Aviation Weather — Reading the Sky Before You Leave the Ground

Lesson 1 — The Atmosphere and Stability

All weather occurs in the troposphere — the lowest layer of the atmosphere, extending from the surface to approximately 36,000 ft at mid-latitudes. Temperature normally decreases with altitude at the standard lapse rate of approximately 2°C per 1,000 ft (3.5°F/1,000 ft). This normal temperature decrease is what allows weather to develop: as air rises, it cools, and eventually moisture condenses into clouds.

Atmospheric stability — the key to predicting weather type

Stability describes how the atmosphere responds when a parcel of air is displaced upward. This single concept predicts whether you'll see smooth stratus layers or towering cumulonimbus — whether your flight will be smooth or turbulent.

Unstable atmosphere: When the actual temperature decreases faster than the standard lapse rate, rising air remains warmer than the surrounding environment and continues to rise on its own. This self-sustaining ascent builds cumulus clouds — the puffy, vertical clouds. In extreme instability, cumulus towers into cumulonimbus. Unstable air means convective development: turbulence, showers, and potentially thunderstorms. The atmosphere is most unstable in summer afternoons when surface heating is greatest.

Stable atmosphere: When the actual temperature decreases more slowly than the standard lapse rate (or increases with altitude — an inversion), rising air quickly becomes cooler than its surroundings and sinks back. Stability suppresses vertical motion and keeps weather in horizontal layers: stratus, fog, haze, and widespread low ceilings. Stable conditions produce smooth flight but poor visibility and IFR ceilings that don't burn off quickly.

Temperature inversions

A temperature inversion occurs when temperature increases with altitude — a complete reversal of the normal lapse rate. Inversions create an extremely stable layer that acts as a lid on the atmosphere below it. Smoke, haze, fog, and pollutants become trapped beneath the inversion layer, often reducing visibility to IFR conditions at the surface while skies above the inversion are clear and smooth. Radiation inversions (the most common type) form overnight when the Earth's surface cools rapidly under clear skies. They typically dissipate 2–3 hours after sunrise as surface heating breaks them down.

Inversions also cause a phenomenon called low-level wind shear — a sudden change in wind speed or direction between the stable surface layer and the air above the inversion. Aircraft on approach or departure can encounter abrupt airspeed changes crossing through an inversion layer.

Lesson 2 — Frontal Systems and Their Weather

A front is the boundary between two air masses with different temperature, moisture, and density characteristics. Most significant weather in the continental US occurs along or near fronts. Understanding what type of front you're dealing with lets you predict what weather to expect — and from how far away.

Cold fronts — fast, violent, narrow

A cold front is the leading edge of a cold air mass actively undercutting and displacing warmer air. Cold fronts move faster than warm fronts — typically 20–35 knots — and produce weather concentrated in a narrow band extending only 50–100 miles either side of the surface front. The atmosphere is violently unstable along a cold front: cumulonimbus, thunderstorms, squall lines, heavy precipitation, and gusty winds are all possible.

The good news about cold fronts: they pass quickly. A cold front moving at 30 knots will clear a given location in 2–6 hours. After cold front passage, conditions improve rapidly — temperature drops, dew point drops, winds shift (veer — clockwise in the Northern Hemisphere), visibility improves dramatically, and ceilings rise. The cold, dry post-frontal air often produces the clearest, smoothest flying of the week.

The bad news: prefrontal weather can extend 100–200 miles ahead of the surface front position as warm moist air is destabilized. Don't assume that because you're "ahead of the front" you're in safe air.

Warm fronts — slow, broad, insidious

A warm front is the leading edge of warm air gradually overriding retreating cold air. Because warm air is less dense, it can't plow through cold air the way a cold front does — instead it rides up over the cold air at a very shallow angle, creating a cloud and precipitation shield that extends hundreds of miles ahead of the surface front.

The sequence of clouds as a warm front approaches from hundreds of miles away: cirrus (high, wispy ice crystals — the first warning sign, up to 1,000 miles ahead) → cirrostratus (thin veil creating halos around sun and moon) → altostratus (gray overcast, sun visible as if through frosted glass) → nimbostratus (dark, low, thick layer with continuous rain). By the time you see low ceilings and rain, the surface front may still be 200–300 miles away.

Lesson 3 — Clouds and What They Tell You

Clouds are the atmosphere's weather report — visible evidence of what the air is doing right now. Every cloud type tells you something specific about the conditions around it. Learning to read clouds is a skill that supplements all your weather products and works anywhere in the world, with no technology required.

Cloud classification by altitude and form

High clouds (above 20,000 ft): Composed primarily of ice crystals. Cirrus are thin, wispy streaks — beautiful to look at, but often the first indicator of an approaching warm front. Cirrostratus forms a thin veil across the sky that creates halos around the sun and moon — a reliable warm front sign. High clouds alone rarely ground VFR flights, but they demand your attention as weather scouts.

