs) | High | >20,000 ft | Ice sheet; halo around sun/moon |
| Cirrocumulus (Cc) | High | >20,000 ft | Mackerel sky; ice + supercooled water |
| Altostratus (As) | Middle | 6,500–20,000 ft | Thick grey sheet; rain/snow; sun/moon dull |
| Altocumulus (Ac) | Middle | 6,500–20,000 ft | White/grey patches; often wave-like |
| Nimbostratus (Ns) | Low–Middle | Surface–10,000 ft | Dark rain cloud; continuous rain/snow; poor vis |
| Stratocumulus (Sc) | Low | Surface–6,500 ft | Most common cloud; grey lumpy patches; light rain |
| Stratus (St) | Low | Surface–6,500 ft | Uniform grey layer; drizzle/fog |
| Cumulus (Cu) | Low base | Variable | Fair weather (small) or towering |
| Cumulonimbus (Cb) | Low base, extends to tropopause | Base 1,500–6,000 ft | Major hazard — thunderstorm cloud |
Aviation Weather Hazards
Icing
In-flight icing is one of the most dangerous weather hazards. Ice accumulation reduces lift, increases drag, weight, and stall speed. It can block pitot tubes and static ports. Avoid flight into known icing conditions unless equipped and certified.
| Icing Type | Description | Severity |
|---|---|---|
| Clear ice (glaze ice) | Freezing rain or large supercooled droplets; transparent, hard | Most dangerous — hard to remove |
| Rime ice | Small supercooled droplets; white, brittle | Moderate — easier to remove |
| Mixed ice | Combination of clear and rime | Very dangerous |
| Carburetor icing | Forms in carb venturi; can occur in +20°C, humid conditions | Can cause engine failure |
Icing risk conditions: clouds, rain, drizzle in 0°C to −20°C range; most severe at −2°C to −10°C.
Thunderstorms
NEVER fly through a thunderstorm. Avoid by 20+ nm or as per applicable regulations.
Hazards include: severe turbulence, windshear, icing, hail (up to 20 nm from storm), lightning, microbursts, heavy rain reducing visibility to near-zero.
Fog Types
| Fog Type | Formation | Location |
|---|---|---|
| Radiation fog | Clear night, light wind, high humidity → ground cools | Valleys, flat terrain |
| Advection fog | Warm moist air moves over cold surface | Coastal areas, spring/summer |
| Hill fog | Orographic lifting of moist air | Hills and mountains |
| Mixing fog | Warm rain falling into cold air (frontal) | Warm fronts |
| Arctic (sea) smoke | Cold air over relatively warm water | Maritime/polar regions |
Windshear
- Rapid change in wind speed and/or direction over a short distance
- Most dangerous at low altitude (takeoff/approach)
- Sources: thunderstorms, microbursts, frontal boundaries, temperature inversions, terrain effects
- Low-level windshear alert systems (LLWAS) at major airports
- Report any significant windshear encountered to ATC
SIGMETs and AIRMETs
SIGMET (Significant Meteorological Information)
SIGMETs are issued for weather phenomena considered hazardous to all aircraft. There are two types:
| Type | Issuance Criteria | Valid Time |
|---|---|---|
| Non-Convective SIGMET |
|
4 hours (6 hours for tropical cyclone or volcanic ash) |
| Convective SIGMET (US) |
|
2 hours (issued at H+55 each hour; 6 hours for tropical cyclone) |
SIGMET series: SIGMETs are identified by a letter series (e.g., SIGMET ROMEO 1). The series letter resets at 0000Z each day. Letters N and O are reserved for convective SIGMETs. International SIGMETs use WMO headers with FIR identification.
