10.1 Weather Sources, Reports, Forecasts & Local Assessment
Key Takeaways
- Use current authoritative forecast, warning, observation, and aviation information, then compare it with the manufacturer’s limits and the exact time and location of the flight.
- A forecast describes expected conditions over an area and period; an observation describes measured conditions at a place and time. Neither replaces a local site assessment.
- Basic aviation conventions include wind direction and speed, gusts, visibility, cloud amount and height, temperature/dew point, and pressure, but an aerodrome report may not represent an urban A2 site.
- Local assessment must consider wind direction, gust spread, obstacles, precipitation, visibility, cloud development, temperature, surface condition, and how conditions will change.
- If sources disagree, conditions are near limits, or a hazard cannot be observed reliably, add margin, delay, relocate, or cancel rather than select the most convenient number.
Build a Weather Picture, Not One Number
The syllabus starts with obtaining and interpreting weather information. A safe A2 decision uses a current forecast, severe-weather warnings, nearby observations, relevant aviation reports, and a direct local assessment before and during flight.
The Met Office provides UK public forecasts, warnings, and observations. Official aviation reports can add context. Record source and issue time. A screenshot without time, location, units, or update status is weak evidence.
Forecasts and Observations
A forecast predicts likely conditions across an area and period. It contains uncertainty and can smooth local extremes. An observation reports measured conditions at one location and time. It may be precise but becomes stale and may be far from the site.
| Information | Strength | Limitation |
|---|---|---|
| Regional forecast | Trend, timing, broad wind and cloud picture | Can miss street-level effects |
| Weather warning | Significant hazard and expected area/time | Does not clear a courtyard |
| Station observation | Actual measured conditions | Represents that sensor and time |
| Aerodrome report | Standard aviation convention | Airport exposure differs from urban sites |
| On-site observation | Direct local relevance | Short snapshot needing interpretation |
A calm airport report does not clear a rooftop behind a building. A gusty headland station does not prove an inland site is identical. Differences prompt assessment, not selection of the lower number.
Basic Aviation Conventions
Wind
A group such as 24010G20KT means wind from 240 degrees at 10 knots, gusting 20 knots. Convert units before comparing with an m/s limit: one knot is about 0.514 m/s. Do not confuse gust with mean.
Forecast direction may be north-referenced, while streets and buildings channel local wind differently.
Visibility
Visibility may be in metres. In common aviation reporting, 9999 means 10 km or more, not exactly 9,999 m. For VLOS, broad reported visibility is only a start. Haze, glare, rain, low contrast, background, and aircraft size can make practical visual range shorter.
Cloud
Common amounts are FEW, SCT, BKN, and OVC. Heights are often encoded in hundreds of feet above the reporting site. Low cloud can hide the aircraft and contain moisture or icing.
Temperature and dew point
Temperature and dew point close together suggest air nearer saturation, so mist, fog, or low cloud may be more likely when other conditions support it. Pressure supports the wider picture, but the controller’s height display must not be confused with the legal terrain-relative height rule.
A METAR is an aerodrome observation and a TAF an aerodrome forecast. Both are location-specific; neither replaces a site walk.
Local Assessment
At the site assess:
- wind direction, mean, gusts, and lulls over time;
- flags, trees, smoke, rain, and moving cloud;
- corners, roof edges, gaps, tree lines, slopes, and turbulence;
- precipitation and wet surfaces;
- temperature effects on battery and propulsion;
- visibility, glare, and contrast along the route;
- cloud growth, thunder, lightning, or sudden wind shift; and
- whether conditions improve or deteriorate.
Measure in a representative exposed place where safe. A handheld anemometer in a sheltered doorway can under-read roof-height flow. Observe long enough to catch gusts.
Convert Weather into Risk
- Headwind can prevent return and raise energy use.
- Tailwind can increase groundspeed and stopping distance.
- Crosswind can push the aircraft toward a boundary.
- Turbulence can defeat low-speed precision.
- Cold can cause battery sag.
- Heat and low density reduce thrust margin.
- Rain can exceed ingress limits.
- Poor visibility ends VLOS.
- Thunderstorm outflow can change wind before rain arrives.
For a 5 m evaluation, “forecast below maximum” is insufficient. Actual local weather must support predictable control with margin.
Timing and Change
Check setup, repeat flights, and recovery—not only launch time. Define an early stop before forecast deterioration. Reassess when wind changes, gusts rise, cloud builds, precipitation begins, visibility falls, temperature shifts, or the task moves around a building.
Scenarios
METAR versus street. The nearby report shows 8 knots, but the aircraft oscillates behind a warehouse. Local turbulence controls the decision; increase distance or land.
Sources disagree. One app shows 5 m/s while a current official warning describes severe gusts. Review issue times, observe locally, and choose the conservative action.
Dew point close to temperature. Visibility falls toward dusk. Expect possible mist and verify VLOS; a green light does not cure obscuration.
Storm signs. Towering dark cloud, thunder, and a cool gust appear. Land and seek shelter; do not wait for rain.
Remember forecast → observation → local assessment → aircraft effect → people-risk decision.
What is the best use of a METAR for an A2 site among buildings?
What does 24010G20KT broadly mean?
Why is one sheltered wind reading insufficient for roof work?
Sources disagree near a limit. What is sound?