METAR/TAF decoding, density altitude, and the weather hazards that actually affect a small drone.
Exam weight: 11–16% of the exam (7–10 of 60 questions) • lowest pass rate of any area (~62%)
Decode what the test gives you
Weather: Decode METAR/TAF
Code
Read it as
METAR
Current observed airport weather.
TAF
Terminal airport forecast.
18012KT
Wind from 180° at 12 knots.
VRB03KT
Variable wind at 3 knots.
G25KT
Gusts to 25 knots.
10SM
10 statute miles visibility.
BKN020
Broken clouds at 2,000 ft AGL.
OVC008
Overcast at 800 ft AGL.
-RA
Light rain.
+TSRA
Heavy thunderstorm rain.
Ceiling = lowest broken or overcast layer (or vertical visibility into obscuration). FEW/SCT alone are not a ceiling.
Temperature + dew point close together → air near saturation → fog/low clouds more likely.
Hot air is less dense → density altitude rises → aircraft/propeller performance can decrease. Think of it like a propeller "grabbing" air to push itself forward — hot, spread-out air molecules give each spin fewer molecules to grab, so the same power produces less lift/thrust.
Wind shear = rapid change in wind speed and/or direction over a short distance. Thunderstorms can create severe turbulence, downdrafts, gust fronts and hazardous outflow beyond visible rain.
Battery planning: don't fly far downwind without considering the return. A return-leg headwind reduces groundspeed and can consume much more battery.
Getting a weather briefing
A standard aviation weather briefing is available free through Leidos Flight Service (1-800-WX-BRIEF, or 1800wxbrief.com) — the same official source manned-aircraft pilots use. Many EFB apps pull from the same underlying data.
Term
What it is
PIREP
Pilot report of actual conditions encountered in flight — can reveal things a forecast missed (turbulence, icing, actual cloud bases).
AIRMET
Advisory for weather that's a hazard mainly to smaller aircraft (moderate turbulence/icing, extensive mountain obscuration, sustained winds).
SIGMET
Advisory for more severe weather hazardous to all aircraft (severe turbulence/icing, dust storms, widespread sandstorms).
Video
Weather Pt. 1
First of a two-part general weather walkthrough, picking up right after the METAR/TAF decode above.
Video
Weather Pt. 2
Continues directly from Weather Pt. 1 above.
Video
METAR Reports, Explained
A dedicated deep dive on reading METAR reports, reinforcing the decode table at the top of this page.
Don't mix these up
Ceiling vs. 400 ft AGL
These are two completely different numbers that happen to both involve altitude, and the test leans on that overlap.
A weather ceiling is the lowest broken or overcast cloud layer (or vertical visibility into an obscuration) — defined above. It changes hour to hour with the weather, and it's reported in a METAR/TAF.
The 400 ft AGL figure is a Part 107 operating limit on how high the aircraft itself may normally fly (with the structure-radius exception) — a regulation, not a weather observation. See the Regulations area for the full rule.
Trap: a question describing a ceiling of, say, 300 ft is testing whether you know you can't legally operate up to a 400 ft limit underneath it — not asking you to recall the 400 ft number itself. Read whether the question is giving you a weather ceiling or asking about the regulatory altitude limit before you answer.
The baseline everything else is measured against
Standard Atmosphere & Lapse Rates
The FAA defines a standard atmosphere as a reference: at sea level, pressure is 29.92 inHg and temperature is 15°C (59°F). Moving up from there, conditions change at predictable average rates:
Standard temperature lapse rate: about 2°C per 1,000 ft (roughly 3.5°F per 1,000 ft), up to 36,000 ft.
Standard pressure lapse rate: about 1 inHg per 1,000 ft in the lower atmosphere (first ~10,000 ft).
Example: at 4,000 ft in a standard atmosphere — Temperature ≈ 15°C − (4 × 2°C) = 7°C Pressure ≈ 29.92 inHg − (4 × 1 inHg) = 25.92 inHg
Every "standard day" question is really asking you to apply one of these two rates from the sea-level baseline. These numbers also feed directly into density altitude and pressure altitude, below — this is the baseline those calculations correct away from.
Why hot/high/humid hurts performance
Density Altitude
Pressure altitude is height above the standard-pressure (29.92 inHg) reference level. Density altitude is pressure altitude corrected for non-standard temperature — it's the altitude the air actually "feels like" to the aircraft and its propeller, not just a number on the altimeter.
Rule of thumb: density altitude ≈ pressure altitude + 120 ft for every 1°C the actual temperature is above the standard temperature for that altitude (and − 120 ft per °C below standard).
Example: pressure altitude is 3,000 ft, where standard temperature is 9°C (15°C − 3×2°C). If the actual temperature is 29°C — 20°C above standard — density altitude is approximately 3,000 + (20 × 120) = 5,400 ft, even though the altimeter and field elevation never changed.
