Weather Information
Private Pilot ACS · Area I · Task C · 14 CFR 61, 14 CFR 91, FAA-H-8083-25
Everything you need to know about Weather Information for your private pilot checkride. Aligned to FAA-S-ACS-6C Task I-C, covering decoding a metar, decoding a taf, weather products overview.
Decoding a METAR AIM 7-1-30 §
METAR KBWI 141854Z 27018G28KT 10SM FEW045 BKN070 OVC250 18/05 A2998 RMK AO2 SLP152
METAR
Routine hourly observation (SPECI = unscheduled special observation)
KBWI
ICAO airport identifier — Baltimore/Washington International
141854Z
Day 14, time 1854 UTC (Zulu). Always UTC — never local time.
27018G28KT
Wind FROM 270° (west) at 18 kts, gusting to 28 kts. Variable: VRB05KT. Calm: 00000KT.
10SM
Visibility 10 statute miles. <1/4SM = M1/4SM. Fractions: 1 1/2SM.
FEW045
Few clouds at 4,500 ft AGL (heights always in hundreds of feet AGL). FEW=1-2/8, SCT=3-4/8, BKN=5-7/8 (ceiling), OVC=8/8 (ceiling).
BKN070
Broken ceiling at 7,000 ft AGL. BKN or OVC = ceiling. FEW/SCT = not a ceiling.
OVC250
Overcast at 25,000 ft AGL. CB = cumulonimbus; TCU = towering cumulus.
18/05
Temperature / Dewpoint in °C. Spread = 13°C. Closer spread = higher chance of fog/low clouds.
A2998
Altimeter setting 29.98 in Hg. Set this in your altimeter before departure and update en route.
RMK AO2
Remarks: AO2 = automated station with precipitation discriminator. SLP152 = sea level pressure 1015.2 mb.
Decoding a TAF AIM 7-1-31 §
TAF KORD 141730Z 1418/1524 27020KT P6SM BKN030 FM152000 30015KT 6SM -SHRA BKN025 OVC060 TEMPO 1522/1524 2SM TSRA OVC015CB
TAF KORD
Terminal Aerodrome Forecast for Chicago O'Hare. 5 statute miles around the airport.
141730Z
Issued day 14 at 1730Z
1418/1524
Valid period: day 14 from 1800Z through day 15 at 2400Z (24-30 hour TAFs)
27020KT P6SM BKN030
Base conditions: 270/20kts, visibility >6SM, broken at 3,000
FM152000
FROM: new conditions starting day 15 at 2000Z — rapid complete change
TEMPO 1522/1524
Temporary conditions day 15 from 2200–2400Z (lasting <60 min, <50% of the time). During this period expect: 2SM visibility, thunderstorms + rain, overcast at 1,500 with cumulonimbus
BECMG (not shown)
Becoming: gradual change over 2 hours. PROB30/40: 30% or 40% probability of conditions occurring during the time period.
Weather Products Overview AIM 7-1 §
SIGMET and AIRMET Reference §
| Product | Category | Covers | Valid |
|---|---|---|---|
| SIGMET | WS | Severe/extreme turb, severe icing, widespread dust/sandstorm, volcanic ash | 4 hrs (6 hrs tropical) |
| Convective SIGMET | WST | Severe CB activity, embedded TSs, hail ≥3/4", tornado | 2 hrs |
| AIRMET Sierra | WA | IFR (ceiling <1000/vis <3SM over 3000+ sq mi), mountain obscuration | 6 hrs |
| AIRMET Tango | WA | Moderate turbulence, sustained sfc winds ≥30 kts, low-level wind shear | 6 hrs |
| AIRMET Zulu | WA | Moderate icing, freezing level data | 6 hrs |
| CWA | — | ARTCC area — doesn't meet SIGMET criteria but affects safety | 2 hrs |
AIM 7-1-6
GFA — Graphical Forecast for Aviation §
Replaced the area forecast (FA). Available at aviationweather.gov. Updated every 6 hours. Valid for 18 hours in two 6-hour increments.
What it shows: Ceiling and visibility forecasts, precipitation type and intensity, turbulence (low and high altitude), icing (low and high altitude), freezing levels, surface winds, convective outlook.
How DPEs use it: Expect to open aviationweather.gov, select GFA, interpret ceiling/vis and turbulence layers for a proposed route. You should know how to read the interactive layers.
What it shows: Ceiling and visibility forecasts, precipitation type and intensity, turbulence (low and high altitude), icing (low and high altitude), freezing levels, surface winds, convective outlook.
How DPEs use it: Expect to open aviationweather.gov, select GFA, interpret ceiling/vis and turbulence layers for a proposed route. You should know how to read the interactive layers.
AIM 7-1-5; ACS PA.I.C
Winds Aloft Forecast (FB) §
Issued twice daily (based on 00Z and 12Z upper-air data). Covers altitudes from 3,000 ft through FL390.
