Lahore Winter Fog: Reading a TAF Whose Validity Ends Too Early

This is radiation fog. It is a long December night (sunset about 1715, sunrise about 0700), the sky is clear, and the wind is calm, so the boundary layer cools by long-wave loss with no turbulent mixing to deepen or disperse the cooling. The temperature falls from 9 to 7 °C in two hours while the dewpoint rises from 8 to 7 °C, closing the spread to 0 °C - saturation is essentially reached by 2200–2300, consistent with the BECMG 2221/2223 to 600 m.

The 850 hPa temperature of +12 °C over a 7 °C surface means a strong, shallow inversion of about 3 °C in 4,000 ft, which caps the moist boundary layer and traps any condensate. With calm wind the fog forms first as a shallow layer 30–60 m deep, thickens by further cooling, and - critically - persists rather than being lifted, because there is no wind to mix it out. At 7 °C it is not freezing fog.

Dissipation: after sunrise the ground warms and the inversion erodes from below, but with a deep inversion and no mixing, clearance is slow, typically 0900–1000, and patchy fog often reforms the following night. The TAF's 0300 end time does not cover the morning, so a re-issue is needed for the arrival and departure peak.

The TAF is internally consistent, but it is unfavourable at the ETA. From 2300 the aerodrome is forecast to be permanently at 600 m in fog (BECMG), with temporary deteriorations to 200 m and vertical visibility 200 ft between 2200 and 0200 (TEMPO).

At 550 m RVR and DH 200 ft the crew's CAT I minima are already roughly equivalent to the BECMG condition, so even the 'good' phase is marginal. The TEMPO phase is clearly below minima: 200 m visibility with VV002 means the sky is obscured and there is no cloud base on which to fly a CAT I decision height - the approach would be illegal and unsafe. The company's 175 m LVP minima are irrelevant here: they apply only to qualified crews and suitably equipped aircraft, which this flight is not.

The forecast at the 2330 ETA is therefore below minima for a sustained period. The planning assumption must be that a landing at Lahore is unlikely, and the decision should be framed as 'where do we divert' rather than 'can we get in'. Continuing to destination is acceptable only in the sense of positioning for a possible improvement; the crew should be prepared to divert without a second approach.

Islamabad at 5 km in mist with no significant cloud satisfies the alternate weather requirement, but it is a weak choice on two counts. First, it lies in the same airmass only about 270 km north-east of Lahore, under the same inversion, so a regional fog outbreak will probably affect it too - mist at 5 km at 0300 is exactly the precursor pattern seen at Lahore two hours earlier. Second, it is close enough that a diversion there leaves little fuel flexibility if it also deteriorates.

Fuel: one approach plus 30 minutes of holding will not outlast a fog event that the TAF itself implies will persist into the morning. Holding for half an hour in the hope of a 200 m fog clearing is not a credible plan.

Recommendation: either delay departure so that arrival falls after the forecast break-up (roughly 0900–1000), or select an alternate well outside the fog-prone Punjab plain with a large margin over its minima, and carry fuel to reach it after a missed approach without eating final reserve. If no such alternate can be identified within range, the flight should not be dispatched at all. If the commander does continue, the briefing should state that no approach below 550 m RVR will be attempted and that the diversion will be initiated before the holding fuel is consumed.