Sea breeze blows during the day and the land heats up faster than the sea. The air on land becomes less dense (i.e.warmer) and rises so the cooler air over the sea which is denser(cooler) flows in to take the place of the warm air, causing a sea breeze. Land breeze blows during the night from land to sea and the land becomes cooler faster than the sea. The air above the sea becomes less dense (i.e. warmer) and rises.