Based on the observations logged in the table, the most appropriate explanation for the changes in weather is:
A cold front moved into the area.
This is supported by the observation of a "drop in temperature" on Day 3, followed by "clear skies and cold temperatures" on Day 4, which typically occurs when a cold front advances and replaces a warm air mass, leading to a drop in temperatures. The occurrences of "cloudy and rainy" conditions on Day 2 also align with the passage of a cold front, which can bring precipitation before the clearer weather settles in.