A Day at the Beach

The interaction between water and land is never so evident than it is at the beach, any beach. Ponds, lakes and seas all develop the same circulation pattern from water to land and back again, just on differing scales. The bigger the body of water, the larger the effect. Come along and bring your sun lotion for a day at the beach!

The most obvious interaction between water and land is the land/sea breeze (also called a land/lake breeze). It works like this. Water warms and cools much more slowly than land and this has an interesting consequence. During the day, air over land heats up more quickly than air over the adjacent water. The warm air rises over the land and cooler air from over the water moves in to replace it.

Depending on time of year, terrain and location, this sea (or lake) breeze begins sometime from late morning to early afternoon and continues as long as the land is warmer than the sea. The sea breeze reached peak strength when the difference in temperature is greatest, usually in late afternoon. As the temperature difference narrows after sunset, the sea breeze weakens and eventually fades away when the temperatures are equal.

At night, the opposite occurs. Land cools more quickly than water and once the balance shifts and the land is cooler than the water, air cooled over land moves over the water to replace the rising warm air there. While this effect is most obvious on ocean beaches, it is felt even on riverbanks and on the shores of tiny ponds. This land breeze is typically lighter than the daytime sea breeze and it generally fades away around sunrise.

An interaction between rural and urban areas, called a country/city circulation, works in a similar way even without the presence of bodies of water. The temperature difference between the "concrete jungles" of the city and the fields, forests and plains of the country around it is enough to induce a country breeze.

The difference is, a city breeze never develops because all that concrete in the city radiates heat all night long and the country cools faster, so the country breeze continues. A circulation pattern of warm air rising above the city and eventually sinking over the country after it cools is a permanent feature over some major cities.

The Arctic coast of Siberia has an amazing land breeze effect in winter that operates much in the same way as well. Arctic Siberia is brutally cold in winter and the sea is frozen over early in the season. Hereís the trick; the ice insulates the water beneath it from the frigid air.

The water remains warmer than the air and it actually warms the air immediately above the ice to above the ambient land temperature. The result is a winter-long land breeze along the coast! This just proves it only takes a temperature difference, not hot temperatures.

Sea breeze fronts and convection are less obvious results of the interaction between the land and the sea. As the name implies, sea breeze fronts occur with the sea breeze. Air is not just cooled by the water, it gains moisture from it. As it moves over land, it clashes with the warmer, drier air it meets there. Two of the classic indications of a frontal zone are now in place, a temperature difference across the zone and a difference in moisture content (humidity).

Wind direction inland of the sea breeze is often different than the sea breeze direction as well, and now a third indicator is in place. A sea breeze front can often be identified on small-scale weather charts from these three elements alone. There are visible markers as well. When a sea breeze begins, a line of cumulus and/or stratocumulus develops along the forward edge of the cool, moist air.

As the cool air moves inland, so does the line of clouds. The clouds build vertically, rising in the warm air, as they move inland and can develop into rainshowers (sea breeze convection). The next time you go to the beach on a fine summerís day, watch for a stretch of cloudy skies and maybe some rainshowers a few miles before you get there. Donít worry, the sky will clear again by the time you get to the beach. You just passed through the sea breeze front!

Something else a sea breeze brings is also a terrain-induced weather feature. When there are hills or promontories near the sea, the sea breeze flow splits around the obstruction. That creates divergent flow on the seaward side of the terrain. As the airflow gets around the obstacle, however, it rejoins, like water around a rock in a stream. The area where the flow rejoins is called a convergence zone or convergence front and convective clouds grow here. Sometimes only cumulus or towering cumulus, these clouds often develop into rain showers. If circumstances are right, these rainshowers can even become thunderstorms!

When the sea breeze dies, so does the convection. It vanishes like cotton candy in a mist. The same effect works on large-scale wind flow as well. The sea downwind of mountainous islands, for instance, is typically cloudier than the upwind sea and gets more rainfall as well. The convergence zone is responsible for that. In times gone by, sailors often used the clouds created by convergence downwind of islands in the tradewinds to find land!

A day spent at the beach is a fascinating one for weather watchers. Understanding what is taking place when a cool breeze blows in your face from the sea or when you see rainshowers build up miles inland from the water makes it all the more interesting. Nature at work is an awesome thing!

Written by Melody L. Higdon, 14 WS/DOPA