Format: ddsstt — direction (true north), speed in knots, temperature in °C. Example: 2722-04 = wind from 270°, 22 kts, -4°C.
Light & variable: 9900
Over 99 kts: Coded as direction + 50, speed − 100. 731960 = from 230°, 119 kts, -60°C
No temperature: At 3,000 and 6,000 ft (too close to surface for reliable data)
No winds below 1,500 AGL
Format: ddsstt — direction (true north), speed in knots, temperature in °C. Example: 2722-04 = wind from 270°, 22 kts, -4°C.
Light & variable: 9900
Over 99 kts: Coded as direction + 50, speed − 100. 731960 = from 230°, 119 kts, -60°C
No temperature: At 3,000 and 6,000 ft (too close to surface for reliable data)
No winds below 1,500 AGL
AC 00-45H
Hazardous Weather In Depth PHAK Ch.12 §
Thunderstorm Stages §
Cumulus stage: Strong updrafts only. No lightning. No precipitation at surface. Can last 20 min.
Mature stage: Most hazardous. Both updrafts AND downdrafts present simultaneously. Precipitation at surface. Lightning, hail, severe turbulence, possible tornado, microburst outflow. Can last 10–30 min.
Dissipating stage: Downdrafts only. Precipitation decreases. Still dangerous — outflow winds and possible lightning. Cell collapses inward.
Avoidance rule: Avoid by at least 20 nm (do not fly near anvil tops — hail can extend well beyond the visible cloud). Never attempt to fly under a CB.
Embedded thunderstorms: The most dangerous — hidden inside stratiform clouds, invisible on EFB radar without onboard weather radar.
Mature stage: Most hazardous. Both updrafts AND downdrafts present simultaneously. Precipitation at surface. Lightning, hail, severe turbulence, possible tornado, microburst outflow. Can last 10–30 min.
Dissipating stage: Downdrafts only. Precipitation decreases. Still dangerous — outflow winds and possible lightning. Cell collapses inward.
Avoidance rule: Avoid by at least 20 nm (do not fly near anvil tops — hail can extend well beyond the visible cloud). Never attempt to fly under a CB.
Embedded thunderstorms: The most dangerous — hidden inside stratiform clouds, invisible on EFB radar without onboard weather radar.
Icing Types — Temperature Ranges §
Clear ice (glaze ice): 0°C to -10°C. Large supercooled droplets spread before freezing. Dense, hard to remove, severely disrupts airfoil. Most dangerous.
Rime ice: -10°C to -20°C. Small droplets freeze instantly on impact. Rough, opaque, brittle. Less dense than clear ice. Leading edge buildup.
Mixed ice: Combination; occurs in temperature transitions or mixed cloud populations. Unpredictable buildup.
Required conditions: Visible moisture (clouds, rain, drizzle) AND OAT at or below 0°C. Both required simultaneously.
Carburetor ice: Not structural icing — requires only high humidity + 20–70°F temps. No visible moisture required.
Rime ice: -10°C to -20°C. Small droplets freeze instantly on impact. Rough, opaque, brittle. Less dense than clear ice. Leading edge buildup.
Mixed ice: Combination; occurs in temperature transitions or mixed cloud populations. Unpredictable buildup.
Required conditions: Visible moisture (clouds, rain, drizzle) AND OAT at or below 0°C. Both required simultaneously.
Carburetor ice: Not structural icing — requires only high humidity + 20–70°F temps. No visible moisture required.
Fog Types — How They Form §
| Type | Mechanism | Wind | When |
|---|---|---|---|
| Radiation | Ground cools air to dewpoint at night | Calm | Clear nights, fall/winter |
| Advection | Warm moist air moves over cool surface | Required | Coastal, all seasons |
| Upslope | Moist air forced up slope, cools adiabatically | Required | Eastern Rockies, windward slopes |
| Steam (evaporation) | Cold air over warm water — steam evaporates and recondenses | Light | Cold outbreaks, arctic sea smoke |
| Frontal | Rain evaporates, saturates cold air below warm front | Variable | Ahead of warm fronts |
PHAK Ch.12
Mountain Wave & Lenticular §
Formation: Wind ≥25 kts perpendicular to a mountain ridge creates a standing wave on the leeward side. Waves can extend hundreds of miles downwind and to very high altitudes.
Lenticular clouds: Lens-shaped, stationary, form at wave crests. A lenticular cloud is a direct indicator of mountain wave activity.
Rotor zone: Below the lenticular, at the mountain base elevation or lower. Extremely violent horizontal swirling turbulence — most dangerous aviation turbulence outside of a thunderstorm.
Cap cloud: Sits on top of the mountain peak. The mountain is "wearing a cap."
Avoidance: Do not fly below lenticular clouds on the leeward side of mountains. Gain significant altitude before crossing.
Lenticular clouds: Lens-shaped, stationary, form at wave crests. A lenticular cloud is a direct indicator of mountain wave activity.
