10.4 Temperature, Density, Precipitation, Visibility & Icing

UK CAA A2 Certificate of Competency A2 CofC

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1 Introduction & A2 CofC Pathway

1.1 Exam Facts, Format, Pass Mark & Validity 1.2 Flyer ID, Operator ID & Practical Declaration 1.3 Operating Guides, Maintenance & Safe-to-Fly Decisions

2 Open Category Air Law

2.1 Open Category A1 / A2 / A3 & 2026 Near People Rename 2.2 VLOS, 120 m Height Limit & Flight Restriction Zones 2.3 Uninvolved Persons, Assemblies & Giving Way to Manned Aircraft

3 Near People Separation Rules

3.1 UK2/C2 Separation: 30 m Normal & 5 m Low-Speed Mode 3.2 Legacy Aircraft 50 m Separation & MTOM < 2 kg 3.3 The 1:1 Rule, Height vs Horizontal Distance & Safe Distances

4 Operational Ground Risk Mitigations

4.1 Site Selection, Segregation & Pre-Flight Briefing 4.2 Reacting to Approaching People & Abort Decisions 4.3 Weather, Performance & Segregation Checks Before 5 m Ops

5 Technical & Human Ground-Risk Mitigations

5.1 Prop Guards, Parachutes, Geo-Awareness & Other Technical Mitigations 5.2 Kinetic Energy, Mass, Speed & Ground Impact Risk 5.3 Medical Fitness, Alcohol, Medication & Fatigue

6 Aircraft Eligibility & Flight Characteristics

6.1 UK0, UK1 & UK2 Class Marks and A2 Eligibility 6.2 Rotorcraft, Fixed-Wing & Hybrid Operating Envelopes 6.3 Basic Principles of Flight, Stability & Control

7 Practical Procedures, Emergencies & Decisions

7.1 Flight Procedures & Basic Practical Drills 7.2 Emergency & Abnormal Procedures 7.3 Decision-Making, Customer Pressure & Occurrence Reporting

8 UAS Flight Performance Systems

8.1 Mass, Balance, Payload Effects on Performance 8.2 Battery Terminology, Types, Charging & Storage 8.3 Failsafe Modes, Return-to-Home & Lost-Link Behaviour

9 Navigation, Compass & Flyaways

9.1 GNSS Position Hold, Drift & Loss of Positioning 9.2 Compass Errors & Magnetic Interference 9.3 Firmware, Braking Distance & Flyaway Causes

10 Meteorology for Near-People Operations

10.1 Weather Sources, Reports, Forecasts & Local Assessment 10.2 Wind Speed, Gusts & Manufacturer Limits 10.3 Wind Gradient, Masking & Urban Turbulence 10.4 Temperature, Density, Precipitation, Visibility & Icing 10.5 Clouds, Cumulonimbus, Lightning & IP43

10.4 Temperature, Density, Precipitation, Visibility & Icing

Key Takeaways Explain This Topic AI

Hot, humid, high conditions reduce air density and can reduce thrust margin, while cold can improve density but degrade battery power; temperature effects must be considered together.

Visual Line of Sight (VLOS) requires adequate visibility: fog, heavy precipitation, or low cloud can make Open flight impossible even in light wind. Night flight remains possible only when VLOS and all other rules are met and a green flashing light stays activated.

Icing risk on propellers in cold moisture can destroy thrust rapidly — a hard no-go near people.

Build an integrated go/no-go framework that combines weather, aircraft performance, and segregation before any Near People sortie, especially before 5 m operations.

UAS.OPEN.030’s weather–performance–segregation triad is the regulatory backbone of that framework for reduced separation.

Last updated: August 2026

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):

• Factor Density effect Multirotor result
• High temperature Warmer air is less dense Harder hover, shorter endurance, weaker climb

High humidity Moist air is slightly less dense than dry air at the same pressure/temperature (exam-level teaching) Extra performance debit on already hot days

High elevation / low pressure Fewer air molecules per volume Same as “high density altitude” thinking from manned aviation, applied to small UAS

Hot + high + humid combined Worst common stack Motors near limits; early low-battery events; little reserve for abort climbs

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

• Visibility condition Typical A2 decision
• Clear air, aircraft easily seen at planned range Proceed if wind/density/segregation also OK
• Patchy fog, aircraft intermittently lost No-go or delay until VLOS is continuous
• Precipitation that hides the aircraft or people entering the site No-go
• Low cloud base near planned working height Cap height further or cancel; do not “pop up” into cloud

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:

• Weather conditions
• Performance of the unmanned aircraft
• 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)

• Check Go only if…
• Sustained wind & gusts Comfortably below manufacturer max with margin for the task
• Gradient & turbulence Site geometry (upwind preference, lee avoidance) keeps control reliable
• Precipitation / IP No wetting beyond what the aircraft is suited for
• Visibility Continuous VLOS from the control point at planned range/height
• Icing No cold-moisture icing risk
• Density day type Hot/humid/high performance debit understood and acceptable
• Layer B — Performance (link to Chapters 8–9)

• Check Go only if…
• Mass / payload Under manufacturer MTOM and class ceiling (e.g. UK2 < 4 kg including payload)
• Battery Healthy, warm enough, endurance realistic for weather
• Modes Low-speed mode actually active if claiming 5 m
• GNSS / compass / firmware No red flags; urban multipath risk considered
• Failsafe / RTH path Does not default over people; contingency landings clear
• Braking / stop distance Fits inside planned buffers at expected ground speed
• Layer C — Segregation (link to ground-risk chapters)

Check Go only if…

Uninvolved persons Horizontal floors met (30 m normal / 5 m only if fully qualified / 50 m legacy as applicable)

• Overflight No intentional overflight of uninvolved persons
• Access control Site plan keeps people out of the operating volume
• Contingency Abort and landing areas remain clear as weather evolves
• Mid-flight people You will move or land , not “finish the orbit first”

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

• Topic Anchor
• Density killers Hot, humid, high
• Density effect Less lift, harder work, shorter endurance
• Visibility Required for VLOS — fog/precip/low cloud
• Icing Cold moisture on props → thrust loss → no-go
• 5 m triad Weather + performance + segregation + low-speed on
• Framework All layers green, then continuous re-check

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 .