Aviation Weather for Pilots
Weather is the leading cause of general aviation accidents. This reference covers everything a pilot needs to understand and interpret aviation weather: from reading METARs and TAFs to understanding icing risks, turbulence types, SIGMETs, and the International Standard Atmosphere. Use the sections below to navigate to specific topics.
Weather Reference Topics
International Standard Atmosphere (ISA)
The International Standard Atmosphere (ISA) is a standardised model of atmospheric conditions used as a baseline for aviation performance calculations and altimetry. It defines a set of standard values for temperature, pressure, and density at each altitude that are used as reference points when comparing real-world conditions.
ISA Sea Level Standard Values: Temperature 15°C (59°F) · Pressure 1013.25 hPa (29.92 inHg) · Air Density 1.225 kg/m³
Temperature Lapse Rate
In the troposphere, ISA temperature decreases at a rate of 2°C per 1,000 ft (precisely 1.98°C/1,000 ft) with increasing altitude. This continues up to the tropopause at 36,089 ft (11,000 m), where temperature stabilises at −56.5°C and remains constant through the lower stratosphere.
Example: ISA temperature at 8,000 ft = 15 − (8 × 2) = −1°C
ISA Altitude Reference Table
| Altitude (ft) | ISA Temp (°C) | ISA Temp (°F) | Pressure (hPa) | Pressure (inHg) |
|---|---|---|---|---|
| 0 (Sea Level) | +15.0 | +59.0 | 1013.25 | 29.92 |
| 2,000 | +11.0 | +51.8 | 942.1 | 27.82 |
| 4,000 | +7.0 | +44.6 | 875.1 | 25.84 |
| 6,000 | +3.1 | +37.5 | 811.9 | 23.98 |
| 8,000 | −1.0 | +30.2 | 752.6 | 22.22 |
| 10,000 | −4.8 | +23.3 | 696.8 | 20.58 |
| 15,000 | −14.7 | +5.5 | 571.8 | 16.89 |
| 20,000 | −24.6 | −12.3 | 465.6 | 13.75 |
| 30,000 | −44.4 | −47.9 | 300.9 | 8.88 |
| 36,089 (Tropopause) | −56.5 | −69.7 | 226.3 | 6.68 |
ISA Deviation
Real-world conditions rarely match ISA exactly. ISA deviation describes how much warmer or cooler the atmosphere is compared to the standard model:
- ISA+10 means the actual temperature is 10°C warmer than the standard ISA value for that altitude
- ISA−15 means the actual temperature is 15°C colder than ISA standard
Why ISA Matters for Pilots
- Altimetry: Altimeters are calibrated to ISA; significant temperature deviations cause indicated altitude to differ from true altitude
- Performance charts: All aircraft performance data (takeoff distance, climb rate, ceiling) is published using ISA conditions as the baseline
- Density altitude: ISA+temperature increases density altitude — hotter or higher conditions mean longer takeoff runs, reduced climb performance, and higher true airspeeds for the same indicated airspeed
- Engine performance: Piston and turbine engines both lose performance as air density decreases (ISA+ conditions)
Wind Shear & Microburst
Wind shear is a rapid change in wind speed and/or direction over a short distance — either horizontally or vertically. It is one of the most dangerous weather phenomena a pilot can encounter, particularly during approach and departure.
Low-Level Wind Shear (LLWS)
Low-level wind shear occurs below 2,000 ft AGL and is most hazardous during approach and departure phases when the aircraft has limited energy and altitude to recover. Common causes include:
- Frontal passages: Significant wind speed and direction changes across a weather front
- Thunderstorms: Outflow boundaries and gust fronts can cause sudden, severe shear
- Temperature inversions: A shallow layer of cold, stable air under warm air can trap a strong low-level jet
- Terrain features: Hills, ridges, and buildings cause mechanical turbulence and wind flow disruption
Microburst
A microburst is an intense, concentrated downdraft generated by a thunderstorm that spreads outward on reaching the surface. It is among the most lethal hazards in aviation weather.
- Downdraft velocities can exceed 6,000 ft/min
- Horizontal extent is typically less than 2 nautical miles
- Duration is usually only 2–5 minutes, but its effects are sudden and severe
- An aircraft on approach first experiences a headwind (airspeed increase), then a strong downdraft, then a tailwind (airspeed loss) — the combined performance loss can be unsurvivable at low altitude
Microburst escape manoeuvre: Apply TOGA (takeoff/go-around) thrust immediately, rotate to the go-around pitch attitude, do not retract flaps, and declare an emergency. Do not attempt to maintain the glidepath.
Detection & Reporting Systems
| System | Description |
|---|---|
| PIREPs | Pilot reports — the most timely and operationally valuable LLWS reports; always file a PIREP if you encounter wind shear |
| LLWAS | Low Level Wind Shear Alert System — network of anemometers around larger airports that detect surface divergence indicating microburst activity |
| TDWR | Terminal Doppler Weather Radar — dedicated radar at major airports specifically designed to detect microbursts and wind shear near the runway |
| ATIS/AWIS | Wind shear alerts are broadcast on ATIS when reported; listen carefully on approach |
Pilot Recognition of Wind Shear
- Unexpected airspeed changes (increase then decrease, or vice versa)
- Abnormal pitch or power required to maintain the glidepath
- Sudden increase in sink rate without control input
- Localised rain shaft or virga below thunderstorm base
Go/No-Go Weather Decision Framework
Every pilot must develop a systematic approach to weather decision-making before each flight. The following ten-point checklist is designed for VFR pilots, but the principles apply to all operations. Work through each item honestly before committing to a departure.
Key principle: Set personal weather minimums above the legal minimum — then apply them without exception. Self-imposed pressure (passengers, schedules, cost) kills pilots. The aircraft will always be there tomorrow.
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1
Visibility: Is forecast visibility above VFR minima for all route segments, including at destination and any intended alternate? Account for deterioration in forecast.
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2
Cloud base: Is the cloud base high enough to maintain terrain clearance and VMC separation throughout the route, including in valleys or areas of rising ground?
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3
Precipitation: Will you encounter rain, snow, or hail? Is carburettor heat available and functional? Is icing possible inside cloud at your planned altitude?
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4
Icing: Is the temperature between +5°C and −20°C in cloud at any point on route? If yes and you do not have certified de-icing equipment, this is a no-go.
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5
Turbulence: Are SIGMETs or PIREPs reporting moderate or severe turbulence on your route? Is severe turbulence within aircraft structural limits and personal tolerance?
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6
Wind: Are surface winds within aircraft limits and your personal crosswind limit? Check both surface and en-route winds — consider the effect of wind at altitude on groundspeed and fuel.
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7
Thunderstorms: Is there any CB (cumulonimbus) activity forecast within 50 nm of your route? If yes — NO GO. Do not attempt to fly under, through, or between embedded thunderstorms VFR.
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8
Deteriorating weather: Is the trend improving or worsening? Do not launch into forecast deterioration hoping it will improve. If in doubt, do not go.
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9
Alternate: Is your destination forecast above limits for your planned arrival time? Do you have a suitable alternate that is forecast to remain VMC? Have you filed alternate fuel?
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10
Personal minimums: Have you set personal minimums above the legal minima? Apply them on every flight, without exception. Personal minimums should be reviewed and adjusted as experience grows.
METAR Interpretation Examples
Ten real-world METARs decoded with operational context. Five worked examples are presented here; additional examples are available in the interactive METAR decoder.