10.4 Temperature, Density, Precipitation, Visibility & Icing

Beyond Wind: Density, Eyesight, and Ice

Wind and turbulence (Sections 10.2–10.3) dominate many A2 meteorology questions, but they are not the whole subject. Air density, visibility, and icing decide whether the aircraft can produce thrust, whether you can legally and practically keep Visual Line of Sight (VLOS), and whether a cold, wet day is survivable for composite propellers spinning at high RPM. This section closes the meteorology chapter with an integrated go/no-go framework for Near People (A2) operations.

Air Density — Hot, Humid, High

Lift and propeller thrust depend on air density. Denser air gives the rotors more mass to accelerate each second; thinner air means the same RPM produces less thrust. Multirotors then command higher power to hover, climb, and fight wind.

Conditions that lower density (and therefore performance):

Near People meaning

• Less lift / performance margin — abort climbs away from people are sluggish.
• Shorter endurance — RTH and contingency energy disappear earlier than brochure times.
• Higher continuous current — heat in motors/ESCs; cold-battery problems can stack with density problems on odd shoulder-season days.
• Payload sensitivity — a heavy camera that was acceptable on a cool sea-level morning may be unsafe on a hot inland afternoon (link to flight-performance mass teaching).

Exam phrase: hot / humid / high-elevation → lower air density → less lift and performance → multirotors work harder with shorter endurance.

You will not be asked to compute density altitude with a circular slide rule on a typical A2 paper. You will be expected to choose the safer option when a stem describes a hot hillside job with a heavy UK2 and people nearby: reduce payload, shorten the task, wait for cooler air, or cancel.

Density vs wind — do not trade one risk for another

A light-wind hot day is not automatically a green light. Density can steal the climb you need when a person walks into the buffer. Conversely, a cool dense day with severe lee turbulence can still be a no-go. All meteorology factors must clear together.

Visibility and VLOS

Open-category A2 pathway flying is VLOS — the remote pilot must be able to see the unmanned aircraft well enough to monitor its flight path and keep clear of other aircraft, people, and obstacles (within the applicable Open rules). Meteorology that destroys visibility destroys the operation even when the wind rating is perfect.

Threats to visibility

• Fog and mist — especially dawn valley fog and coastal haar
• Heavy rain, snow, or spray — also an IP and camera issue
• Low cloud — aircraft may enter cloud or be lost against grey backgrounds at modest height
• Haze, smoke, and low sun glare — contrast loss at range
• Night or dusk — Open-category night flight is permitted with a continuously activated green flashing light, but darkness can reduce practical VLOS and obstacle/traffic detection

Critical point: apps that show “wind OK” do not measure whether you can see this aircraft against this sky. Do a real visual check from the control position before and during flight. If you must use binoculars or a screen as a substitute for unaided VLOS, you are already outside the Open VLOS model taught for this certificate path.

Visibility also protects ground risk awareness. You cannot segregate what you cannot see. A person stepping from a doorway in fog is invisible until they are inside your planned bubble.

Icing Risk on Propellers

Icing in cold moist air is a classic small-rotor hazard. Supercooled droplets or wet snow can freeze on leading edges of fast-spinning props. Ice:

• Changes blade aerofoil shape and roughness
• Reduces thrust dramatically
• Creates vibration and possible imbalance
• Can shed asymmetrically, worsening control
• May progress in seconds to minutes, not hours

When to suspect icing risk

• Temperatures near or below freezing with visible moisture (cloud, fog, freezing rain, wet snow)
• Climbing into low cloud on a cold day
• Ops in winter spray or lake-effect moisture
• Aircraft feels underpowered without battery explanation; power rises to hold height

Hard operational rule taught for safety exams: if icing conditions are present or likely, do not fly Open multirotor missions near people. There is no practical de-ice system on typical consumer/prosumer camera drones used in A2 work. A sudden thrust loss at low height beside uninvolved persons is an unacceptable ground-risk outcome.

Cold weather also links to battery sag (flight-performance syllabus): density may be higher (good for thrust) while the pack is weaker (bad for available power). Integrated thinking means you evaluate both.

Integrated Go/No-Go Framework for Near People (A2)

UAS.OPEN.030 does not publish a consumer checklist PDF inside the article text, but it does require that before reducing separation to 5 m with low-speed mode on an eligible aircraft, the remote pilot evaluates:

1. Weather conditions
2. Performance of the unmanned aircraft
3. Segregation of the overflown area

That triad is the skeleton of a professional go/no-go model for all Near People flying — not only the 5 m case. Use it every sortie; apply stricter thresholds when you plan the closest distances.

Layer A — Weather (meteorology chapter)

Layer B — Performance (link to Chapters 8–9)

Layer C — Segregation (link to ground-risk chapters)

Decision logic

IF any Layer A critical weather item fails → NO-GO or replan (larger buffers / new site / new day)
ELSE IF any Layer B performance item fails → NO-GO or reconfigure aircraft
ELSE IF any Layer C segregation item fails → NO-GO or wait / cordon / change geometry
ELSE IF intending 5 m → confirm low-speed ON + document the three evaluations mentally/briefly
ELSE → GO with continuous monitoring; be ready to reverse to NO-GO mid-flight

Mid-flight go/no-go: weather is not only a pre-flight stamp. Gust fronts, fog banks, and crowd arrivals can flip a green decision to red in minutes. A2 competence includes landing early.

Worked Go/No-Go Examples

Example 1 — summer city centre, UK2, 5 m ambition.
Wind 4 m/s gusting 6, upwind street available, visibility excellent, temperature 32 °C, heavy cinema payload, continuous shoppers on the pavement.

• Weather wind: OK with margin.
• Density/performance: hot + heavy payload → endurance and climb weak.
• Segregation: fails (uncontrolled shoppers).
  Decision: NO-GO for 5 m; likely NO-GO for any Near People hover until segregation exists. Cooler day, lighter camera, and controlled access required.

Example 2 — winter harbour, light wind, freezing fog.
Wind calm, density good, but aircraft disappears at 40 m and moisture glazes cold surfaces.
Decision: NO-GO — VLOS and icing risk. Wind being under limits is irrelevant.

Example 3 — cool autumn, open car park, mean 5 m/s, building 80 m away, low-speed on, trained crew, empty buffer.
Weather, performance, and segregation all pass for a conservative task.
Decision: GO at appropriate separation (use 30 m unless the full 5 m evaluation remains solid for the whole task). Monitor gradient if height increases.

Example 4 — mean wind OK, lee façade only angle the client accepts, people in doorway.
Turbulence + segregation fail.
Decision: NO-GO or redesign (upwind proxy shots, different day, Specific category if the job truly cannot fit Open).

Memory Table — Closing the Meteorology Chapter

Meteorology on the A2 CofC is not about forecasting for airlines. It is about deciding, in plain language, whether the air, the eyesight, the ice risk, and the people picture still allow a small multirotor to operate Near People without inventing safety that is not there. If any critical box is amber or red, the professional answer — and the exam answer — is to wait, move, reconfigure, or cancel.