Part 107 Weather Study Guide

Weather and Effects on Performance is 5% of the current FAA Part 107 knowledge test — the smallest-weighted area, but a safety-critical one (UAG_Information_Bulletin.pdf, p. 4) — and maps to ACS Area of Operation III, split into Task A (Sources of Weather) and Task B (Effects of Weather on Performance) (uas_acs.pdf, pp. 11–12). Density altitude shows up again, with worked arithmetic, in the Loading & Performance guide.

Task A: Sources of weather information

A remote pilot’s preflight weather picture is built from surface observations (manual, or automated via ASOS/AWOS), aviation routine weather reports (METAR), terminal aerodrome forecasts (TAF), and weather charts, all available through an internet weather briefing such as aviationweather.gov (remote_pilot_study_guide.pdf, pp. 15, 21; uas_acs.pdf, p. 11).

Decoding a METAR

Figure 12 gives five real METAR/SPECI examples. Take this one apart field by field: METAR KINK 121845Z 11012G18KT 15SM SKC 25/17 A3000.

Field	Raw text	Decoded
Wind	11012G18KT	110° true at 12 knots, gusting to 18 knots
Visibility	15SM	15 statute miles
Sky condition	SKC	Sky clear
Temperature/dew point	25/17	25°C / 17°C — always Celsius, separated by a slash
Altimeter	A3000	30.00” Hg — an “A” plus a four-digit group read with an implied decimal point

(remote_pilot_study_guide.pdf, pp. 16–17; sport_rec_private_akts.pdf, Figure 12, p. 2-12.)

Three more rules Figure 12’s other reports test directly:

• Sky-condition heights are in hundreds of feet AGL. SCT007 SCT250 (from Figure 12’s KLAX report) is two scattered layers — 700 ft. and 25,000 ft. — not 70,000 ft. and 2,500,000 ft. BR is the weather contraction for mist, consistent with that report’s 6SM visibility (sport_rec_private_akts.pdf, Figure 12, p. 2-12; remote_pilot_study_guide.pdf, p. 17).
• SPECI is a report type, not a location code. A SPECI is an unscheduled special report issued any time between routine METARs to capture a rapid weather change — not a routine METAR, and not the same thing as the “COR” (corrected) modifier (remote_pilot_study_guide.pdf, p. 16).
• Runway visual range is coded as “R,” the runway number, a slash, then the visibility in feet — R04/2200 is a 2,200-foot visual range on Runway 4, not a wind group (remote_pilot_study_guide.pdf, p. 16).

Decoding a TAF

A TAF forecasts conditions within 5 statute miles of an airport, issued four times a day (0000Z, 0600Z, 1200Z, 1800Z) for a 24- or 30-hour period, using the same field codes as a METAR (remote_pilot_study_guide.pdf, pp. 18–19).

Figure 15’s KMEM TAF begins KMEM 121720Z 1218/1324 20012KT 5SM HZ BKN030. The valid-period group reads day-then-hour on each side of the slash, always in UTC: 1218/1324 runs from the 12th at 1800Z to midnight UTC on the 13th — annotated 24, equivalent to 0000Z on the 14th (remote_pilot_study_guide.pdf, p. 18; sport_rec_private_akts.pdf, Figure 15, p. 2-15).

That same TAF includes PROB40 1220/1222 1SM TSRA OVC008CB — a 40% probability, not a certainty, between 2000Z and 2200Z on the 12th, of 1SM visibility in a thunderstorm with heavy rain and an 800-ft. overcast ceiling of cumulonimbus clouds (remote_pilot_study_guide.pdf, pp. 16, 19).

Figure 15’s second TAF, for Oklahoma City, shows the difference between the two change-group codes pilots mix up most:

• TEMPO — a temporary fluctuation expected to last under an hour at a time, within the stated window. TEMPO 0513/0516 1 1/2SM BR forecasts brief dips to 1.5SM visibility in mist between 1300Z and 1600Z on the 5th.
• BECMG — a gradual change that becomes established by the end of its window. BECMG 0522/0524 20013G20KT 4SM SHRA OVC020 forecasts conditions settling into wind 200° at 13 kt. gusting 20, 4SM visibility in rain showers, and an 2,000-ft. overcast ceiling by 2400Z on the 5th.

