If you have spent much time at the beach during the summer at the beach, absorbing UV radiation to darken your skin or just beachcombing, you've probably noticed that at around 3:00 p.m. there often is a strong steady wind blowing in from the water. This steady wind, the sea breeze, is a result of the uneven heating during the daytime between the land and the adjacent water. At night the wind often reverses direction and blows from the land to the water (a land breeze). Land and sea breezes are referred to as direct thermal circulations. Let's examine the sea breeze.
An object will heat up or cool down depending on its total energy budget. If the object gains more energy than it looses, the object warms. How much the object's temperature increases depends on its thermal properties -- heat capacity and heat (or thermal) conductivity. During the day the land, which has a low specific heat and is a poor conductor, heats much more quickly than water. As the land warms up, the air next to it heats by conduction and rises, warming the air above the land by convection. As the air rises it generates a pressure gradient , and thus a pressure gradient force , generating the thermal circulation. To understand this, let's first imagine that the land, sea and the air above them are at the same temperature and that the isobars are parallel. As pressure is defined as the weight of the air molecules above us, the isobars must decrease in magnitude with increasing altitude (Figure 1). In this simplified model, the temperature of the ocean and the air above the ocean are not changing and thus initially no air molecules are moving. As air over the land warms (due to absorption of solar energy, and conduction to a thin air layer above) it rises and there is a vertical displacement of air molecules to a higher altitude (Figure 2). These rising thermals of air change the orientation of the initial isobars.
For example, the height of the 980 mb isobar in Figure 1 must increase if we are putting air molecules above this height. The result is that above the surface, the isobars begin to slope upward. Now, at a given altitude (say 100 meters), the pressure is no longer the same over the ocean as it is over land, it is higher over land. Note, at this time in our simple model, the surface pressures over the land and ocean are the same as we have not transported molecules horizontally. But now we have a horizontal pressure gradient above the surface (Figure 3), and thus a pressure gradient force generating a wind from over the land out towards the ocean (Figure 4). This wind removes air molecules from over the land (less molecules; thus lower pressure at the surface) out over the ocean increasing the surface pressure over the water. Thus at the surface over land low pressure is developing while over the ocean high pressure is developing, generating a horizontal pressure gradient force at the surface acting from over the ocean towards the land. To replace this surface air moving from over the water towards land, air sinks from above, completing the circulation (Figure 5). This sinking air moves air molecules towards the surface, causing the pressure above the surface to lower as the molecules from above descend. Notice the slope of the pressure surfaces in Figure 5. As the temperature difference between the land and water increases throughout the afternoon, the circulation increases in strength and winds pick up, reaching a maximum in the middle to late afternoon. Over land the distance between two isobars (i.e., 980 and 960 mb) is greater than over the ocean. This difference is what keeps the circulation moving and is due to the air over land being warmer than the air over the ocean.
The important concept is that heating (or cooling) of a column of air leads to horizontal differences in pressure, generating a pressure gradient force which causes the air to move and a circulation to develop. During the evening, the land cools faster than the water and the process is reversed (Figure 6). The net result is a land breeze, surface winds blow from the land out to sea.
If you are not at the beach to feel the sea breeze, you can monitor its existence and intensity by analyzing satellite imagery. A rising parcel of air expands, and cools and the relative humidity increases -- conditions favorable for the formation of clouds. For this reason, the upward branch of the sea breeze is often visible from satellite pictures in the form of cumulus clouds. During the day, the upward branch moves inland and is an indication of the strength of the sea breeze. If the atmospheric conditions are favorable for the formation of thunderstorms, the sea breeze may provide just enough lifting to cause thunderstorms to develop. An example of such a case is given in a sequences of satellite images accessed below. Before going to the sequence of satellite images, or satellite loop, let's inspect some of the individual scenes. These images were made using the visible channel of the GOES-8 imager which has a spatial resolution of 1 km. In all the images, which have been enlarged to twice their original size, the imagery is showing the coast of North Carolina. The first image is at 1545 UTC, which is approximately 8:45 am local sun time (0845 LST). Notice that along the North Carolina coast, there are few clouds over the Atlantic Ocean, and many scattered clouds over land. There is a distinct cloud-free boundary at the coast line. By 1815 UTC (1115 LST) the clouds have moved inland, marking the boundary of the upward branch of the sea breeze, or the sea breeze front. As the day progresses and the temperature difference between the land and ocean increases, the circulation gets stronger and the sea breeze front penetrates farther inland (2015 UTC). On this day the atmosphere is susceptible convective activity and the lifting associated with the sea breeze is enough to 'kick off' convection by 2145 UTC (1545 LST).
The evolution of the sea breeze phenomena is demonstrated in a satellite loop of a sea breeze from approximately 1545 UTC (8:45 am) through 2215 (3:15 pm) at half-hour intervals.
Whenever large land and water bodies are adjacent to one another, sea breezes may develop and may cause thunderstorms. Florida's abundant summertime rainfall is a result of sea breezes. One sea breeze front advances from the east and one from the Gulf of Mexico side.
If the synoptic scale winds are weak, lake breezes may develop due to the differential heating between the lake and the surrounding land. The lake breezes are often observed around the Great Lakes.
Questions for thought.
