Weather Theory
AWH / PHAK Ch 12 • Theory, METAR/TAF, Briefings
Theory
Application
Key Takeaways
- Standard atmosphere at sea level is 15°C and 29.92" Hg; temperature falls ~2°C and pressure ~1" Hg per 1,000 ft.
- All weather forms in the troposphere (surface to ~36,000'); the tropopause caps it.
- Flying high-to-low pressure or warm-to-cold without resetting the altimeter leaves true altitude lower than indicated.
- Air-mass codes combine moisture + temperature — c/m (dry/moist) and P/T (polar/tropical); e.g. cP = cold and dry.
- Stable air brings stratiform clouds, steady precip, and a smooth ride; unstable air brings cumuliform clouds, showers, and turbulence.
Atmosphere Basics
Composition of the Atmosphere
- Nitrogen: 78%
- Oxygen: 21%
- Water vapor: 0-5% (variable)
- Decreases with altitude
Layers of the Atmosphere
- Troposphere: 0-36,000'
- All weather occurs here
- Temp lapse: 2°C/1,000'
- Tropopause: boundary layer
Standard Atmosphere (ISA)
- Sea level temp: 15°C/59°F
- Sea level pressure: 29.92" Hg
- Pressure lapse: ~1"/1,000'
- Base for performance charts
Pressure & Altimetry
- High→Low = lower than indicated
- Low→High = higher than indicated
- Hot→Cold = lower than indicated
- Always get current altimeter setting
Weather Theory
Air Masses
Large bodies of air with uniform temperature and moisture. Continental (c) = dry, Maritime (m) = moist, Polar (P) = cold, Tropical (T) = warm. Example: cP = continental polar (cold, dry).
- cP — Continental Polar: Cold, dry, stable
- cT — Continental Tropical: Hot, dry, unstable (summer)
- mP — Maritime Polar: Cool, moist, unstable
- mT — Maritime Tropical: Warm, moist, usually unstable
Fronts
Boundaries between air masses. Weather severity depends on temperature/moisture differences, front speed, and slope.
- Cold Front (▲▲▲ (blue), 25-30 mph) — Narrow band of intense weather, cumuliform clouds, gusty winds, rapid temp drop; visibility poor in precip, good after passage
- Warm Front (●●● (red), 10-15 mph) — Wide band (600+ mi) of steady precip, stratiform clouds, gradual temp rise, poor visibility, possible fog
- Stationary Front (▲●▲● (alternating), Little movement) — Similar to warm front but prolonged, can persist for days
- Occluded Front (▲●▲● (purple), Variable) — Complex weather, combines characteristics of cold and warm fronts
Stability vs Instability
Stable air resists vertical movement; unstable air promotes it. Stability drives cloud type, precipitation, and turbulence.
| Stable Air | Unstable Air |
|---|---|
| Stratiform clouds (layers) | Cumuliform clouds (vertical) |
| Steady precipitation | Showery precipitation |
| Smooth air (little turbulence) | Rough air (turbulence) |
| Continuous fog/haze | Good visibility (except in precip) |
| Poor visibility in smoke/haze | Fair to poor visibility in precip |
Lifting Mechanisms
Four mechanisms cause air to rise: Convective (surface heating), Orographic (terrain), Frontal (air mass boundaries), Convergence (air masses meeting).
- Convective — Surface heating: Thermals, cumulus clouds, afternoon storms
- Orographic — Terrain forcing: Mountain waves, updrafts/downdrafts
- Frontal — Air mass boundaries: Organized weather along fronts
- Convergence — Air masses meeting: Rising motion, cloud formation
Temperature Inversions
An inversion is when temperature INCREASES with altitude instead of the normal decrease. Inversions trap pollutants, fog, and haze below them — poor visibility near the surface, clear above.
- Surface (Radiation) Inversion — Forms on clear, calm nights as ground cools; traps radiation fog; breaks after sunrise
- Frontal Inversion — Warm air overriding cold air at a front; associated with warm front weather
- Subsidence Inversion — Descending air in a high warms; caps convection; traps haze/smog
Fronts in Cross-Section
The same four cities, spaced 600 miles apart, under each structural front type — watch how the cloud sequence, precipitation, and the surface chart and METARs change as the front moves through.
Key Takeaways
- Highs = descending, diverging air → clear skies, good visibility, and light winds; Lows = rising, converging air → clouds, precip, and turbulence.
- In the Northern Hemisphere air spirals clockwise and outward around a high, counter-clockwise and inward around a low (Coriolis deflects it right).
- Wind is driven by the pressure-gradient force: tight isobars mean strong wind, wide isobars mean light wind.
