- a. The whistle of a passing train is often heard more clearly on a quiet misty night. This phenomenon is typically due to atmospheric conditions like temperature inversions that often occur on clear, still nights, especially with ground-level mist or fog. In an inversion, air temperature increases with altitude for a layer, causing sound waves to refract (bend) downwards towards the ground, enhancing propagation over distance. - b. Velocity of sound in air is not directly proportional to humidity in air. The speed of sound in air increases slightly with humidity, but the relationship is not a simple linear proportionality. The speed primarily depends on temperature (proportional to the square root of the absolute temperature) and is also affected by the composition of the air (humidity changes the average molar mass).