Rotor zone: Below the lenticular, at the mountain base elevation or lower. Extremely violent horizontal swirling turbulence — most dangerous aviation turbulence outside of a thunderstorm.
Cap cloud: Sits on top of the mountain peak. The mountain is "wearing a cap."
Avoidance: Do not fly below lenticular clouds on the leeward side of mountains. Gain significant altitude before crossing.
Microburst — The Silent Killer §
Definition: A localized downdraft that diverges outward when it hits the ground. Horizontal outflow can exceed 45 kts within a diameter of 1–2 nm. Lasts only 2–5 minutes but can be deadly during that window.
Sequence on approach:
1. Aircraft enters headwind → airspeed increases → pilot reduces power
2. Aircraft passes through the core downdraft
3. Aircraft exits into tailwind → airspeed drops dramatically → pilot already at low power
4. Aircraft cannot accelerate fast enough at low altitude → collision with terrain
Avoidance: Do not attempt to continue approach through wind shear or loss of airspeed. Go around immediately at the first indication — low-level wind shear alert, airspeed fluctuation, or unusual glidepath deviation.
Sequence on approach:
1. Aircraft enters headwind → airspeed increases → pilot reduces power
2. Aircraft passes through the core downdraft
3. Aircraft exits into tailwind → airspeed drops dramatically → pilot already at low power
4. Aircraft cannot accelerate fast enough at low altitude → collision with terrain
Avoidance: Do not attempt to continue approach through wind shear or loss of airspeed. Go around immediately at the first indication — low-level wind shear alert, airspeed fluctuation, or unusual glidepath deviation.
AIM 7-1-24
Stability & Cloud Type §
Stable air: Resists vertical motion. Produces stratiform clouds, steady precipitation, poor visibility in fog and haze, smooth flight.
Unstable air: Encourages vertical development. Produces cumuliform clouds, convective showers, good visibility in precipitation, turbulent flight.
Cloud type reference:
• Cirrus / cirrostratus / cirrocumulus: High (16,000+), ice crystals
• Altostratus / altocumulus: Middle (6,500–20,000)
• Stratus / nimbostratus: Low (<6,500), liquid water
• Cumulus: Convective, all levels
• CB: Tall enough to span all levels — the thunderstorm cloud
Unstable air: Encourages vertical development. Produces cumuliform clouds, convective showers, good visibility in precipitation, turbulent flight.
Cloud type reference:
• Cirrus / cirrostratus / cirrocumulus: High (16,000+), ice crystals
• Altostratus / altocumulus: Middle (6,500–20,000)
• Stratus / nimbostratus: Low (<6,500), liquid water
• Cumulus: Convective, all levels
• CB: Tall enough to span all levels — the thunderstorm cloud
PHAK Ch.12
Go / No-Go Decision Making ACS PA.I.C.R1 §
Personal Weather Minimums §
DPEs frequently ask about personal minimums. You must have a thoughtful answer that goes beyond "VFR minimums."
Establish in advance: Write them down, share them with passengers, commit to them before the flight. Example minimums for a new private pilot:
• Ceiling: 3,000 ft or higher at departure and along route
• Visibility: 5 SM or better
• Winds: <15 kts, crosswind <8 kts
• No known icing, no convective activity within 50 nm of route
Stick to them: Get-there-itis, passenger pressure, and sunk-cost thinking are the primary killers. If conditions breach your minimums, the flight does not depart — regardless of external pressure.
Establish in advance: Write them down, share them with passengers, commit to them before the flight. Example minimums for a new private pilot:
• Ceiling: 3,000 ft or higher at departure and along route
• Visibility: 5 SM or better
• Winds: <15 kts, crosswind <8 kts
• No known icing, no convective activity within 50 nm of route
Stick to them: Get-there-itis, passenger pressure, and sunk-cost thinking are the primary killers. If conditions breach your minimums, the flight does not depart — regardless of external pressure.
EFB Weather Limitations §
NEXRAD radar delay: Data displayed on EFBs (ForeFlight, Garmin Pilot) has a 5–20 minute delay from the NWS radar sweep. Storms move. Do not navigate around storm cells using EFB radar as a real-time avoidance tool.
Use EFB radar for: Strategic planning, understanding the overall storm pattern, determining if you are clearly outside all convective activity.
Use onboard Stormscope / XM for: Real-time (or near real-time) strike detection. Still not a substitute for avoidance — all convective weather should be avoided.
Weather briefing requirement: FSS or an approved EFB briefing counts as a proper weather briefing for regulatory purposes.
Use EFB radar for: Strategic planning, understanding the overall storm pattern, determining if you are clearly outside all convective activity.
Use onboard Stormscope / XM for: Real-time (or near real-time) strike detection. Still not a substitute for avoidance — all convective weather should be avoided.
Weather briefing requirement: FSS or an approved EFB briefing counts as a proper weather briefing for regulatory purposes.
AIM 7-1-11; ACS PA.I.C