- Below ~2,000' AGL friction slows the surface wind and angles it ~30° across the isobars toward low pressure; winds aloft parallel the isobars.
- Flying high-to-low pressure (or hot-to-cold) without resetting the altimeter leaves you lower than indicated — "High to Low, Look Out Below."
Pressure Systems
How pressure drives weather
Atmospheric pressure is the weight of air above a given point. Differences in surface heating create pressure gradients that drive all large-scale weather. Knowing high and low pressure characteristics is key to anticipating weather along your route.
| Property | High Pressure (H) | Low Pressure (L) |
|---|---|---|
| Surface Air | Air descends and diverges at the surface. Descending air warms and dries, suppressing cloud formation. | Air converges at the surface and rises. Rising air cools, condenses, and forms clouds and precipitation. |
| Rotation | Clockwise (Northern Hemisphere) — air spirals outward | Counter-clockwise (Northern Hemisphere) — air spirals inward |
| Vertical Motion | Subsidence (descending air) — air compresses and warms | Convergence & lifting — air rises, cools, and condenses |
| Typical Weather | Clear skies, good visibility, calm/light winds, fair weather | Clouds, precipitation, reduced visibility, stronger winds, turbulence |
| Cloud Type | Few to none; subsidence inversion may trap haze/smog at lower levels | Cumuliform (unstable) or stratiform (stable lifting) — towering Cu/CB possible |
| Stability | Stable air — smooth flying conditions, stratiform clouds if any | Unstable air — bumpy/turbulent, convective activity likely |
Key Pressure Concepts
Coriolis, pressure gradient, friction, and altimeter errors
Coriolis Effect
Earth's rotation deflects moving air to the right in the Northern Hemisphere. That's why wind doesn't flow straight from high to low pressure — it curves, creating the clockwise/counter-clockwise circulation around pressure systems.
- Deflects right (NH)
- Stronger at higher speeds
- Zero at equator
- Max at poles
Pressure Gradient Force
Air always moves from high to low pressure. The closer the isobars (lines of equal pressure) on a surface analysis chart, the stronger the wind. Wide spacing = light winds, tight spacing = strong winds.
- Tight isobars = strong wind
- Wide isobars = light wind
- Drives all wind
Surface Friction & Wind
Below about 2,000' AGL, surface friction slows wind and angles it across isobars toward low pressure at roughly 30° over land. Above the friction layer, wind flows roughly parallel to the isobars — geostrophic where they're straight, gradient wind around curved highs and lows. That's why surface winds differ from winds aloft.
- Friction layer: ~2,000’ AGL
- Surface wind crosses isobars ~30°
- Aloft: parallel to isobars
Pressure & Altimeter Errors
Flying from high to low pressure without updating your altimeter setting makes it read higher than your true altitude — you're lower than you think. Remember: "High to Low, Look Out Below."
- High → Low pressure: True altitude is LOWER than indicated
- Low → High pressure: True altitude is HIGHER than indicated
- Hot → Cold temperature: Same effect — true altitude is lower
- “High to Low, Look Out Below”
- “HALT” — High Altimeter = Low Temperature
Diurnal (Daily) Pressure Changes
Daytime surface heating creates local low pressure (thermals, convection, afternoon thunderstorms). Overnight cooling creates local high pressure and stable conditions — which is why the smoothest VFR flying is typically early morning.
- Morning: calm, stable
- Afternoon: thermals, gusty
- Max heating ~3 PM local
- CB peak: late afternoon
Sea Breeze & Land Breeze
Differential heating between land and water creates local pressure differences and predictable coastal winds.
| Sea Breeze (Daytime) | Land Breeze (Nighttime) |
|---|---|
| Land heats faster → air rises over land → cooler air flows in from the sea. Wind blows from sea to land. | Land cools faster → air sinks over land → air flows out toward the warmer sea. Wind blows from land to sea. |
DPE Quick-Hitters
Common oral exam questions on pressure systems
Q: Which way does air circulate around a low in the Northern Hemisphere?
A: Counter-clockwise, inward, and upward.
Q: What kind of weather is associated with a high pressure system?
A: Generally fair weather — clear skies, good visibility, light winds. Subsidence can trap haze.
Q: You're flying from an area of high pressure into low pressure. What happens to your altimeter?
A: Your altimeter reads higher than your true altitude. You're lower than you think. "High to Low, Look Out Below."
Q: What causes wind?
A: Pressure gradient force (unequal heating of the Earth's surface), modified by Coriolis effect and surface friction.
Q: When would you expect the most turbulence on a clear day?
A: Mid-afternoon (~2-4 PM local) when surface heating and thermals are at their peak.
