The best explanation for the student's observations is: A cold front moved into the area.
Here's the reasoning:
- The observation of "sunny and warm" on Day 1 suggests stable weather conditions.
- The "cloudy and rainy" on Day 2 indicates a disturbance in the weather pattern, commonly associated with the arrival of a cold front.
- A "drop in temperature" on Day 3 is typical following the passage of a cold front.
- Finally, "clear skies and cold temperatures" on Day 4 suggests that the cold front has passed, leading to clearer and colder conditions.
These observations align well with the effects of a cold front moving through the area.