Wind Shift During a Crossing
You are tracking a slow-moving weather system. At launch, surface wind is 180°/8 kt, and the 925 hPa wind is 200°/15 kt. A nearby aerodrome reports a 1500 ft wind of 220°/20 kt. Your planned track requires a heading of 090° to reach a valley landing site 25 km east. Two hours into the flight, the surface wind veers to 240°/12 kt and the 1500 ft wind backs to 190°/25 kt. The temperature-dewpoint spread at the surface is 2°C, and the valley floor is 300 m below your current altitude. You have 45 minutes of fuel remaining above reserve and no alternate landing site within 8 km except a ploughed field.
1. Explain how the reported wind changes affect your ability to reach the valley site, and what the backing aloft wind implies.
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At launch the winds were broadly south-westerly, giving a track roughly east with drift. After the shift, the surface wind veers to 240°, which pushes the balloon towards 060° rather than 090°, and the 1500 ft wind backs to 190°, which is almost a headwind component for an easterly track. Backing aloft while the surface veers indicates a change in the vertical wind profile, often associated with a frontal passage or a developing low. The pilot should expect increasing shear and turbulence between layers, and the effective drift may be slower than planned. Reaching the valley site 25 km east in 45 minutes requires an average ground speed of about 33 km/h, which the current winds do not support. The pilot should plan to land short rather than press on.
2. Assess the significance of the 2°C temperature-dewpoint spread and the valley 300 m below, and state a decision.
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A 2°C spread is close to saturation, so any lifting or cooling could produce cloud or fog. Descending into a valley 300 m below exposes the balloon to cooler, denser air and often to drainage flow and reduced visibility. Combined with the ploughed field as the only nearby landing option, the risk of a low-level landing in poor visibility or a hard landing on rough ground is high. The decision should be to land while still over open, suitable ground, before entering the valley, and to accept the ploughed field only if it is confirmed clear of obstacles and the surface wind is manageable. Continuing toward the valley with 45 minutes of fuel and no alternate is not justified.