Local Wind Patterns
Terrain-Induced Winds
Sea/Land Breeze
Caused by differential heating between land and water.
| Day | Night |
|---|---|
| SEA BREEZE: During day, land heats faster than water. Air rises over land, cooler air from sea flows in. Wind blows from sea toward land. | LAND BREEZE: At night, land cools faster. Air sinks over land and flows toward warmer water. Wind blows from land toward sea. |
- Hazard: Can cause significant wind shifts near coastlines, especially during afternoon
Valley/Mountain Breeze
Caused by differential heating of mountain slopes.
| Day | Night |
|---|---|
| VALLEY BREEZE (Anabatic): Sun heats slopes, air rises along mountainsides. Wind flows UP the valley. | MOUNTAIN BREEZE (Katabatic): Slopes cool rapidly, cold dense air drains DOWN into valley. |
- Hazard: Katabatic winds can be strong and gusty; valley airports may have morning low clouds/fog from pooled cold air
Chinook/Foehn Winds
Warm, dry downslope winds on the lee side of mountains.
| Day | Night |
|---|---|
| Moist air forced up windward side loses moisture through precipitation. Descending on lee side, air warms at dry adiabatic rate (faster than it cooled). | Can occur day or night when conditions exist. |
- Hazard: Very rapid temperature changes (40-50°F in hours), gusty turbulent conditions, rapid snow melt
Mountain Waves
Standing waves in the atmosphere, like water waves behind a rock in a stream.
| Day | Night |
|---|---|
| Form when stable air flows over mountains at 25+ knots perpendicular to ridgeline. Waves extend downwind and can reach extreme altitudes. | Can persist as long as wind conditions exist. |
- Hazard: SEVERE turbulence in rotor zone (below mountaintop level on lee side), strong updrafts/downdrafts, lenticular clouds indicate presence
Key Takeaways
- Density altitude is pressure altitude corrected for temperature — high temperature, high elevation, and high humidity all raise it and cut performance.
- High density altitude means a longer takeoff and landing roll, reduced climb, less engine power, and a higher true airspeed for the same indicated airspeed.
- On hot days at elevation, density altitude can run 2,000–4,000' above field elevation — always calculate it before flight.
- When temperature and dewpoint converge (a small spread), the air nears saturation and fog or low clouds become likely.
- Estimate cloud base AGL ≈ temp–dewpoint spread (°C) × 400 ft, or (°F) × 230 ft.
Density Altitude
Pressure altitude corrected for non-standard temperature — the altitude the aircraft "feels" it is flying at, based on air density.
Formula
DA = PA + (120 × (OAT - ISA Temp))
Factors Affecting Density Altitude
- High Temperature — ↑ Density Altitude; Air is less dense, reduced performance
- High Altitude — ↑ Density Altitude; Less air molecules, reduced lift/power
- High Humidity — ↑ Density Altitude; Water vapor is lighter than dry air
- Low Pressure — ↑ Density Altitude; Fewer air molecules
Performance Effects
- Longer takeoff roll required
- Reduced rate of climb
- Higher true airspeed for same indicated airspeed
- Longer landing roll
- Reduced engine power output (normally aspirated)
- Reduced propeller efficiency
Temperature-Dewpoint Relationship
When temperature equals dewpoint, the air is saturated (100% relative humidity) and condensation occurs. The "spread" between them shows how close the air is to saturation.
Key Rules:
- Temp-Dewpoint Spread: Small spread (≤5°F) indicates high humidity and likely fog/low clouds
- Cloud Base Estimation: Cloud base (AGL) ≈ Spread (°C) × 400 ft OR Spread (°F) × 230 ft (based on the ~2.5°C/1,000 ft temperature–dewpoint convergence rate, not the standard lapse rate)
- Fog Formation: Fog likely when spread is 5°F or less and decreasing, especially with calm winds
- Dew vs Frost: Dew forms when dewpoint is above 32°F; frost forms when dewpoint is at or below 32°F
Examples:
- Temp 22°C, DP 18°C, Spread 4°C — High humidity, potential fog. Cloud base ≈ 1,600' AGL
- Temp 25°C, DP 10°C, Spread 15°C — Low humidity, no fog risk. Cloud base ≈ 6,000' AGL
Key Takeaways
- Clouds are grouped by height: high (cirro-, 16,500–45,000'), middle (alto-, 6,500–23,000'), low (strato-, surface–6,500'), and clouds with vertical development (cumulo-).
- High cirrus and cirrostratus (sun/moon halo) often precede a warm front by 12–24 hours.
- Low stratus and nimbostratus bring low ceilings, poor visibility, and steady precipitation.
