Radiation fog and a BECMG that may not happen
November, 06:50Z. You are booked to fly LIRF (Roma Fiumicino) to LIRQ (Firenze Peretola), 40 minutes, in a C172, VFR only. Off-blocks were planned for 07:10Z; a passenger is waiting at Firenze.
LIRF METAR 070650Z: 00000KT 0300 FG VV001 08/08 Q1022. LIRF TAF 070500Z 0706/0715 00000KT 0800 FG BKN002 BECMG 0708/0710 4000 BR SCT008. LIRQ METAR 070650Z: 09003KT 1200 BR BKN004 09/08 Q1021. LIRQ TAF: TEMPO 0706/0709 3000 BR.
The synoptic situation is a broad, slow-moving high with almost no pressure gradient over central Italy. Local sunrise is about 06:15Z. The previous two mornings at both aerodromes followed the same pattern: fog until about 09:30Z, then 4–6 km in mist by 10:30Z.
1. Using the METAR and the synoptic situation, assess how much faith to put in the BECMG 0708/0710 group.
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Treat it as the least reliable part of the TAF. Radiation fog is a local, self-generating phenomenon: it forms under clear skies, light wind and strong nocturnal cooling, and it dissipates when the sun warms the ground enough to raise the temperature above the dewpoint or to break the inversion. That process is driven by insolation, not by a synoptic feature, and models handle it poorly, which is why the TAF reaches for BECMG and why BECMG here is a change expected within a two-hour window rather than a promise at 08:00Z.
The observations argue for a slower burn. At Roma the wind is calm, the temperature and dewpoint are identical at 08/08 – saturated air – and the vertical visibility is 100 ft. With no pressure gradient there is nothing to mix the fog out mechanically, so the only mechanism is solar heating, and in late November the sun is weak. The two previous mornings did not clear until about 09:30Z. Sunrise is 06:15Z, so the TAF's 4000 m by 08:00–10:00Z means clearing within two to four hours of sunrise, which is optimistic against the local history. Expect improvement, but nearer 09:30–10:30Z.
2. The clouds of the TAF are irrelevant to you: explain the VFR and practical constraints you face at both aerodromes right now, without assuming any change.
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Start with the ceiling. LIRF is reporting VV001 – the sky is obscured, the vertical visibility is 100 ft and the horizontal visibility is 300 m. No VFR flight can take off or land from that: you cannot satisfy the cloud-clearance requirements, you cannot see the runway environment, and the aerodrome is effectively closed to visual flight. LIRQ is better but still not usable: BKN004 with 1200 m is a ceiling of 400 ft.
The numbers you must apply are the VFR minima of the airspace you are in – typically 5 km visibility and a 1500 ft ceiling around a controlled aerodrome, with lower figures only in specific low-level class G cases. Check them in the current AIP rather than from memory: the point is that 1200 m and BKN004 are outside any of them.
The practical constraint is the passenger. A 40-minute flight with fog at both ends and a forecast that may or may not verify is not a schedule problem; it is a decision about whether to sit at Fiumicino and watch successive METARs, or cancel and let the passenger drive. Give yourself a hard time limit, not an open-ended wait.
3. At 09:15Z LIRF reports 0800 BR OVC002 and LIRQ reports 2000 BR BKN005. The BECMG window has passed. Do you go?
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No. Two facts decide it. First, the improvement is real but a long way short of the forecast: LIRF is at 800 m in mist with an overcast at 200 ft, when the TAF wanted 4000 m and SCT008 by 10:00Z. The forecast verified in direction and failed in magnitude, which is the classic radiation-fog outcome and a reason to distrust the rest of the TAF. Second, LIRQ at 2000 m with BKN005 gives you a 500 ft ceiling at destination, and the TEMPO in its TAF expires at 09:00Z, so the improvement you were relying on has now lapsed.
Add the trend logic: the fog at Roma formed under a flat pressure pattern and will only burn off with insolation, so the next METARs are the only reliable forecast you have. Watch for the temperature-dewpoint spread opening up and the vertical visibility becoming a real cloud base.
Decision: hold on the ground, set a firm cut-off – if LIRF is not at least 5 km with a 1500 ft ceiling by 10:30Z, cancel. Do not take off 'to have a look', because with a 300 m start there is no diversion back into Roma if Firenze goes down.