Weather for Remote Pilots

Weather is the most-tested Area III material, and it's two skills: understanding what the atmosphere is doing, and decoding the reports that describe it (METAR/TAF - next lesson).

Your legal minimums (memorize cold)

• Visibility: 3 statute miles, measured from the control station (where you're standing), not "how far you can see the drone."
• Cloud clearance: 500 ft below, 2,000 ft horizontally from any cloud.
• The remote pilot in command is responsible for checking all of it before flight. (Any "who is responsible" question: the RPIC.)

What makes weather

Uneven solar heating of the Earth's surface is the primary cause of all weather changes - the exam phrases it as "variation of solar heating." Warm air rises (updrafts, thermals); cool air sinks. Rising warm air cools, its moisture condenses: clouds.

• Thermals (rising columns off hot surfaces) are the primary cause of low-level turbulence. "What condition causes turbulence?" → uneven heating of the Earth's surface.
• Wind = air moving from high pressure to low pressure. Global winds: pressure differences plus the Earth's rotation.
• Wind shear = a sudden, drastic change in wind speed or direction over a small distance. It can occur at any altitude (that's the exam answer) but is most dangerous close to the ground. Microbursts - violent thunderstorm downdrafts - are the killer case.
• Wind and drones: control authority loss, faster battery drain (motors fight the wind), and the return-to-home trap: flying downwind feels easy until the drone can't fight its way back upwind.

Air masses and fronts

An air mass is a large body of air that takes on the character of the surface it forms over (over water: moist; over desert: hot and dry) - and it changes character as it moves. Where two air masses meet, the boundary is a front. That's the exam definition: the boundary between two different air masses is called a front.

• Cold front: cold dense air shoves warm air up fast → cumulus buildups, thunderstorms, gusty winds. Cold fronts move faster than warm fronts. Hearing "cold front" in a forecast is your red flag.
• Warm front: warm air slides gently over cold → low stratus, steady rain, reduced visibility.
• Stationary front: neither side wins → lingering clouds and rain.
• Occluded front: a cold front overtakes a warm front → complex, unstable weather.
• The most recognizable sign a front has passed: a rapid change in temperature. Second sign: the wind shifts (direction and speed). Not humidity, not cloud cover - temperature.

Stable vs unstable air

The one contrast that decodes half the questions:

Stable air feels nicer and is smoother to fly in, but it traps haze and fog - poor visibility. Unstable air is bumpy but clear. The exam banks on you guessing that backwards.

Clouds

• Low clouds: surface to ~6,500 ft AGL.
• Stratus = gray, flat, layered; stable air; drizzle and mist; visibility killer; threat to VLOS and legal minimums.
• Cumulus = puffy, flat base, cauliflower top; fair weather in unstable air - unless they keep building vertically into cumulonimbus.

Thunderstorms: three stages, in order

1. Cumulus stage - strong updrafts build the cloud. No precipitation yet (the updraft holds the moisture up), essentially no lightning.
2. Mature stage - the most dangerous. The tell: updrafts and downdrafts coexist. Precipitation begins falling (that's what marks the start of the stage), lightning, hail (updrafts recycling ice), microbursts, gust fronts spreading along the ground.
3. Dissipating stage - downdrafts everywhere; the cold outflow cuts off the warm inflow that fed the storm, and it starves.

Part 107 doesn't name a standoff distance from storms, but visibility and cloud-clearance rules make flying near one effectively illegal as well as dumb.

Density altitude

The concept the FAA uses to ask "do you understand performance?"

• Air density drops with heat, altitude, and humidity (humid air is LESS dense - water vapor is lighter than the O2/N2 it displaces; most people guess backwards).
• Density altitude is the altitude the air "feels like." Hot, humid, high → the air behaves like thinner air from a higher elevation: high density altitude = thin air = less lift, less prop efficiency, more battery, worse performance.
• Direction check, because it reads inverted: HIGH density altitude = BAD. LOW density altitude (cool, dry, low) = dense air = the drone flies great.

Dew point, fog, and frost

• Dew point = the temperature at which air becomes saturated. The spread between temperature and dew point is your fog early-warning: 27°C/17°C is comfortable; when the spread closes to near zero, expect fog, mist, low cloud - and the end of your legal 3 SM.
• Temperature = dew point → 100% relative humidity → fog/cloud forms.
• Fog is a cloud at ground level. Dew = condensation on surfaces; frost = vapor straight to ice.
• Dew/fog form best on clear, calm nights (ground radiates heat away fastest); wind and cloud cover prevent it.
• Fog and drones: visibility gone, moisture in motors and sensors, degraded GPS/radio. Fog = you don't fly.

Traps to drill

• 3 SM is measured at the control station, not at the drone.
• Density altitude direction: high = bad. Humidity lowers density.
• Stable = poor visibility. Unstable = good visibility.
• Thunderstorm stage order and the mature-stage tell (updrafts + downdrafts, precipitation starts).
• Front passage = temperature change first, wind second.
• Wind shear: any altitude.