Aviation weather essentials

What the weather means

Weather is one of the most important factors affecting flight safety. It influences aircraft performance, route planning, fuel consumption, takeoff and landing distances, and operational decisions. Pilots, dispatchers, air traffic controllers and maintenance personnel all rely on weather information to identify hazards before they become threats. Understanding how to interpret weather reports and recognize dangerous conditions is a core aviation skill.

The goal is not simply to read the weather. It is to understand what the weather means for aircraft operations.

METAR

Aircraft crews primarily use two types of airport weather products: the METAR and the TAF. A METAR is a routine aviation weather observation describing current conditions at an airport. METARs are usually issued hourly, though special reports may be issued whenever significant weather changes occur. Think of a METAR as a weather snapshot at a specific moment. It provides information such as:

• Wind direction and speed
• Visibility
• Cloud cover
• Temperature
• Dew point
• Altimeter setting
• Significant weather phenomena

TAF

A terminal aerodrome forecast (TAF) predicts weather conditions around an airport for a specified period, typically 24 or 30 hours. Think of a TAF as the airport's weather schedule for the coming day. A TAF includes forecasts for:

• Wind
• Visibility
• Clouds
• Weather phenomena
• Temporary or expected changes

Reading a METAR

Consider this example: KSFO 021856Z 29015G25KT 10SM FEW015 BKN200 18/12 A2992. A pilot can quickly visualize the weather from this compact report. Breaking it down:

• KSFO: San Francisco International Airport.
• 021856Z: observation made on the 2nd day of the month at 1856 UTC.
• 29015G25KT: wind from 290° at 15 knots, gusting to 25 knots.
• 10SM: visibility 10 statute miles.
• FEW015: few clouds at 1,500 ft.
• BKN200: broken cloud layer at 20,000 ft.
• 18/12: temperature 18°C, dew point 12°C.
• A2992: altimeter setting 29.92 inHg.

Temperature and dew point

One of the most important weather relationships in aviation is the spread between temperature and dew point. When the two values are close together, the air is nearly saturated. For example, with a temperature of 10°C and a dew point of 9°C, fog or low clouds may form easily. The smaller the spread, the greater the risk of reduced visibility. Conditions become favorable for:

• Fog formation
• Low cloud development
• Reduced visibility
• Dew and frost formation

High and low pressure systems

Many aviation weather hazards are associated with fronts and pressure systems.

• High-pressure systems generally bring stable air, good visibility, lighter winds and reduced cloud cover. They are often associated with favorable flying conditions.
• Low-pressure systems usually bring rising air, clouds, precipitation, stronger winds and turbulence. Many significant weather events are associated with low-pressure areas.

Fronts

A front is the boundary between two different air masses. Understanding frontal weather helps crews anticipate changing conditions along a route.

• Cold fronts often produce thunderstorms, gusty winds, turbulence and rapid weather changes.
• Warm fronts commonly produce extensive cloud layers, widespread precipitation, icing conditions and reduced visibility.

Thunderstorms

Thunderstorms are among aviation's most dangerous weather hazards. No aircraft is designed to safely operate through the most intense portions of a thunderstorm. A thunderstorm contains multiple hazards simultaneously, including:

• Severe turbulence
• Hail
• Lightning
• Heavy rainfall
• Wind shear
• Microbursts
• Icing
• Strong updrafts and downdrafts

The thunderstorm life cycle

• Cumulus stage: growing clouds, strong updrafts and developing instability. The storm is building energy.
• Mature stage: the most dangerous stage, with heavy rain, lightning, hail, severe turbulence, and powerful updrafts and downdrafts. Most aviation weather hazards occur during this phase.
• Dissipating stage: the storm begins weakening. Downdrafts dominate and precipitation gradually decreases. Although the storm is weakening, turbulence and wind hazards may still exist.

Microbursts

A microburst is an intense, localized downdraft that impacts the ground and spreads outward. An aircraft on approach may encounter, in sequence:

• A sudden headwind increase.
• Increased lift.
• An airspeed gain.
• A strong downdraft.
• A rapidly developing tailwind.
• Significant airspeed loss.

Avoid by a wide margin

This combination can be extremely dangerous close to the ground. For this reason, pilots avoid thunderstorms by a substantial margin rather than attempting to fly through them.