- Cumuliform clouds signal instability; towering cumulus means turbulence.
- Cumulonimbus (Cb) is the thunderstorm cloud — severe turbulence, hail, icing, lightning, and microbursts.
- Lenticular and rotor clouds mark mountain-wave activity — expect severe turbulence near terrain, even in smooth-looking air.
- Cloud shape reads the air: layered (stratiform) = stable, smooth, steady precip, rime icing; heaped (cumuliform) = unstable, turbulent, showery, clear icing.
Cloud Types by Altitude
Appearance and hazard, by family
| Cloud Type | Altitude | Appearance | Hazard |
|---|---|---|---|
| Cirrus (Ci) | 16,500-45,000'Cirro- | Thin, wispy, ice crystals, "mare's tails" | Generally none, may indicate approaching front |
| Cirrocumulus (Cc) | 16,500-45,000'Cirro- | Small white puffs, "mackerel sky" | May indicate approaching weather |
| Cirrostratus (Cs) | 16,500-45,000'Cirro- | Thin sheet, sun/moon halo | Often precedes warm front by 12-24 hours |
| Altostratus (As) | 6,500-23,000'Alto- | Gray/blue sheet, sun dimly visible | May produce light precip, often precedes warm front |
| Altocumulus (Ac) | 6,500-23,000'Alto- | White/gray patches, "sheep in sky" | If castellanus (tower) type = instability aloft |
| Stratus (St) | Surface-6,500'Strato- | Uniform gray layer, drizzle | Low ceilings, reduced visibility |
| Stratocumulus (Sc) | Surface-6,500'Strato- | Lumpy gray layer | Generally light turbulence, light precip |
| Nimbostratus (Ns) | Surface-6,500'Strato- | Dark gray, continuous precip | Low ceilings, poor visibility, steady rain/snow |
| Cumulus (Cu) | VerticalCumulo- | Puffy, flat base, vertical growth | Fair weather = little hazard, towering = turbulence |
| Cumulonimbus (Cb) | VerticalCumulo- | Thunderstorm cloud, anvil top | SEVERE: turbulence, hail, icing, lightning, microbursts |
Cirrus (Ci)
High Clouds · Cirro-
Thin, wispy, ice crystals, "mare's tails"
Hazard: Generally none, may indicate approaching front
Cirrocumulus (Cc)
High Clouds · Cirro-
Small white puffs, "mackerel sky"
Hazard: May indicate approaching weather
Cirrostratus (Cs)
High Clouds · Cirro-
Thin sheet, sun/moon halo
Hazard: Often precedes warm front by 12-24 hours
Altostratus (As)
Middle Clouds · Alto-
Gray/blue sheet, sun dimly visible
Hazard: May produce light precip, often precedes warm front
Altocumulus (Ac)
Middle Clouds · Alto-
White/gray patches, "sheep in sky"
Hazard: If castellanus (tower) type = instability aloft
Stratus (St)
Low Clouds · Strato-
Uniform gray layer, drizzle
Hazard: Low ceilings, reduced visibility
Stratocumulus (Sc)
Low Clouds · Strato-
Lumpy gray layer
Hazard: Generally light turbulence, light precip
Nimbostratus (Ns)
Low Clouds · Strato-
Dark gray, continuous precip
Hazard: Low ceilings, poor visibility, steady rain/snow
Cumulus (Cu)
Clouds with Vertical Development · Cumulo-
Puffy, flat base, vertical growth
Hazard: Fair weather = little hazard, towering = turbulence
Cumulonimbus (Cb)
Clouds with Vertical Development · Cumulo-
Thunderstorm cloud, anvil top
Hazard: SEVERE: turbulence, hail, icing, lightning, microbursts
Stratiform vs Cumuliform
What the cloud's shape tells you about the air
| Trait | Stratiform — layered | Cumuliform — heaped |
|---|---|---|
| Air | Stable — resists vertical motion | Unstable — rises freely |
| Shape | Flat, uniform sheets and layers | Puffy, heaped, vertical towers |
| Ride | Smooth | Turbulent, bumpy |
| Precipitation | Steady, continuous, widespread | Showery, intermittent, localized |
| Visibility | Often poor — haze trapped below | Good between buildups |
| Icing | Rime — small droplets, long exposure | Clear / mixed — large droplets, severe but brief |
| Ceilings | Low, uniform | Higher, variable bases |
Reading the Sky
The free forecast overhead — what each cloud is telling you
| In the sky | What it’s telling you |
|---|---|
| Halo around the sun or moon (cirrostratus) | A warm front is approaching — weather often within 12–24 hours. |
| Cumulus building tall by late morning | Unstable air — expect afternoon turbulence and possible thunderstorms. |
| Stratus lowering and thickening | Ceilings dropping — VFR deteriorating toward IFR. |
| Smooth, lens-shaped clouds near mountains (lenticular) | A mountain wave aloft — severe turbulence, even in clear, calm-looking air. |
