Part 107 Ground School

Lesson

M4 Weather, METARs, TAFs

12:30-1:30 (60 min)

METAR order

  • Type (METAR or SPECI), station, day and time in Zulu
  • Wind (true direction, knots, G for gust), visibility in SM
  • Weather and obscurations, sky condition, temp/dew point, altimeter, remarks

Sky condition

  • FEW, SCT, BKN, OVC, heights in hundreds of feet AGL
  • VV: vertical visibility into an obscuration
  • Ceiling: lowest BKN or OVC layer, or vertical visibility

Weather codes

  • Intensity: minus light, no sign moderate, plus heavy
  • Descriptor then type: TS, SH, FZ; RA, SN, DZ; BR (mist), FG (fog), HZ
  • RMK AO2: automated station with precipitation sensor

TAF

  • Forecast for 5 SM around the airport, usually 24 or 30 hours
  • Valid period written as DDHH/DDHH
  • FM (from), TEMPO (temporary), BECMG (becoming), PROB30/40

Fog and moisture

  • Small temp/dew point spread means fog or low clouds likely
  • Radiation fog: clear, calm night, low terrain
  • Advection fog: moist air moving over a cooler surface, needs wind
  • Upslope and steam fog round out the list

Density altitude

  • Standard: 15 C and 29.92 inHg at sea level, about 2 C cooler per 1,000 ft
  • High, hot, humid: higher density altitude, less lift and thrust
  • Drone: shorter flight time, weaker climb, sluggish response

Stable vs unstable air

  • Stable: stratus clouds, smooth air, poor visibility, steady precipitation
  • Unstable: cumulus clouds, turbulence, good visibility, showers
  • Inversion: temperature increases with height, traps haze and smoke, smooth air but low visibility

Thunderstorms

  • Needs moisture, unstable air, lifting
  • Cumulus stage: updrafts; mature: precipitation begins, up and down drafts, most hazardous; dissipating: downdrafts
  • Microburst: intense localized downdraft, short lived, severe wind shear

Frost

  • Forms when a surface cools to or below the dew point of the air around it and that dew point is below freezing
  • Disrupts airflow over props and wings, reduces lift; remove before flight

Weather sources

  • aviationweather.gov for METAR, TAF, and more
  • 1800wxbrief.com for a briefing
  • Phone apps are fine in practice; the test asks about FAA sources

Drill

METAR and TAF decoding

Times are Zulu (UTC). In October, Ann Arbor is on EDT, so local = Z minus 4 hours.

Decoding key (hand out with the samples)

Group Example Meaning
Type and station METAR KARB Routine report, Ann Arbor Municipal. SPECI = special (unscheduled) report
Date/time 221753Z 22nd of the month, 1753 UTC
Wind 24012G20KT From 240° true, 12 kt, gusting 20. 00000KT = calm
Visibility 10SM, 1 1/2SM, 1/4SM Statute miles
Weather -SHRA, +TSRA, BR, FG Light rain showers, thunderstorm with heavy rain, mist, fog
Sky SCT035 BKN060 OVC080 Scattered at 3,500 ft, broken at 6,000, overcast at 8,000 (hundreds of ft AGL)
Vertical visibility VV002 Sky obscured, 200 ft vertical visibility (counts as a ceiling)
Temp/dew point 14/06, M02/M05 °C, M = minus
Altimeter A3001 30.01 inHg

Ceiling = the lowest layer reported as broken (BKN), overcast (OVC), or vertical visibility (VV). Scattered and few are not ceilings.

What to answer

Questions for each METAR (1 to 4): What is the local time? What is the wind? What is the ceiling? Can you legally fly a Part 107 job right now, and what is your maximum altitude?

Questions for the TAF (5): What period does it cover? What conditions are forecast at 0800Z on the 23rd? When does the wind first gust?

Sample 1 of 5

METAR KARB 221753Z 24012G20KT 10SM SCT035 BKN060 14/06 A3001

Sample 2 of 5

METAR KDTW 221753Z 18006KT 3SM BR OVC008 11/10 A2992

Sample 3 of 5

SPECI KYIP 221812Z 27025G35KT 1 1/2SM +TSRA BKN030CB OVC050 22/19 A2978

Sample 4 of 5

METAR KARB 220953Z 00000KT 1/4SM FG VV002 08/08 A3010

Sample 5 of 5

TAF KDTW 221720Z 2218/2324 21010KT P6SM SCT050 FM230200 23015G25KT P6SM BKN040 TEMPO 2306/2310 3SM -SHRA BKN020

Today's reports

Paste the KDTW, KARB, KYIP METARs and TAFs here the morning of class.

Saved in this browser only.

Check for understanding

  1. METAR shows FEW008 SCT020 BKN035 OVC080. What is the ceiling?

    Reveal answer

    3,500 ft AGL, the lowest broken layer.

  2. Hot, humid afternoon at a high-elevation site. What happens to density altitude and drone performance?

    Reveal answer

    Density altitude goes up, performance goes down: less thrust, shorter endurance.

  3. Which thunderstorm stage is characterized by downdrafts only?

    Reveal answer

    Dissipating stage.