Which factor contributes most significantly to the formation of sleet?

The formation of sleet is primarily influenced by rapid temperature changes, which is the correct choice in this scenario. Sleet occurs when there is a layer of warm air situated above a layer of cold air, allowing ice pellets to form. The process begins when snowflakes fall through the warm layer, causing partial melting. As these partially melted snowflakes continue to descend and enter the colder air layer below, they refreeze into small ice pellets before reaching the ground. Rapid changes in temperature cause this layer of warm air to fluctuate, leading to the conditions necessary for sleet formation.

High humidity, while contributing to overall precipitation, does not directly affect the specific conditions required for sleet. Freezing rain and high wind speeds may impact other types of precipitation but are not key factors in the specific development of sleet. Understanding the importance of temperature profiles in the atmosphere is crucial to grasping how different forms of frozen precipitation, including sleet, occur.