Fog Bank Under the Inversion
At 0645 in late October, a balloon pilot launches from a river valley airport at 400 ft MSL. Surface temperature is 8°C, dew point 6°C, wind calm, sky clear. The forecast calls for a strong high-pressure system, light winds, and a pronounced morning inversion. The pilot plans a one-hour flight to 2,500 ft AGL. During the climb, the balloon passes through a layer of haze at 300 ft AGL. At 1,200 ft AGL, the outside air temperature is 10°C and the dew point is 7°C. To the north, about 2.5 NM away, the pilot sees a low, thin fog bank forming over the river and moving slowly south at about 2 knots. The balloon's ground speed is 4 knots toward the north. There are several open fields within 1 mile. The pilot has 45 minutes of propane remaining.
1. Calculate the temperature-dew point spread at the surface and at 1,200 ft AGL, and explain what the vertical temperature profile indicates.
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Surface spread = 8°C − 6°C = 2°C. At 1,200 ft AGL, spread = 10°C − 7°C = 3°C. The temperature increases from 8°C at the surface to 10°C at 1,200 ft, which is an inversion - temperature increasing with height. Inversions are stable and trap moisture, haze, and smoke near the ground. A spread of 2°C at the surface is close to saturation; with clear skies, calm wind, and high pressure, radiative cooling can easily cool the surface air to the dew point. The visible fog bank is stronger evidence than the numerical spread: moisture is already condensing nearby. The 3°C spread at 1,200 ft does not mean the surface is safe, because the fog is forming below and moving with the shallow surface flow.
2. Assess the risk of the fog bank reaching the balloon's flight path. Use the closing speed and distance to support your answer.
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Closing speed = balloon 4 knots north + fog 2 knots south = 6 knots. Distance is 2.5 NM, so time to encounter = 2.5 ÷ 6 = 0.42 hour, or about 25 minutes. The pilot has 45 minutes of propane, so fuel is not the limiting factor; visibility is. The fog is low and thin now, but inversions can thicken fog quickly. If the balloon stays at 2,500 ft AGL, it may be above the fog, but the pilot will not be able to see landing sites or avoid obstacles during descent, and the fog may rise. The risk is high and the time margin is short. The pilot should not continue north toward the fog bank.
3. What decision should the pilot make and why? Include the effect of the inversion on fog formation and on the pilot's ability to land.
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Land now at the nearest suitable open field, before the fog arrives. The pilot should choose a field with good access, avoid trees and power lines, and descend while visibility is still good. The inversion supports fog: clear skies and calm winds allow cooling to the dew point; the inversion traps the cool, moist air and prevents mixing. Once fog forms, the pilot may lose sight of the ground and be unable to judge drift or choose a landing spot. Climbing above the fog is not a safe plan because the fog top is uncertain and the balloon must eventually descend through it. The 45-minute propane reserve is not a reason to press on; it is a reason the pilot has time to land deliberately now. After landing, secure the balloon and wait for the fog to clear.