| Ragged, churning clouds in a ridge’s lee (rotor) | Violent low-level turbulence — avoid. |
| Pouches sagging under a storm anvil (mammatus) | Severe turbulence in and around the storm. |
| Low, ragged, fast-moving shreds (scud) | Low ceilings and reduced visibility, usually with precipitation. |
Cloud Reports (METAR)
Coverage, height, and significant types
Coverage (Oktas)
| Code | Sky cover |
|---|---|
| SKC/CLR | Clear (0/8) |
| FEW | 1–2/8 |
| SCT | 3–4/8 |
| BKN | 5–7/8 (ceiling) |
| OVC | 8/8 (ceiling) |
| VV | Vertical vis (obscured) |
Heights
| Example | Decodes to |
|---|---|
| FEW045 | Few at 4,500′ AGL |
| SCT100 | Scattered at 10,000′ AGL |
| BKN012 | Broken at 1,200′ (ceiling) |
| OVC008 | Overcast at 800′ (ceiling) |
Hundreds of feet AGL. Ceiling = lowest BKN/OVC/VV layer.
Significant Types
| Tag | Means |
|---|---|
| CB | Cumulonimbus (thunderstorm) |
| TCU | Towering cumulus (building) |
Appended to a layer, e.g. BKN040CB.
Key Takeaways
- Thunderstorms need moisture, a lifting mechanism, and unstable air — stay 20 NM from severe cells and never fly under or between them.
- Structural icing needs visible moisture plus a surface at or below 0°C; clear (glaze) ice is the most dangerous, and most trainers aren't certified for known icing.
- Carb icing can strike on warm, humid days (20–70°F) — use full carb heat; fuel-injected engines aren't susceptible.
- Fog forms when the temp/dewpoint spread is ≤5°F; radiation fog burns off by mid-morning, but advection fog can persist for days.
- Wind shear — especially thunderstorm microbursts — can suddenly rob airspeed and altitude on approach; go around immediately.
Weather Hazards
Key hazards every pilot must recognize and avoid
Cause:
Needs three ingredients: moisture, a lifting mechanism (front, terrain, convection), and unstable air (steep lapse rate)
Stages:
- Cumulus Stage — updrafts only, building cumulus towers, rain not yet reaching surface. Typically 15 min.
- Mature Stage — strongest updrafts AND downdrafts coexist. Most severe weather: heavy rain, hail, lightning, microbursts, possible tornadoes. First rain on the surface marks this stage.
- Dissipating Stage — downdrafts dominate, rain decreasing, anvil top spreads. Still hazardous (wind shear, turbulence beneath).
Hazards:
- Embedded thunderstorms (hidden in cloud layers) are especially dangerous for VFR pilots
- Squall lines can extend hundreds of miles with no gaps
- If caught: maintain attitude, keep wings level, accept altitude changes, reduce to maneuvering speed (Va)
- The gust front can precede the storm by 15+ miles on the surface
Cause:
Needs two conditions at once: (1) visible moisture (clouds, rain, drizzle) and (2) aircraft surface temperature at or below 0°C
Large supercooled drops, 0°C to -10°C. Most common in rain or near fronts
Most dangerous — hard, heavy, transparent, conforms to the surface, hard to see and remove. Can double drag quickly.
Small supercooled drops, -15°C to -20°C. Common in stratiform clouds
Milky white, rough, granular. Disrupts airflow over wings. Easier to remove than clear ice but accumulates fast in dense clouds.
Variable drop sizes, temps near -10°C to -15°C
Combination of clear and rime characteristics. Rough with some clear areas.
Forms on cold-soaked aircraft when OAT is near dewpoint and below freezing
Not structural icing per se, but frost as thin as sandpaper can cut lift by 30% and raise stall speed. Must be removed before flight.
Effects on Aircraft:
- Increased weight and drag
- Decreased lift (airfoil shape disrupted)
- Increased stall speed
- Reduced engine power (blocked air intake)
- Reduced propeller efficiency
- Pitot/static system blockage → erroneous instruments
- Most single-engine trainers are NOT certified for flight into known icing — exit immediately
- Climb or descend to a different altitude to find warmer air (above freezing)
- Turn on pitot heat in potential icing conditions
- If ice accumulates: increase speed to compensate for higher stall speed, land ASAP
- Use carb heat proactively — carb icing can occur even without visible structural icing
Cause:
Fuel vaporization absorbs heat + venturi effect drops pressure/temperature. Combined effect can drop the air temperature by up to 40°F (22°C) inside the carburetor, causing moisture to freeze on the throttle plate.