Icing

Icing can significantly degrade aircraft performance and controllability. Even relatively small ice accumulations can:

• Increase drag
• Reduce lift
• Increase stall speed
• Add weight
• Restrict flight controls
• Affect engine performance

Conditions required for structural icing

Supercooled water remains liquid below 0°C until it contacts the aircraft and freezes. Structural icing generally requires:

• Visible moisture
• Aircraft surface temperatures at or below freezing
• Supercooled water droplets

Types of ice

• Clear ice: forms from larger supercooled droplets. It is smooth, transparent or glass-like, dense, heavy and difficult to remove, and it spreads beyond protected surfaces. Clear ice is particularly hazardous because it significantly alters wing shape.
• Rime ice: forms from small supercooled droplets. It is rough, opaque and milky white, and builds primarily on leading edges. While lighter than clear ice, it can still seriously affect performance.
• Mixed ice: combines characteristics of both clear and rime ice. It is often irregular in shape, difficult to predict and difficult to remove, and is frequently considered one of the most hazardous icing conditions.

Supercooled large droplet (SLD) icing

SLD icing can form beyond the areas protected by deicing or anti-icing systems. It is particularly dangerous because it may exceed the aircraft's certified icing protection capability. This can result in ice accumulation:

• Behind wing leading edges
• Behind tail leading edges
• Outside protected zones

Wind shear

Wind shear is a rapid change in wind speed, wind direction, or both over a short distance. It can occur:

• Near thunderstorms
• Around frontal systems
• In temperature inversions
• Near mountainous terrain
• Around jet streams

Low-level wind shear

The most dangerous type occurs close to the ground during takeoff, final approach and landing. During these phases, aircraft have limited altitude and time available for recovery. Because of this risk, many airports use specialized wind-shear detection systems.

Turbulence

Turbulence ranges from mild bumps to severe aircraft motion. Common causes include:

• Convective turbulence: associated with thunderstorms, strong surface heating and convective activity.
• Mechanical turbulence: caused by airflow over buildings, hills, mountains and terrain obstacles.
• Clear air turbulence (CAT): occurs without visible cloud indications, often near jet streams, upper-level fronts and strong wind gradients. It can be difficult to predict because it may occur in otherwise clear skies.

Visibility hazards

Reduced visibility affects both visual and instrument operations. Common causes include:

• Fog: the most significant visibility hazard. It can reduce visibility dramatically while creating relatively calm surface conditions.
• Rain: heavy rain may reduce visibility and increase runway contamination.
• Snow: creates both visibility and surface contamination hazards.
• Smoke, dust and haze: these can significantly reduce visual references, especially near the horizon.

Instrument approach minima

When weather falls below published minimums, operations may be delayed, diverted or canceled. Low visibility directly affects:

• Takeoff requirements
• Landing minimums
• Alternate airport requirements

Aircraft performance and weather

Weather affects aircraft performance even when visibility is excellent.

• Hot temperatures: hot air is less dense, resulting in reduced engine performance, reduced propeller efficiency, reduced lift and longer takeoff distances.
• High-altitude airports: higher elevations create the same effect because air density decreases with altitude.

Hot-and-high conditions

Combining high airport elevation with high temperatures creates particularly challenging performance conditions. Aircraft may require:

• Longer runways
• Reduced payload
• Modified departure procedures

Key takeaways

• METARs report current weather. TAFs forecast future weather.
• A small temperature-dew point spread often indicates potential fog or low cloud formation.
• Thunderstorms contain multiple hazards, including turbulence, hail, lightning, wind shear and microbursts.
• Structural icing generally requires visible moisture and freezing temperatures.
• Clear, rime, mixed and SLD icing affect aircraft differently but all reduce performance.
• Low-level wind shear is particularly dangerous during takeoff and landing.
• Clear air turbulence may occur without visible warning.
• Poor visibility influences flight planning, approach minimums and airport operations.
• Hot, high-density-altitude conditions significantly reduce aircraft performance.

The bottom line

Weather affects every phase of flight, from preflight planning to landing. Pilots must interpret METARs and TAFs accurately, identify developing hazards, and understand how weather impacts aircraft performance. Of all aviation weather threats, thunderstorms, icing, wind shear and low visibility remain among the most dangerous because they can rapidly reduce an aircraft's performance margins and challenge even experienced crews. Understanding these hazards is essential to making safe operational decisions.