Weather

Why this area is harder than it looks

A 55-pound aircraft at 400 ft AGL lives in the part of the atmosphere that crewed aviation spends the least time in — the surface boundary layer, where terrain, buildings and heating dominate and where the wind at your launch point tells you very little about the wind at altitude.

The knowledge test does not test that nuance. It tests whether you can read a METAR, decode a TAF, and say what density altitude does to performance. Learn the products first; the boundary-layer judgement is what keeps the aircraft in one piece afterwards.

Sources of weather — UA.III.A

Where a remote pilot gets a briefing (K1)

There is no Part 107 requirement to obtain a formal briefing — but § 107.49(a)(1) requires you to assess local weather conditions before flight, and you cannot assess what you have not looked at. In practice:

• 1-800-WX-BRIEF / Flight Service — a standard briefing works for sUAS. Say you are a Part 107 operation.
• aviationweather.gov — METARs, TAFs, graphical forecasts. This is the source the test assumes.
• ASOS/AWOS by phone or radio — the actual observation at the field, current to the minute.
• Consumer weather apps — useful for a general picture, but they are not aviation products and they do not carry ceiling, visibility or altimeter setting in aviation format.

METAR — the observation (K2)

An aviation routine weather report: what the weather is, usually issued hourly, at a specific field.

Two traps the test likes. METAR cloud heights are AGL, not MSL. And the wind in a METAR is true north, while a tower reading you the wind on the radio gives it magnetic.

Sky cover thresholds, in eighths of the sky: FEW 1–2, SCT 3–4, BKN 5–7, OVC 8. A ceiling is the lowest BKN or OVC layer — SCT is not a ceiling, which matters because § 107.51(b) requires you to stay 500 ft below the cloud.

TAF — the forecast (K3)

A terminal aerodrome forecast covers roughly a 5 statute mile radius of the airport, normally for 24 or 30 hours, issued four times a day. Same coding as a METAR, plus change groups:

• FM (from) — a rapid, permanent change at that time.
• BECMG — a gradual change over the stated window.
• TEMPO — temporary, under an hour at a time, expected to occupy less than half the period.
• PROB30/40 — a 30 or 40 percent probability.

A TAF is a forecast for an airport. If you are launching from a field ten miles away, it is guidance, not an observation.

Weather charts (K4)

• Surface analysis — fronts, pressure systems, isobars. Isobars packed close together mean a strong pressure gradient, which means wind.
• Graphical Forecasts for Aviation (GFA) — the replacement for the old area forecast; clouds, weather and winds by time slice.
• Convective outlook — thunderstorm risk areas.

ASOS and AWOS (K5)

Automated Surface Observing System and Automated Weather Observing System — the machines that generate the observation, broadcast continuously on a discrete frequency and usually available by telephone. ASOS is the joint FAA/NWS system and generally the more capable; AWOS comes in tiers (A, 1, 2, 3) reporting progressively more.

For a remote pilot this is the most useful single source: it is the current altimeter setting, wind and ceiling at a field near you, and it updates every minute rather than every hour.

Effects of weather on performance — UA.III.B

Density altitude (K1a)

Pressure altitude corrected for non-standard temperature — the altitude the aircraft thinks it is at. High, hot and humid all raise it, and every one of those reduces the thrust a propeller can produce.

On a small multirotor the effect is blunt and immediate: less thrust margin, longer to accelerate, a noticeably shorter flight time as the motors draw more current to hold the same hover. A battery that gives 22 minutes at a cool sea-level field can give appreciably less on a hot afternoon at a high field, and the aircraft will not warn you.

Wind and currents (K1b)

The wind at 400 ft is frequently stronger than the wind you feel standing at the controls, and it is not necessarily from the same direction. Three effects worth carrying:

• Downwind return. An aircraft that flew out into wind has been drawing less current than it will need to come home. Plan the return leg before the battery decides it for you.
• Mechanical turbulence. Wind over buildings, tree lines and terrain breaks into rotors and eddies downwind of the obstruction.
• Maximum groundspeed. § 107.51(a) caps groundspeed at 87 knots (100 mph). A tailwind can put you there without the aircraft ever exceeding its own airspeed limit.

Stability, pressure and temperature (K1c)

Stable air resists vertical motion: stratiform cloud, smooth flying, poor visibility, steady precipitation. Unstable air encourages it: cumuliform cloud, turbulence, good visibility, showery precipitation. The standard lapse rate is 2°C per 1,000 ft; the closer the actual lapse rate is to that, the more stable the air.

Air masses and fronts (K1d)

A front is the boundary between two air masses, and the weather is on the boundary. Cold fronts move fast and lift the warm air abruptly — narrow band, violent weather, rapid clearing behind. Warm fronts ride up gently — wide band, low ceilings, poor visibility, and they take a long time to pass. Expect a wind shift at any frontal passage.

Thunderstorms and microbursts (K1e)

Three ingredients: moisture, unstable air, and a lifting action. Three stages: cumulus (updrafts only), mature (updraft and downdraft together — the dangerous stage, marked by the onset of rain at the surface), and dissipating (downdrafts only).

A microburst is a concentrated downdraft under a cell, with downdrafts to 6,000 fpm, a headwind-to-tailwind shift approaching 90 knots, typically 2.5 NM across and lasting about 15 minutes. Nothing this class of aircraft can do will survive one, so the only answer is distance and time.

Tornadoes (K1f), hail (K1h), lightning (K1k)

• Tornadoes form with severe thunderstorms; a funnel aloft is a tornado whether or not it has reached the ground.
• Hail can be thrown out the top and downwind of a cell and fall in clear air — being clear of the cloud is not being clear of hail.
• Lightning is the definition of a thunderstorm: no lightning, no thunderstorm. It is also a real risk to the aircraft's electronics and to the person holding the controller.

Icing (K1g)

Structural ice needs visible moisture and a surface at or below freezing. Small unmanned aircraft have no anti-ice or de-ice of any kind, and ice on a propeller destroys thrust and balance long before it adds meaningful weight. The mitigation is not to fly there.

Fog (K1i)

Cloud at the surface, formed as temperature and dew point converge. A small temperature/dew-point spread in a METAR is your warning. Radiation fog forms on clear, calm nights over land; advection fog forms when moist air moves over a cooler surface and can arrive with wind, so it does not need a calm night to catch you out.

Ceiling and visibility (K1j)

These are the numbers Part 107 turns into a legal limit, not just a comfort question. § 107.51 requires 3 statute miles of visibility from the control station, and cloud clearance of 500 ft below and 2,000 ft horizontally. Remember that a ceiling is the lowest broken or overcast layer — a scattered layer is not a ceiling.