AIRMET (Airmen's Meteorological Information — US)
AIRMETs are issued for moderate hazards primarily affecting light aircraft and VFR flight. Issued every 6 hours (updated as needed) covering the conterminous US:
| AIRMET Type | Name | Specific Criteria |
|---|---|---|
| Sierra (S) | IFR and Mountain Obscuration | Ceiling <1,000 ft AND/OR visibility <3 SM affecting ≥3,000 sq mi; mountains obscured |
| Tango (T) | Turbulence | Moderate turbulence; sustained surface winds ≥30 kt; non-convective low-level windshear (LLWS) |
| Zulu (Z) | Icing | Moderate icing (not associated with thunderstorms); freezing level heights |
AIRMET vs SIGMET distinction for icing and turbulence: AIRMET = moderate icing/turbulence. SIGMET = severe or extreme icing/turbulence. Severe turbulence associated with thunderstorms triggers a Convective SIGMET, not an AIRMET.
CWA — Center Weather Advisory
A CWA is issued by an FAA Center Weather Service Unit (CWSU) for weather that is occurring or is expected within the next 2 hours. Key points:
- Valid for 2 hours maximum
- Issued for current or imminent weather that has not yet been captured by a SIGMET or convective SIGMET
- Not intended for pre-flight planning — it is an in-flight advisory only
- Covers individual ARTCC (en-route center) airspace areas
Winds and Temperatures Aloft Forecast (FB)
Winds and temperatures aloft forecasts are coded in a compact format. Standard 4-digit groups encode direction and speed; temperature is appended for most altitudes.
Standard Decode (4-digit wind group)
High-Speed Wind Encoding (≥100 kt)
When the wind speed is 100 kt or more, the tens of degrees of direction have 50 added to them, and 100 is subtracted from the speed. Temperature is omitted for the highest levels.
Temperature Sign Convention
Temperatures are prefixed with a sign only when positive (e.g., +06). Negative temperatures have no prefix (e.g., 04 = −4°C). Above 24,000 ft all temperatures are assumed negative; the sign is omitted.
| Coded Group | Decoded |
|---|---|
| 9900 | Light and variable — speed <5 kt |
| 9901 | Calm — speed 0 kt |
| 2318+06 | 230° at 18 kt, +6°C |
| 2318-04 | 230° at 18 kt, −4°C |
| 731960 | 230° at 119 kt, −60°C (high speed encoding) |
Surface Analysis Charts — Symbology
Wind Barbs
| Symbol | Wind Speed |
|---|---|
| Circle (no barbs) | Calm |
| Short half-barb | 5 kt |
| Full barb | 10 kt |
| Full barb + half-barb | 15 kt |
| Two full barbs | 20 kt |
| Pennant (filled triangle) | 50 kt |
| Pennant + full barb | 60 kt |
Barbs point in the direction the wind is blowing from. The staff extends downwind; barbs are drawn on the left side of the staff (in the Northern Hemisphere convention).
Constant Pressure Chart Altitudes
| Pressure Level | Approximate Altitude | Use |
|---|---|---|
| 850 mb | ~5,000 ft | Low-level temperature, moisture, winds |
| 700 mb | ~10,000 ft | Low–mid level moisture, icing, cloud tops |
| 500 mb | ~18,000 ft | Mid-tropospheric steering flow; jet stream position |
| 300 mb | ~30,000 ft | Jet stream cores; upper level divergence/convergence |
| 250 mb | ~34,000 ft | Upper troposphere; high-altitude flight planning |
| 200 mb | ~39,000 ft | Near tropopause; jet stream analysis |
Station Plot Model (Surface Charts)
Observations on surface analysis charts are plotted using a station model. Key elements around the station circle:
| Position | Element |
|---|---|
| Centre circle | Station; filled to indicate sky cover (oktas) |
| Above circle (left) | Temperature (°F in US, °C international) |
| Below circle (left) | Dewpoint |
| Upper right | Sea-level pressure (tens, units, tenths of mb — add 9 or 10 prefix) |
| Right of circle | Pressure tendency symbol + 3-hour change (tenths mb) |
| Lower right | Present weather symbol |
| Staff from circle | Wind direction (from) + speed (barbs) |