The formation of a seabreeze is one of the major contributors to high levels of ozone along the New Hampshire coast. During hot summer days, the sun heats the land. This in turn heats the air above it, which then begins to rise. The water temperature remains relatively constant however, keeping the air above it cool. As the hot air over the land surface rises, a low-pressure area relative to the pressure over the water is generated. As a result, the air over the water is free to move sideways toward the land to occupy the low-pressure area. A thermally driven circulation pattern forms due to this pressure gradient and is known as a seabreeze.
The following figure shows a particularly harmful case in which the pollution generated in the Boston area is blown onto the New Hampshire coastline. The lines indicate flow contours of the wind (wind direction). The sharp discontinuity of the contours along the New Hampshire and Maine seacoast show how far the seabreeze penetrated inland. The majority of the ozone exceedances on the coast occur on a south-southeast wind direction just like this one. This strengthens the evidence suggesting that the seabreeze is a major factor in air pollution transport from out of state.