High density altitude (hot, humid, and/or high-elevation air) means air molecules are more spread out. A propeller or rotor "grabs" fewer molecules per revolution, so it produces less lift and thrust for the same power setting — takeoff distance increases and climb performance drops, exactly when a hot summer afternoon at a high-elevation site makes it most likely to happen.
Video
Air Density, Pt. 3
Explains air density and how it drives the density-altitude math covered above.
Know which stage is which
Thunderstorm Life Cycle
Every thunderstorm moves through three stages, and the test specifically checks whether you know which stage has downdrafts.
Stage
What's happening
Cumulus (building)
Updrafts dominate; the cloud is actively growing. Little or no precipitation yet.
Mature
Both updrafts and downdrafts are present at the same time — precipitation begins falling, and the downdraft forms alongside the still-active updraft. This is the most hazardous stage: heavy rain, frequent lightning, strong gusty winds, and the greatest turbulence.
Dissipating
Downdrafts dominate and overcome the updraft; rainfall decreases in intensity. Lightning can still be a hazard even as the storm winds down.
Trap: a question asking "which stage has downdrafts" is testing the mature stage specifically — not dissipating. Downdrafts first appear in the mature stage, alongside the updraft; dissipating is when downdrafts take over completely.
Stability, saturation, and the two fog types that matter
Atmospheric Stability & Fog
Stable air resists vertical motion — it tends to produce smooth air, but also poor visibility, haze, fog, and low stratus-type clouds. Unstable air readily rises — it tends to produce turbulent, bumpy air, but good visibility and cumulus-type cloud buildups. Fog itself forms when air cools to its dew point (or moisture is added until the dew point rises to meet the temperature) — the same temperature/dew-point-spread idea introduced above.
Type
How it forms
When it clears
Radiation fog
Clear skies, light or calm wind, and overnight radiational cooling chill moist air near the ground to its dew point.
Typically burns off shortly after sunrise, once the ground warms.
Advection fog
Warm, moist air moves horizontally over a colder surface (e.g. onshore flow over cold ground or water), cooling the air from below.
Depends on the wind/air-mass flow, not the time of day — can persist for hours or days, day or night, as long as the flow continues.
Trap: "fog always clears by morning" is only true of radiation fog. Advection fog is driven by wind and air-mass movement, so it can sit over an area all day.
Observed on the 15th of the month, at 1753 Zulu (UTC) time.
18012G20KT
Wind from 180° at 12 knots, gusting to 20 knots.
10SM
Visibility 10 statute miles.
BKN020
Broken clouds at 2,000 ft AGL — this is the ceiling (lowest broken/overcast layer).
OVC035
A higher overcast layer at 3,500 ft AGL — reported, but the 2,000 ft broken layer is still the ceiling since it's lower.
22/18
Temperature 22°C, dew point 18°C — only a 4°C spread, consistent with the broken/overcast layers already present.
A2992
Altimeter setting 29.92 inHg — the standard-day pressure reference.
Image placeholder: annotated full METAR string, each group labeled
Worked this way, a full METAR is just the individual codes from the top of this page strung together in a fixed order: station → time → wind → visibility → clouds (low to high) → temperature/dew point → altimeter.
Video
METARs and TAFs, Together
Ties METARs and TAFs together after the full METAR decode above — good last pass before the rapid recall below.
Final review — Weather
Rapid Recall
Ceiling = lowest BKN/OVC layer (or vertical visibility into obscuration) • FEW/SCT ≠ ceiling • Ceiling ≠ the 400 ft AGL operating limit
Standard atmosphere: 29.92 inHg / 15°C (59°F) at sea level • lapse rates: ~2°C (3.5°F) per 1,000 ft temperature, ~1 inHg per 1,000 ft pressure
Density altitude = pressure altitude corrected for non-standard temperature • rule of thumb ≈ 120 ft per °C above standard • hot/high/humid = higher DA = less propeller/rotor performance
Thunderstorm stages: cumulus (updraft only) → mature (updraft AND downdraft — most hazardous) → dissipating (downdraft dominant)
Stable air → smooth, poor visibility, fog/haze/low clouds • Unstable air → turbulent, good visibility, cumulus buildups
Radiation fog: clear + light/calm wind + overnight cooling, clears near sunrise • Advection fog: warm moist air over cold surface, wind-driven, can persist any time of day
Close temp/dew-point spread → saturation → fog/low clouds more likely
Free briefings: Leidos Flight Service, 1-800-WX-BRIEF / 1800wxbrief.com • PIREP = pilot report • AIRMET = hazard to small aircraft • SIGMET = hazard to all aircraft
Wind shear = rapid wind speed/direction change over a short distance • thunderstorms can produce severe turbulence/downdrafts/gust fronts beyond the visible rain
Plan the return leg: a downwind-out flight means a headwind-back flight — budget extra battery for it