(remote_pilot_study_guide.pdf, pp. 18–19; sport_rec_private_akts.pdf, Figure 15, p. 2-15.)

Other weather charts

A winds-and-temperatures-aloft forecast (FD) codes direction (true), speed, and temperature by altitude for each station — below 24,000 ft. as a six-digit group like 2532-08 (250° true, 32 kt., −8°C), and above 24,000 ft. with the sign dropped entirely under a “TEMPS NEG ABV 24000” note, since temperatures up there are always negative (sport_rec_private_akts.pdf, Figure 17, p. 2-16; remote_pilot_study_guide.pdf, p. 19). A weather depiction chart shades IFR areas (ceiling <1,000 ft. and/or visibility <3 mi.) and contours MVFR areas (ceiling 1,000–3,000 ft. and/or visibility 3–5 mi.) without shading (sport_rec_private_akts.pdf, Figure 18, p. 2-17). A low-level SIGWX prognostic chart’s legend marks moderate-or-greater turbulence with a yellow dashed line, distinct from the solid blue freezing-level line and the red IFR ceiling/visibility outline (sport_rec_private_akts.pdf, Figure 19, p. 2-18).

Task B: Effects of weather on performance

Density altitude

Density altitude is the altitude in the standard atmosphere that corresponds to the air’s actual density, and it’s the real measure of how the air will perform — not the field elevation on a chart (remote_pilot_study_guide.pdf, p. 21). High density altitude (thin air, poor performance) results from high elevation, low pressure, high temperature, high humidity, or some combination; low density altitude (dense air, better performance) comes from the opposite conditions. Pressure and density are directly proportional at a constant temperature; temperature and density are inversely related at a constant pressure; and since pressure drops faster than temperature as altitude increases, density reliably decreases with altitude (remote_pilot_study_guide.pdf, pp. 21–22). Moist air is less dense than dry air, so rising humidity also raises density altitude, though it’s a smaller factor than pressure or temperature (remote_pilot_study_guide.pdf, p. 22). The Loading & Performance guide works a full pressure-altitude calculation from Figure 8’s conversion table.

Wind, obstructions, and wind shear

Ground obstructions — buildings, canyons, mountains — break up smooth wind flow into gusts that shift rapidly in speed and direction; in mountainous terrain, the leeward side of a ridge produces increasingly turbulent downslope air that can push an aircraft toward the terrain (remote_pilot_study_guide.pdf, p. 23). Wind shear is a sudden, drastic change in wind speed and/or direction over a very small area; it’s commonly tied to frontal passages, thunderstorms, temperature inversions, and strong upper-level winds (greater than 25 kt.), and it’s especially hazardous at low altitude because there’s no room to recover (remote_pilot_study_guide.pdf, p. 23). The most severe form, a microburst, is a convective downdraft typically 1–2 miles across and about 1,000 ft. deep, lasting 5–15 minutes, capable of 6,000 fpm downdrafts and 30–90 knot headwind-to-tailwind swings — a small UA caught in one can experience a performance-boosting headwind followed by a hard downdraft followed by a rapid tailwind shift, which can drive it into the ground (remote_pilot_study_guide.pdf, p. 23).

Atmospheric stability

A stable atmosphere resists vertical motion and dampens disturbances, producing stratiform clouds, continuous precipitation, smooth air, and generally fair-to-poor visibility in haze; an unstable atmosphere lets vertical motion amplify, producing cumuliform clouds, showery precipitation, rough turbulent air, and good visibility except where it’s degraded by blowing obstructions (remote_pilot_study_guide.pdf, p. 25). A temperature inversion — air warming with height instead of cooling — traps weather and pollutants beneath it, and if humidity is high, that trapped layer can turn into clouds, fog, haze, or smoke that cuts visibility (remote_pilot_study_guide.pdf, p. 24).