Middle clouds (6,500–20,000 ft): Altostratus is the gray uniform overcast that makes the sun look like it's behind frosted glass — rain is typically 12–24 hours away under a thickening altostratus. Altocumulus forms in waves or patches; when altocumulus castellanus (towers developing from the altocumulus layer) appears in the morning, afternoon thunderstorms are highly likely — this is one of the most reliable convective forecasting signs.

Low clouds (surface to 6,500 ft): Stratus is the flat, featureless gray layer that creates IFR conditions. It forms in stable air and can persist for days. Stratocumulus (lumpy rolls of gray cloud) is the most common cloud type — it creates overcast but often with higher ceilings than stratus. Nimbostratus is the dark, low, continuous-rain cloud — true IFR with poor visibility underneath.

Clouds with vertical development: Cumulus clouds range from harmless "fair weather cumulus" (flat bases, limited vertical extent) to threatening towering cumulus (TCU) to full cumulonimbus (CB — the thunderstorm cloud). The critical sign to watch: rate of vertical development. A cumulus that was 2,000 ft tall an hour ago and is now 8,000 ft is developing rapidly — give it room and time to develop fully before deciding whether to route around it.

Lesson 4 — Decoding METARs

The METAR (Meteorological Terminal Air Report) is the standard format for reporting current weather conditions at airports. Issued hourly (and as Special METARs when conditions change significantly), METARs are the most current and reliable source of actual conditions at a specific location. Every pilot must be able to decode one from memory.

Complete METAR decode — every group explained

VFR flight categories

Category	Ceiling	Visibility	Implication
VFR	Above 3,000 ft AGL	Greater than 5 SM	Normal VFR flight
MVFR	1,000–3,000 ft AGL	3–5 SM	Marginal — caution, above legal VFR minimums but low
IFR	500–999 ft AGL	1–3 SM	Below VFR minimums — instrument conditions
LIFR	Below 500 ft AGL	Less than 1 SM	Low IFR — severe restriction

Lesson 5 — TAFs, PIREPs, and Winds Aloft

Terminal Aerodrome Forecast (TAF)

A TAF provides a 24–30 hour forecast for conditions at a specific airport in METAR-like format. TAFs are issued four times daily (0000, 0600, 1200, 1800 UTC) and are the primary forecast tool for planning individual flights.

PIREPs — Pilot Reports

PIREPs (Pilot Reports) are the most valuable real-time weather intelligence available. A pilot who just flew through your planned route 30 minutes ago can tell you exactly what icing level they encountered, where turbulence was, what the cloud tops were, and what visibility was like — information no forecast model can match.

PIREPs are filed by pilots via radio to ATC or FSS and collected in the weather system. Routine PIREPs (UA) report normal conditions; urgent PIREPs (UUA) report hazardous conditions and are broadcast immediately. Always check recent PIREPs before flights where turbulence, icing, or convective activity is possible. A single PIREP reporting severe icing at your planned altitude is worth more than any model forecast for that flight.

Winds aloft forecast (FB Winds)

Winds aloft forecasts give predicted wind direction, speed, and temperature at various altitudes (3,000 through 53,000 ft) at specific reporting points. Key rules for reading winds aloft:

• Direction is true — not magnetic. Apply variation to convert to magnetic heading for navigation use.
• Calm or light and variable is coded as 9900 in the raw data.
• Speeds above 99 kts are coded by adding 50 to the direction and subtracting 100 from the speed (e.g., "7545" = direction 250°, speed 145 kts).
• No winds reported below 1,500 ft AGL or within 2,500 ft of the station elevation — surface conditions are too variable.
• Temperature is important for icing awareness — identify the altitude where temperature is near 0°C and avoid that layer in visible moisture.

Lesson 6 — AIRMETs and SIGMETs

Aviation weather advisories are issued by the Aviation Weather Center (AWC) to alert pilots to conditions that may be hazardous. Knowing the difference between AIRMET types and SIGMETs — and which ones require immediate action for VFR pilots — is a critical written test topic and a real-world safety skill.

AIRMETs — Airmen's Meteorological Information

AIRMETs are issued for conditions that may be hazardous to light aircraft and VFR operations, covering broad areas. They are updated every 6 hours with amendments as needed. There are three types, each with a distinct letter designator:

SIGMETs — Significant Meteorological Information

SIGMETs cover conditions hazardous to all aircraft — not just light aircraft. They are more serious than AIRMETs. Non-convective SIGMETs cover: severe turbulence, severe icing, volcanic ash, and tropical cyclones. They use the identifier WS followed by a number (e.g., SIGMET ROMEO 3).

Convective SIGMETs (WST) are issued for hazardous thunderstorm activity: severe thunderstorms (surface winds over 50 kts, hail at the surface 3/4" or larger, or tornadoes), lines of thunderstorms, and embedded thunderstorms. Convective SIGMETs are valid for up to 2 hours (6 hours for tropical systems).

Lesson 7 — Structural Icing

Structural icing forms when supercooled water droplets — liquid water existing at temperatures below 0°C — contact the airframe and freeze on impact. This is distinct from carburetor ice (covered in Module 3), which can form in warm temperatures. Structural icing requires both visible moisture and sub-freezing temperatures.

