What mechanism causes the cooler air over water to move towards land during a sea breeze?

The phenomenon of a sea breeze occurs due to the temperature differences between the land and the water. During the day, the land heats up more quickly than the water, resulting in warmer air over the land and cooler air over the water. This temperature difference leads to a situation where the warmer air over land rises, creating a low-pressure area above the land.

As the warmer air rises, cooler air over the water moves in to replace it, leading to a breeze that flows from the water towards the land. This movement of cooler, denser air towards the area of lower pressure over land is what characterizes a sea breeze. It emphasizes how temperature gradients can drive air movement and create localized wind patterns.

The other options relate to different concepts that do not directly describe the movements associated with a sea breeze. Nighttime cooling typically results in the opposite effect, where the cooler land can lead to land breezes at night. Thermal expansion of air does play a role in creating airy spaces, but it is the pressure differences driven by temperature differences that primarily cause the movements associated with a sea breeze. Pressure differences are indeed involved, but they are the result of the initial temperature differences, making the latter the more direct answer.