At night, the land temperature falls to below that of the ocean and becomes less dense. Therefore it begins to rise (1, above right). The rising air creates a weak low pressure area due to a decrease in air mass at the surface (2). As the air cools, it begins to collect resulting in an increase in pressure, creating a "high" (3).
These differences in pressures over the water, both at the surface and aloft are greater than the differences in pressures over land at the same elevations over land (4 and 5). Therefore, as the atmosphere seeks to reestablish the equal pressure both onshore and offshore, two high pressure to low pressure airflows develop; the onshore flow aloft (6) and surface offshore, called the land breeze (7).