3.2 Wind, Turbulence, and Wind Shear

Reading and Managing Wind

A small drone has a low mass-to-surface-area ratio, so it is far more wind-sensitive than a manned aircraft. Part 107 sets no numeric wind limit — the Remote PIC must judge whether conditions are safe for the specific aircraft. The exam tests how you interpret wind data and anticipate turbulence.

Surface Wind vs. Winds Aloft

• Surface wind is measured at 33 ft (10 m) AGL at weather stations.
• Direction is reported as where the wind comes FROM: a "270 wind" blows from the west toward the east.
• Friction slows surface wind and backs its direction; winds aloft are stronger and more from the right.
• Rule of thumb: wind at 2,000-3,000 ft AGL can be 2-3 times the surface speed — relevant if your drone climbs near the 400 ft ceiling on a windy day.

What Reports Wind From What North

High-value trap: "Surface" products (METAR, TAF, ATIS) use magnetic north; winds aloft use true north. The exam loves this distinction.

The Four Turbulence Types

Mechanical example: Inspecting the downwind face of a tall building places the drone squarely in the rotor of eddies shedding off the structure. Stand off farther or work the windward side when possible.

Convective example: A parking lot heats faster than an adjacent lawn, so a thermal rises over the asphalt while sinking air forms over the grass — your drone can balloon up over one and sink over the other on a hot afternoon.

Wind Shear

Wind shear is a sudden change in wind speed and/or direction over a short distance. It can occur at any altitude and is associated with:

• Frontal boundaries (especially fast-moving cold fronts)
• Thunderstorms and their gust fronts
• Temperature inversions (shear at the top of the inversion layer)
• Terrain features and obstacles

Microbursts

A microburst is a small, intense downdraft from a thunderstorm that spreads outward at the surface.

• Diverging outflow can exceed 45 knots
• Footprint typically less than 2.5 miles across, lasting 5-15 minutes
• A drone hit by the outflow can be slammed into the ground
• They can occur even from a storm that looks distant — stay grounded near any convective activity

Wind Effects and Mitigations

Crosswinds and Battery

In a crosswind a GPS-stabilized drone automatically crabs (angles) into the wind to hold its track, which raises power consumption and shortens flight time. Always fly the headwind leg outbound so the easier tailwind leg is on the way home, when the battery is lower.

Estimating Wind Without an Anemometer

Most field decisions rely on observation, not instruments. A drone that suddenly drifts or refuses to hold position is telling you the wind aloft exceeds the surface reading you felt at launch. Watch flags, smoke, treetops, and water surfaces; if the wind is already moving large branches at the surface (roughly 20-25 kt), conditions at 200-400 ft are likely well beyond a lightweight drone's comfortable envelope. Because winds aloft can be 2-3 times the surface speed, a calm launch site does not guarantee a calm operating altitude.

Reading Wind in a Coded Report

Wind appears early in a METAR as a five-character group. 18012KT is from 180 degrees at 12 kt; VRB03KT is variable at 3 kt, common in light, near-calm conditions; 00000KT is dead calm. A gust group such as 24018G31KT (from 240 degrees, 18 kt gusting 31 kt) is the real planning number — gust spread of 13 kt warns of mechanical and thermal mixing that can flip a small drone. Always plan against the gust value, never the steady value.

Common Wind Traps

• "A north wind blows toward the north." No — wind is named for where it comes FROM, so a north wind blows from north toward south.
• Forgetting the north reference switch. METAR/TAF/ATIS use magnetic north; winds aloft (FB) and charts use true north.
• Assuming Part 107 has a wind speed limit. It does not; the Remote PIC must judge the limit for the specific aircraft.
• Treating turbulence as a high-altitude-only problem. Mechanical turbulence near buildings and convective thermals are strongest in exactly the low altitudes drones use.

For the exam: Microbursts, wind shear, and the leeward turbulence rule appear repeatedly — link each to its cause (thunderstorm downdraft, abrupt wind change, flow over an obstacle) rather than memorizing isolated definitions.