Temperature Range:
OAT 20°F to 70°F (-7°C to 21°C), most likely 50–70°F with high humidity. Can occur even on warm summer days!
Signs:
Fixed-pitch prop: gradual RPM decrease. Constant-speed prop: manifold pressure drop. Engine roughness, eventual power loss if uncorrected.
Types of Carb Icing:
Most common. Forms on the throttle plate/butterfly valve, especially at low power settings.
Occurs as fuel vaporizes and absorbs heat in the venturi.
Visible moisture freezes on the air filter/intake. Occurs when flying through freezing rain or wet snow.
Cause:
Forms when temperature and dewpoint converge (spread ≤ 5°F). Fog is essentially a surface-level cloud with visibility below 1 SM.
Clear night skies, calm winds (< 5 kts), moist air, small temp/dewpoint spread
Forms after sunset as the ground cools by radiation. Thickest near dawn, burns off within a few hours of sunrise. Most common in valleys and low areas.
Warm, moist air moves over a cold surface (land or water). Winds 5–15 kts.
Can form any time — day or night. Can persist for days. Very common along the coast and Great Lakes. Can be carried inland by wind.
Moist air forced upward along a terrain slope, cooling adiabatically to dewpoint
East slopes of the Rockies are notorious. Can extend for hundreds of miles.
Very cold air passes over much warmer water (temp difference > 20°F)
Arctic regions, early mornings over lakes in autumn. Usually thin and low.
Warm rain from aloft falls through cold surface air, raising humidity to saturation
Often tied to approaching warm fronts. Can form quickly ahead of the front.
- Monitor temp/dewpoint spread — if ≤ 4°F and converging, expect fog formation
- Radiation fog: if reporting "FG" at your destination at dawn, expect improvement by mid-morning
- Advection fog: DO NOT expect it to burn off — it can persist or thicken
- If caught above fog: divert to a fog-free field. Do not attempt to descend through fog VFR.
- Cloud base formula: (Temp – Dewpoint in °F) × 230 = approximate cloud base AGL
Cause:
A sudden change in wind speed and/or direction over a short distance, horizontally or vertically
Sources:
- Thunderstorm microbursts — most dangerous. Downdraft spreads outward on contact with ground, creating headwind-to-tailwind shift.
- Frontal boundaries — especially fast-moving cold fronts with 30+ kt wind speed difference across the front
- Temperature inversions — wind speed/direction can change dramatically across the inversion layer
- Low-level jets — narrow bands of strong winds (30–60 kts) at 1,000–3,000' AGL, common at night in the Central U.S.
- If you suspect wind shear on approach: go around immediately — do not try to salvage the approach
- PIREPs of wind shear are critical — report any encounter to ATC
- Avoid takeoff and landing within 30 minutes of a thunderstorm passage
- LLWAS (Low-Level Wind Shear Alert System) warnings from ATC should be taken very seriously
Cause:
Irregular air motion from eddies and vertical currents, driven by various atmospheric phenomena.
Surface heating creates rising air columns. Strongest on hot summer afternoons over dark surfaces (parking lots, plowed fields). Eases after sunset.
Wind flowing over surface obstacles (buildings, trees, terrain). Worst with strong winds. Creates eddies downwind.
Temperature and wind differences across a front. Worst with fast-moving cold fronts. Narrow band of turbulence.
Wind shear near the high-altitude jet stream. Can occur in clear skies with no visual warning. Mainly affects jets, but important to understand.
Strong winds (>25 kts) perpendicular to mountain ridges create oscillating waves downwind. Lenticular clouds mark the crests. Rotor turbulence below the wave crests is most severe — can be extreme.
Wingtip vortices from heavy aircraft. Greatest behind heavy, clean (gear up), slow aircraft. Vortices sink ~500 fpm and drift with the wind. Wait 3 minutes or extend upwind.
Intensity Levels:
- Light: slight erratic changes in altitude/attitude. Occupants may feel slight strain against belts.
- Moderate: changes in altitude/attitude occur but aircraft remains in positive control. Occupants feel definite strain against belts.
- Severe: large, abrupt changes in altitude/attitude. May momentarily lose control. Objects thrown about. Occupants forced violently against belts.
- Extreme: aircraft violently tossed, practically impossible to control. May cause structural damage.
