Which weather condition is likely to occur on a clear, relatively still night?

On a clear, relatively still night, the likelihood of a temperature inversion occurring is high due to the specific atmospheric conditions present during this time. A temperature inversion happens when a layer of warm air traps cooler air close to the ground. Under clear skies, the ground loses heat quickly through radiation, cooling the air directly above it. If there is stillness and little to no wind, this cooler air is not mixed with the warmer air above, leading to an inversion.

In contrast, the other weather conditions listed are typically associated with different atmospheric dynamics. Heavy snowfall usually requires specific moisture-laden conditions, often associated with low-pressure systems or fronts, rather than a still and clear night. Thunderstorms typically form in conditions of rising heat and moisture, which would not be present on a calm and clear night. Tornadoes also need significant atmospheric instability and wind shear, conditions not conducive to a tranquil night with clear skies. Thus, the correct answer reflects the unique atmospheric behavior observed during clear, calm nights.