"Pilot error combined with bad weather is the biggest cause of accidents." That’s the line from the syllabus and it’s borne out by every operator’s incident log. Most weather-related drone losses aren’t catastrophic conditions — they’re marginal conditions misjudged. This briefing walks through what we look at before every survey flight and how we make the Go / No-Go call.

The atmosphere up to where we work

Commercial drone operations live in the bottom slice of the troposphere — the layer from the surface up to roughly FL360 (about 11 km). The troposphere is where almost all weather happens, where temperature decreases with altitude at a standard lapse rate of roughly 2°C per 1,000 ft (or 6.5°C per 1,000 m).

The standard ICAO atmosphere reference values are pressure 1013.25 hPa and temperature 15°C at sea level. Synoptic charts visualise the actual conditions: isobars are lines of equal pressure. Closer isobars mean a steeper pressure gradient — and that means stronger wind.

Wind: direction, speed, gust

Wind is the horizontal movement of air from high to low pressure. The wind a pilot experiences on site — surface wind — is slower than upper-level winds because of friction with terrain, vegetation and buildings.

Three numbers matter:

  • Direction — expressed in degrees from true north (e.g. 90° is wind blowing from the east).
  • Speed — in knots (1 kt = 1.15 mph = 1.852 km/h) for aviation purposes, m/s in scientific.
  • Gust — brief, sudden increases above the steady wind speed.

The gust value is the one operators get wrong. Even a moderate steady wind with a gust 8–10 kt above can push the aircraft outside its operational envelope momentarily. The rule we apply: peak gust value must sit inside the airframe’s rated maximum wind tolerance, with margin. If the gust forecast brushes the envelope, the answer is No-Go.

Local winds

Standard met forecasts cover synoptic-scale wind. The local-scale wind at a specific site can be very different:

  • Anabatic winds — upslope, daytime, caused by sun heating mountain slopes.
  • Katabatic winds — downslope, nighttime, cold dense air flowing down cooled slopes.
  • Sea / land breezes — convective circulation between land and water at differential heating rates.
  • Venturi effect — airflow accelerates through narrow gaps between obstacles. Often catches operators flying near tall buildings or between hills.

And there’s wind gradient: wind speed typically increases with height above ground. The wind at the operator’s position can be very different from the wind at 50 m altitude. Masking by buildings, trees or hills causes localised wind shadows and sudden gusts as the aircraft crosses the masked / unmasked boundary.

Turbulence: four flavours

Turbulence is irregular air movement caused by variations in wind speed and direction. Five common types for surface-level work:

  • Mechanical — air disrupted over buildings, trees and terrain. The default urban-survey turbulence.
  • Convective — rising warm air from daytime surface heating or thunderstorms. Often associated with cumulonimbus clouds.
  • Orographic — air moving over mountains, producing mountain waves and rotors on the leeward side.
  • Frontal — sharp transition zones between air masses of different temperature and density.
  • Clear-air — jet stream turbulence at high altitudes; mostly irrelevant for drone ops but a useful concept for understanding hazard prediction.

Cloud, by type and altitude

Cloud classification by shape:

  • Cirrus — thin, wispy, high-altitude. Generally indicates fair weather, possibly changing.
  • Cumulus — fluffy, cotton-like. Fair-weather cumulus is benign; growing cumulus signals convective activity.
  • Stratus — uniform grey layers. Often associated with drizzle and reduced visibility.
  • Cumulonimbus — thunderstorm clouds. Capable of severe turbulence, lightning, very strong updrafts and downdrafts, heavy rain and hail. Hard No-Go.

By altitude: low-level (0–6,500 ft) covers stratus and stratocumulus; mid-level (6,500–20,000 ft) altostratus and altocumulus; high-level (above 20,000 ft) cirrus and cirrostratus.

Precipitation and IP ratings

Most consumer-grade drones have no ingress protection rating. Light rain can damage motors, electronics and gimbals; heavier rain almost certainly will. Commercial airframes with an IP-rated body can fly in light precipitation, but two cautions apply:

  1. The IP rating typically covers the airframe only — the controller, screens and payload may not be IP-rated and can fail.
  2. Some airframe configurations compromise the IP rating (e.g. with certain payloads or accessories attached).

The IP code itself is two digits: first digit is solid particle protection (0–6), second is water protection (0–9). IP55, for example, means dust-protected and protected against water jets from any direction. A DJI Mavic 3T has no IP rating; the larger Matrice 30T is IP55. A DJI Matrice 4D-series airframe is also IP55.

Temperature

Cold reduces battery capacity and peak discharge current — less flight time, less thrust headroom. Most consumer-grade batteries should not be launched cold; if winter operations are scheduled, batteries are pre-warmed in the vehicle and not deployed below about 5°C.

Heat is the other end. High temperatures can soften propellers, accelerate battery ageing, and contribute to thermal runaway risk during charging. Flight in direct strong sunlight with batteries above 45°C is not recommended.

In humid cold, icing can form on propeller leading edges and the airframe, changing weight and aerodynamics. Moisture plus sub-zero temperatures is enough; you don’t need actual precipitation.

Visibility and what you can’t see

VLOS — visual line of sight — depends on you being able to see the aircraft unaided. The maximum VLOS distance varies with conditions:

  • Aircraft size and visual conspicuity
  • Any onboard lighting
  • Sun glare, fog, drizzle, haze
  • Pilot eyesight
  • Terrain and obstacles between you and the aircraft

The often-cited 500 m practical maximum applies in clear weather with a mid-size aircraft. In poor visibility it can drop substantially — and the pilot is required to maintain VLOS at all times, which means staying inside whatever the day’s actual limit is.

Mist reduces visibility to no worse than 1 km; fog is the same phenomenon at a denser concentration, reducing visibility below 1 km. Either is a No-Go for routine VLOS work.

The information sources

For pre-flight weather:

  • Met Office — primary forecaster, including aviation-specific products.
  • METAR — routine aerodrome weather reports issued every 30 or 60 minutes.
  • TAF — Terminal Aerodrome Forecast, expected weather trends.
  • SIGMET / AIRMET — significant and moderate hazardous-weather advisories.
  • Local hill / sea / urban observations — the site itself, on the day.

The arrival check on site — recce the surroundings, check imminent weather, measure surface wind with an anemometer, assess gust, look at the sky — is the final filter. If the on-site reality doesn’t match the morning’s forecast, the forecast loses.