- Reduce to maneuvering speed (Va) in turbulence — protects the airframe
- Keep wings level, maintain attitude, accept altitude variations
- Convective turbulence: fly in the morning before surface heating builds
- Mountain wave: stay well above ridge level (at least 50% above ridge height), approach ridges at a 45° angle for easy escape
- Wake turbulence avoidance: land beyond the heavy aircraft's touchdown point, take off before their rotation point
Cause:
Eruptions inject fine particulate matter that can travel thousands of miles. Invisible at night or in clouds.
Ash melts in the combustion chamber and solidifies on turbine blades. Even piston engines lose power from abrasive ash.
Abrasive particles sandblast the windscreen, cutting visibility to near-zero.
Fine ash can block pitot tubes and static ports, causing instrument failures.
Cause:
High temperature, high altitude, and high humidity reduce air density — the aircraft performs as if at a much higher altitude.
Effects on Aircraft:
- Longer takeoff roll (reduced lift and engine power)
- Reduced rate of climb
- Higher true airspeed for same indicated airspeed
- Longer landing roll at destination
- Engine power output decreases (less dense air for combustion)
- At a 1,200 ft field on a 95°F day, density altitude can easily exceed 3,500 ft
- Use the Koch Chart or E6B to calculate density altitude before every summer flight
- Fly early morning when it's cooler — density altitude can drop 1,000' or more
- Reduce weight if performance is marginal — less fuel, fewer passengers
- Do not exceed max gross weight and expect normal performance in high DA
Key Takeaways
- METAR is the routine hourly observation; SPECI is an off-schedule update issued when conditions change significantly.
- TAF is the terminal forecast (valid 24–30 hours) and uses FM (rapid change), TEMPO, BECMG, and PROB to describe timing.
- Codes stack intensity + descriptor + phenomenon — e.g. '-RA' is light rain, 'FZRA' freezing rain, '+TSRA' a heavy thunderstorm with rain.
- Flight categories: VFR (>3,000' & >5 SM), MVFR (1,000–3,000' and/or 3–5 SM), IFR (500–999' and/or 1–3 SM), LIFR (<500' and/or <1 SM).
- AIRMETs: Sierra = IFR/mountain obscuration, Tango = turbulence, Zulu = icing; SIGMETs warn of severe hazards for all aircraft.
Flight Categories
| Category | Ceiling | Visibility |
|---|---|---|
| VFR | > 3,000′ | > 5 SM |
| MVFR | 1,000–3,000′ | 3–5 SM |
| IFR | 500–999′ | 1–3 SM |
| LIFR | < 500′ | < 1 SM |
The category is the more restrictive of the two — VFR requires both ceiling AND visibility.
Ceiling >3,000' AND visibility >5 SM
Ceiling 1,000-3,000' AND/OR visibility 3-5 SM
Ceiling 500-999' AND/OR visibility 1-3 SM
Ceiling <500' AND/OR visibility <1 SM
AIRMET Types
Advisories for light aircraft - moderate intensity hazards
| AIRMET | Meaning | Mnemonic |
|---|---|---|
| AIRMET Sierra | IFR conditions and mountain obscuration | S = See (visibility) |
| AIRMET Tango | Turbulence (moderate) and sustained surface winds ≥30 kts | T = Turbulence |
| AIRMET Zulu | Icing (moderate) and freezing level heights | Z = Zero (freezing) |
IFR conditions and mountain obscuration
Turbulence (moderate) and sustained surface winds ≥30 kts
Icing (moderate) and freezing level heights
Weather Products Reference
| Product | Description | Validity |
|---|---|---|
| METAR | Routine hourly aviation weather observation | Current conditions |
| SPECI | Special METAR when conditions change significantly | Current conditions |
| TAF | Terminal Aerodrome Forecast | 24-30 hours |
| AIRMET (WA) | Hazards for light aircraft (moderate) | 6 hours |
| SIGMET (WS) | Significant hazards for ALL aircraft | 4 hours (6 for volcanic ash) |
| Convective SIGMET (WST) | Severe thunderstorms, tornadoes | 2 hours |
| PIREP (UA/UUA) | Pilot weather report (routine/urgent). PIREPs are the ONLY direct confirmation of turbulence, icing, and cloud tops. File via ATC or FSS - your reports help other pilots! | Time of observation |
| Graphical Forecast for Aviation (GFA) | Web-based graphical regional forecast at aviationweather.gov; replaced the textual Area Forecast (FA) in 2019 | Hourly snapshots out to 18 hours |
| Winds/Temps Aloft (FB) | Forecast winds and temperatures at altitude | 6, 12, 24 hours |
| Prog Charts | Surface and significant weather prognostics | 12, 24, 36, 48 hours |
Routine hourly aviation weather observation
Special METAR when conditions change significantly
Terminal Aerodrome Forecast
Hazards for light aircraft (moderate)
Significant hazards for ALL aircraft
Severe thunderstorms, tornadoes
Pilot weather report (routine/urgent). PIREPs are the ONLY direct confirmation of turbulence, icing, and cloud tops. File via ATC or FSS - your reports help other pilots!