Why icing is so dangerous

Ice accumulation on an airframe is insidious and cumulative. Even small amounts of ice change the wing's aerodynamic profile in ways that have catastrophic consequences:

• Lift reduction: Ice disrupts smooth airflow over the upper wing surface, reducing lift — sometimes by 30% or more with only 1/4 inch of ice accumulation.
• Stall speed increase: Contaminated wing surfaces stall at higher angles of attack with less warning — stall speed can increase dramatically and unpredictably.
• Weight increase: Ice is heavy. A full coat of ice on a small GA aircraft can add hundreds of pounds.
• Drag increase: Ice roughens the surface enormously, increasing drag by 200% or more in severe cases.
• Propeller damage: Ice on propellers creates vibration and reduces thrust. Chunks of ice thrown off the prop can strike the fuselage.
• Pitot blockage: Ice blocking the pitot tube kills the airspeed indicator — hence pitot heat.

Icing risk conditions

The icing risk zone is defined by two simultaneous conditions: visible moisture (clouds, freezing rain, freezing drizzle) AND temperatures between approximately +2°C and −20°C. The +2°C upper threshold accounts for the cooling effect of evaporation — the aerodynamic heating that would prevent ice from forming above this temperature.

The highest risk zone is between 0°C and −10°C where supercooled large droplets (SLD) are most common. SLD produces the most severe icing because larger droplets spread further back on the wing surface before freezing, affecting areas the anti-ice system may not protect.

Icing types and their appearance

Clear ice (glaze ice) forms from freezing rain or large supercooled droplets. It is smooth, hard, and nearly transparent — difficult to see. It conforms closely to the airfoil shape initially but can form a dangerous horn or double horn shape on the leading edge. Clear ice is the most dangerous icing type because it is hardest to detect visually and spreads across more of the wing surface.

Rime ice forms from small supercooled droplets in stratiform clouds. It is white, opaque, and brittle — rough but confined mainly to the leading edge. Less aerodynamically disruptive than clear ice per unit thickness, but significant accumulations are still dangerous.

Mixed ice combines characteristics of both clear and rime, forming in conditions where droplet size varies. Common in flight through clouds near the freezing level.

Lesson 8 — Thunderstorms and the Go/No-Go Decision

Thunderstorm formation

A thunderstorm requires three ingredients acting simultaneously: sufficient moisture (water vapor to fuel cloud growth and precipitation), a lifting mechanism (a front, orographic lift, surface heating, or converging winds to force air upward), and atmospheric instability (the lapse rate allows air to continue rising once lifted). Remove any one ingredient and the thunderstorm cannot form or sustain itself.

Thunderstorm hazards and avoidance

Turbulence: Both inside and outside the thunderstorm. Clear air turbulence extends up to 20 nm from severe cells. The anvil cloud can extend hundreds of miles downwind — don't fly under an anvil assuming you're safe because the cell is far away.

Hail: Can be thrown outward from a storm up to 20 nm. Hailstones the size of golf balls or larger have been documented. Even small hail at 150 knots airspeed can destroy propellers, windshields, and leading edges.

Wind shear and microbursts: Thunderstorm outflows create dangerous low-level wind shear. A microburst is a concentrated downdraft that creates an outflow of wind in all directions at the surface — capable of producing wind shear of 100 kts in a 1.5-mile span. Aircraft on approach or departure encountering a microburst may lose airspeed so rapidly that recovery is impossible.

Lightning: Strikes can damage aircraft structure, avionics, and temporarily blind the pilot. Although modern aircraft are designed to survive lightning strikes, small GA aircraft are less protected than transport category aircraft.

The go/no-go weather decision framework

Weather go/no-go decisions fail most often not because pilots lack information, but because they allow external pressure to override what the information clearly indicates. The framework below separates the data collection from the decision to reduce rationalization.

Step 1 — Get a complete standard briefing. Call 1800wxbrief.com or use Leidos Flight Service. An app weather check is not a standard briefing. The briefer asks structured questions that ensure you receive all applicable information — AIRMETs, SIGMETs, TFRs, NOTAMs — that an app may not surface.

Step 2 — Check AIRMETs and SIGMETs for your route. Active Sierra on your route means IFR conditions are present. Active Zulu means icing. Convective SIGMET means thunderstorms. These are objective, operationally significant flags.

Step 3 — Review recent PIREPs. What are pilots actually reporting along your route right now? A pirep of severe turbulence at your planned altitude from 30 minutes ago is information no forecast can substitute for.

Step 4 — Assess the trend. Is weather improving or deteriorating? A ceiling at 1,800 ft that was 3,000 ft an hour ago is more concerning than a ceiling at 1,800 ft that was 1,500 ft an hour ago.

Step 5 — Identify alternates. Where would you divert if your destination goes IFR? Check their weather too.

Step 6 — Apply your personal minimums — the ones you set before you checked weather. If conditions are at or below your personal minimums, the decision is already made. No re-evaluation needed.

Step 7 — Give yourself explicit permission to cancel. Say it out loud if necessary: "The weather doesn't support this flight today." Landing is always an option. The sky sometimes isn't.