10.5 Weather Observations: Reading the Sky and the Site
Key Takeaways
- Dust devils, willy-willies and circling birds are visible indications of thermal activity and of turbulence that will not appear in any forecast.
- Wind gradient is the increase in wind speed with height above the surface, so the wind at 100 m AGL is routinely stronger than the wind felt at the launch point.
- Windshear is a sudden change in wind speed or direction over a short distance, and it is most severe near fronts, thunderstorm outflows and terrain.
- Precipitation static is an electrostatic charge built up by flight through precipitation or dust, which produces radio noise and degrades the C2 link.
The Forecast Is Not the Weather
A forecast describes an area. A remote pilot operates at a point, at a height of a few tens of metres, often in the middle of terrain and structures that the forecast model never resolved. Schedule 4 therefore asks for a separate skill: weather observation — recognising, on site and with your own eyes, the indications of conditions that will affect the flight.
Indications of Thermals, Turbulence and Dust Devils
Thermals are rising columns of warm air produced by uneven surface heating. Over a dark ploughed paddock, a bitumen carpark or a rock outcrop, the surface heats faster than its surroundings, the air above it warms and rises, and cooler air flows in beneath. For a small RPA a thermal is a sudden, invisible updraught followed by compensating sink at its edges.
Visible indications:
- Dust devils (locally, willy-willies) — rotating columns of dust and debris, from a metre to tens of metres across. A dust devil is a violently rotating thermal made visible by the material it has picked up, and a small multirotor caught in one can be flipped. Treat one as a no-go for the area it is tracking through.
- Circling birds gaining height without flapping — they are working a thermal, and they are marking it for you.
- Cumulus cloud forming over a particular feature on a hot day, marking the top of a thermal column.
- Dust, chaff or loose vegetation lifting in a rotating pattern rather than blowing steadily downwind.
- Shimmer or heat haze over a dark surface.
Mechanical turbulence indications are different: a windsock that is steady one moment and slack the next, tree canopies moving inconsistently across a site, ripples and cat's-paws travelling erratically across water, and — most reliably — a large gust spread in a METAR. The difference between the mean and the gust is a direct turbulence indicator: 27015G25KT has a spread of 10 knots and describes a turbulent flow even though 15 knots sounds modest.
Wind Gradient
Wind gradient is the increase in wind speed with height above the surface. Friction slows the air in contact with the ground, so surface wind is lighter than wind a hundred metres up. The gradient is steeper over rough surfaces — trees, buildings, broken terrain — and shallower over water and smooth ground.
The operational consequence is direct and frequently underestimated: the wind at your operating height is stronger than the wind you feel at the launch point. A hand-held anemometer reading 12 knots at head height can easily correspond to 20 knots at 100 m AGL. An aircraft launched in "acceptable" wind can find itself unable to make headway against the wind at height — the classic scenario where a drone at the far end of a survey line cannot get home.
Practical technique:
- Check the forecast wind at height, not just the surface wind.
- Do a wind check climb: hover at 20 m, then at your operating height, and watch the tilt angle and the current draw. A markedly greater tilt at height is the gradient made visible.
- Plan the outbound leg downwind and the return upwind, so the leg with the strongest headwind is flown with the most battery remaining — not the least.
Windshear
Windshear is a sudden change in wind speed or direction over a short distance, horizontally or vertically. Where wind gradient is a gradual change with height, windshear is an abrupt one, and it is the more dangerous.
Where remote pilots meet it:
- Thunderstorm outflow. The most violent form. A downburst spreading out beneath a cell produces an intense, rapidly changing wind that arrives ahead of any rain — which is why the storm 15 km away that has not reached you is already a reason to land.
- Frontal passage. A cold front brings an abrupt wind shift, often through 60 degrees or more, together with a sharp increase in speed.
- Terrain and structures. On the lee side of a ridge, a building or a treeline, the flow separates and the wind can reverse direction over a few metres.
- The top of a sheltered area. Climbing out of the lee of a building into the free stream produces a step change in wind that the aircraft meets all at once.
The symptoms in flight are a sudden change in the aircraft's attitude or drift with no stick input, an abrupt change in current draw, and — on a fixed-wing — a rapid airspeed change. The response is to descend below the shear layer if that is where the calm air is, or to leave the area, and not to persist with the mission while the aircraft is being thrown around.
Convection, Humidity and Precipitation Static
Three further phenomena from Schedule 4 topic 1 deserve their own note.
Convection is the vertical transport of heat by rising air, and it is the engine behind thermals, cumulus cloud and thunderstorms. Its practical signature for a remote pilot is a daily cycle: calm at dawn, building through late morning as surface heating strengthens, peaking in the mid-to-late afternoon, and easing after sunset. That cycle is the single best argument for scheduling precision RPA work in the early morning — the air is at its most stable, the wind is lightest, and convective turbulence has not yet developed.
Rain and humidity. Most commercial RPA are not rated for flight in rain. Water ingress into motors, ESCs and connectors causes corrosion and short circuits, and moisture on a camera lens or a sensor window ruins the data. Humid air is also slightly less dense than dry air at the same temperature and pressure, which contributes marginally to density altitude. Beyond the aircraft, high humidity raises the risk of fog and reduces the temperature–dew point spread.
Precipitation static (P-static) is the electrostatic charge that builds on an airframe flying through rain, snow, dust or ash. As the charge dissipates it produces broadband radio noise, which degrades reception — the classic symptom is a crackling, noisy radio and a falling link-quality indicator during or after flight through precipitation. On a small RPA the effect is modest, but in dusty conditions it is a real and under-recognised contributor to link degradation, and it is another reason not to fly through precipitation.
Building an On-Site Observation Routine
Before every flight, spend two minutes on observation rather than only on the forecast:
- Look at the sky. Cloud type, cloud amount, whether cumulus is building, whether anything is developing upwind.
- Look at the wind indicators. Trees, flags, smoke, water surface, dust. Are they steady or gusty? Is the direction consistent across the site?
- Measure the wind at head height with an anemometer, and mentally add the gradient for your operating height.
- Look for thermal indicators — dust devils, circling birds, shimmer over dark surfaces.
- Look upwind for anything approaching: a shower, a cell, a frontal cloud line.
- Compare with the forecast. If what you see does not match what was forecast, trust your eyes and re-check the latest observation before launching.
A remote pilot measures 12 knots with a hand-held anemometer at head height. The aircraft's published wind limit is 18 knots and the planned operating height is 100 m AGL. What should the pilot consider?
A rotating column of dust is seen moving across the operating area on a hot afternoon. What does it indicate and what should the pilot do?
What is precipitation static and how does it affect an RPA operation?