Web-based graphical regional forecast at aviationweather.gov; replaced the textual Area Forecast (FA) in 2019
Forecast winds and temperatures at altitude
Surface and significant weather prognostics
Sample METAR Decode
Flight Category: VFR
Ceiling >3,000' (few at 4,500'), Visibility >5 SM (10 SM)
METAR Weather Codes
Decoding Present Weather
Intensity
| Code | Meaning |
|---|---|
| - | Light |
| (no symbol) | Moderate |
| + | Heavy |
| VC | Vicinity (5-10 SM from station) |
Descriptors
| Code | Meaning |
|---|---|
| MI | Shallow |
| PR | Partial |
| BC | Patches |
| DR | Low drifting |
| BL | Blowing |
| SH | Showers |
| TS | Thunderstorm |
| FZ | Freezing |
Precipitation
| Code | Meaning |
|---|---|
| RA | Rain |
| SN | Snow |
| DZ | Drizzle |
| GR | Hail (>1/4") |
| GS | Small hail/snow pellets |
| PL | Ice pellets |
| IC | Ice crystals |
Obscuration (Visibility Reducers)
| Code | Meaning |
|---|---|
| BR | Mist (vis 5/8-6 SM) |
| FG | Fog (vis <5/8 SM) |
| FU | Smoke |
| HZ | Haze |
| DU | Dust |
| SA | Sand |
Other Phenomena
| Code | Meaning |
|---|---|
| SQ | Squall |
| FC | Funnel cloud/tornado |
| SS | Sandstorm |
| DS | Duststorm |
Example Decodes:
-RA = Light rain
+TSRA = Heavy thunderstorm with rain
VCSH = Showers in vicinity
FZRA = Freezing rain
BR = Mist (moderate visibility)
TAF Change Indicators
Understanding Forecast Changes
Sample TAF Decode
Aviation Municipal (illustrative), issued 19th at 1720Z, valid 19th 1800Z to 20th 1800Z
Wind 270° at 12kt gusting 20kt, visibility >6SM, few clouds at 4,000'
FROM 20th at 0200Z: wind 310° at 8kt, >6SM, scattered at 8,000'
TEMPORARILY between 0800-1200Z: 4SM vis, light rain showers, broken at 3,000'
Quick Reference: Code Order
METAR weather codes follow this order:
- Intensity (+/-)
- Descriptor (TS, SH, FZ...)
- Precipitation (RA, SN, GR...)
- Obscuration (FG, BR, HZ...)
Example: +TSRA = Heavy (intensity) + Thunderstorm (descriptor) + Rain (precipitation)
Key Takeaways
- Flight Service (1-800-WX-BRIEF) is the official preflight briefing — Standard, Abbreviated, or Outlook.
- AviationWeather.gov is the primary free, authoritative U.S. source (METARs, TAFs, GFA, AIRMETs/SIGMETs).
- EFBs like ForeFlight are convenient but not an approved sole source — confirm against an official brief for §91.103.
Weather Services
Where to get weather information
Flight Service (1-800-WX-BRIEF)
Main source for preflight weather briefings and flight plan filing
- Standard Briefing: Complete weather picture, file flight plan
- Abbreviated Briefing: Update or supplement previous briefing
- Outlook Briefing: Planning briefing 6+ hours in advance
AWOS/ASOS
Automated surface weather observations
- ASOS: Automated Surface Observing System - more comprehensive
- AWOS: Automated Weather Observing System - basic observations
- Both provide: wind, visibility, clouds, temp/dewpoint, altimeter
AviationWeather.gov
NWS Aviation Weather Center — the primary free, authoritative U.S. source for aviation weather.
- METARs & TAFs
- Graphical Forecasts for Aviation (GFA)
- AIRMETs, SIGMETs, PIREPs
- Winds & temperatures aloft
- Surface analysis & prog charts
ForeFlight (EFB)
The most widely used electronic flight bag in GA — bundles weather imagery, charts, and flight planning. Convenient, but confirm against an official source for your §91.103 self-brief.
- Graphical overlays: radar, satellite, icing, turbulence
- METARs & TAFs
- Charts & flight planning
- Not an approved sole source — supplement with Flight Service or aviationweather.gov
Training aid only — verify all data against your POH and current FAA publications.