Start with the big picture, not the numbers

Before you look at a single METAR, spend two minutes on the surface pressure chart. It tells you where the weather is coming from and how fast it is moving. In the UK, most of our weather arrives from the Atlantic, so a deepening low to the west or north-west is an immediate warning. Look at the spacing of the isobars: tight spacing means strong winds, and a kink or trough can mean squally showers. Fronts matter too. A warm front typically brings a gradual lowering of cloud and steady rain or drizzle. A cold front brings a sharper band of heavy rain, a wind veer, and often a rapid improvement followed by showers. If the chart shows a warm sector over your route, expect low cloud, poor visibility, and drizzle. If a ridge of high pressure is building, you can be more optimistic, but watch for mist and fog forming overnight in light winds.

• Low pressure nearby: expect wind, cloud, and precipitation.
• Cold front passage: timing is critical; conditions can change in minutes.
• Warm sector: classic low cloud and visibility problems.
• Ridge: often good, but radiation fog risk in autumn and winter.

Decoding METARs: the here and now

A METAR is a snapshot, usually issued every 30 minutes or hourly. Take this typical example: EGLL 121150Z 24012KT 9999 SCT025 BKN040 12/08 Q1013 NOSIG. Decode it in groups. EGLL is the aerodrome. 121150Z means the 12th day of the month at 11:50 UTC. 24012KT is wind from 240 degrees at 12 knots. 9999 means visibility 10 km or more. SCT025 BKN040 means scattered cloud at 2,500 ft and broken cloud at 4,000 ft. 12/08 is temperature 12°C and dewpoint 8°C. Q1013 is the QNH in hectopascals. NOSIG means no significant change is expected in the next two hours.

Do not just read the numbers; read the relationships. A dewpoint spread of 4°C or less with a light wind and clear skies is a fog warning. A gusting wind, shown as 24015G28KT, changes your crosswind and turbulence planning. CAVOK means visibility 10 km or more, no cloud below 5,000 ft, and no cumulonimbus or significant weather. NOSIG is helpful, but not a guarantee. If the METAR shows BR (mist), FG (fog), SHRA (showers of rain), or TS (thunderstorm), treat those as decision points, not decoration.

TAFs: planning the next few hours

A TAF is a forecast for an aerodrome, typically covering 9 to 30 hours. It uses change groups that tell you when and how the weather will evolve. BECMG means a gradual, permanent change. TEMPO means temporary fluctuations, usually lasting less than an hour at a time. PROB30 or PROB40 means a 30% or 40% probability of a condition occurring. For example: TAF EGLL 121100Z 1212/0123456789KT 9999 SCT025 BECMG 1214/1216 26018G30KT TEMPO 1216/1222 5000 SHRA BKN012. This tells you that after 14:00 to 16:00 UTC, the wind will strengthen and veer, with temporary periods of 5 km visibility, showers of rain, and broken cloud at 1,200 ft.

• BECMG: expect a lasting change; plan for it.
• TEMPO: brief but potentially disruptive; keep an escape route.
• PROB: not certain, but if the downside is serious, treat it as real.
• Timing: compare the TAF period with your departure and arrival times.

Always cross-check the TAF against the latest METAR and the surface chart. If the TAF says improvement but the METAR shows fog and the chart shows a stationary front, believe your eyes and the trend, not the optimistic forecast.

Hazards that change the decision

UK aviation weather rarely gives you a single clean reason to cancel. It gives you a stack of small clues. Low cloud and poor visibility are the most common. For VFR flights, a cloud base below 1,500 ft or visibility below 5 km should make you pause. For IFR flights, approach minima and alternate requirements matter. Wind is next: a crosswind component above your personal limit, or gusts that exceed the steady wind by 10 knots or more, reduces your margin. In winter, look for FZRA (freezing rain), SN (snow), and FZFG (freezing fog). In summer, look for CB (cumulonimbus), TS, and SHRA. Icing is a serious risk in cloud at temperatures between 0°C and -20°C, especially in stratiform cloud. Turbulence is likely near fronts, in strong gusts, and downwind of high ground.

The go/no-go decision

A safe go/no-go decision is a chain of evidence. Start with the surface chart, move to METARs and TAFs, then add your own limits: recency, aircraft performance, fuel, alternate, and the possibility of turning back. Write down your personal minimums before you need them. If any single element is outside those minimums, do not launch hoping it will improve. If two or three elements are marginal, the answer is almost certainly no. If the weather is good but the trend is deteriorating, plan an earlier return or a different route. A briefing is not a box-ticking exercise; it is how you stay ahead of the aircraft. Read it, question it, and make the call on the ground, where you still have options.