Air masses and fronts

A front is the boundary zone between two air masses with different temperature, humidity, and wind characteristics (remote_pilot_study_guide.pdf, p. 26; faa-h-8083-25c.pdf, p. 12-16).

Front type	Typical movement	Weather pattern
Warm	Slides over cooler air, 10–25 mph	Advance warning over days; cirriform/stratiform clouds, fog, then steady light-to-moderate precipitation before passage
Cold	Plows under warmer air, 25–30 mph (up to 60 mph)	Fast approach with little warning; cumulonimbus, squall lines, sudden storms, gusty winds, hail or tornadoes possible
Stationary	Neither air mass advances	A mixture of warm- and cold-front weather that can persist for days
Occluded	A fast cold front overtakes a slow warm front	Warm-front weather followed immediately by cold-front weather

(faa-h-8083-25c.pdf, pp. 12-18–12-21.)

Thunderstorms, icing, and fog

A thunderstorm cell runs through three stages. In the cumulus stage, an updraft that can exceed 3,000 fpm builds the cloud and carries liquid water above the freezing level, creating an icing hazard. In the mature stage — when precipitation first reaches the surface — downdrafts up to 2,500 fpm coexist with updrafts up to 6,000 fpm, producing the storm’s most violent turbulence, a gusty “plow wind” surface surge, a sharp temperature drop, and a rapid pressure rise. In the dissipating stage, downdrafts dominate and the cell dies out, leaving harmless cloud remnants (remote_pilot_study_guide.pdf, pp. 26–27).

Structural icing needs two things at once: the aircraft flying through visible moisture (rain or cloud droplets), and a surface temperature at or below 0°C — which aerodynamic cooling can produce even when the surrounding air is a few degrees warmer (remote_pilot_study_guide.pdf, p. 26).

Fog is a cloud sitting on the surface, forming when air near the ground cools to its dew point. Radiation fog forms on clear, calm nights as the ground radiates heat away, typically in low-lying valleys, and burns off as the sun warms the surface; under 20 ft. thick, it’s called ground fog. Advection fog forms when warm, moist air moves over a colder surface, and unlike radiation fog it needs wind to form. Ice fog forms the same way as radiation fog but at much colder temperatures (around −25°F or colder), mostly in arctic regions (faa-h-8083-25c.pdf, p. 12-15).

Ceiling and visibility

A ceiling is the lowest broken (5/8–7/8 sky coverage) or overcast cloud layer, or the vertical visibility into an obscuration like fog; visibility is the greatest horizontal distance a prominent object can be seen with the naked eye. Both are reported in every METAR and by automated weather stations (remote_pilot_study_guide.pdf, p. 28).

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Drill this guide’s material with Weather practice questions, print the METAR/TAF decoder cheat sheet, see how density altitude feeds into weight decisions in Loading & Performance, or check the exam’s full topic breakdown at What’s Actually on the Part 107 Test.

Frequently asked questions

Is the wind direction in a METAR true or magnetic?

True. A METAR wind group gives direction in tens of degrees true, not magnetic, followed by speed in knots and a "G" for the peak gust (remote_pilot_study_guide.pdf, p. 16).

Use the METAR/TAF decoder cheat sheet →

What's the difference between a METAR and a SPECI?

A METAR is the routine, regularly-scheduled report. A SPECI is an unscheduled special report that can be issued any time between routine METARs to capture a rapid weather change (remote_pilot_study_guide.pdf, p. 16).

Drill weather questions →

What does TEMPO mean in a TAF, and how is it different from BECMG?

TEMPO forecasts a temporary fluctuation expected to last under an hour at a time within its stated window. BECMG forecasts a gradual change that becomes established by the end of its window (remote_pilot_study_guide.pdf, pp. 18–19).

Use the METAR/TAF decoder cheat sheet →

Why does high humidity increase density altitude?

Water vapor is lighter than air, so moist air is less dense than dry air at the same temperature and pressure — less-dense air means a higher density altitude and reduced performance, though humidity is a smaller factor than pressure or temperature (remote_pilot_study_guide.pdf, p. 22).

Read the Loading & Performance guide →