At the surface, the sun warms both the ground and ocean at the same rate. However, since the heat in the ground is not absorbed well it returns it heat to warm the air. The warmed air, with its decreased density, begins to rise (1). The rising air creates a weak low pressure area (called a thermal low) due to a decrease in air mass at the surface (2). Typically, from 3,000 to 5,000 feet (1,000 to 1,500 meters) above this low pressure, as the air cools, it begins to collect resulting in an increase in pressure, creating a "high" (3).
These differences in pressures over land, both at the surface and aloft are greater than the differences in pressures over water at the same elevations (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 offshore flow aloft (6) and surface onshore, called the sea breeze (7).