Coastal weather flying
Transition zones between water and land make for unique low-altitude
operations that can often cause showers, turbulence, fog and sometimes icing.
By Karsten Shein
Comm-Inst, Climate Scientist
During the day, the sun heats the land more quickly than the water, creating an onshore sea breeze flow and convection over the land. At night, the rapid cooling of the land reverses the sea breeze circulation, moving clouds and showers offshore.
Perfectly lined up on the glide- path, the flightcrew team was starting to relax. They were dropping their CEO's family off for a week in the tropics. And since their next work trip wasn't scheduled until Monday morning, they were already thinking about laying on the beach themselves before returning to their company headquarters in the much colder north. But as they visualized the warm sand between their toes, the aircraft suddenly pitched upward, followed by an equally jarring downward roll.
The pilots quickly recovered control, but the corrections led to a rougher than expected landing. As they helped unload the baggage, they had to endure an earful from the CEO's wife who had chosen final approach as a good time to freshen her lipstick to look her best once she alighted.
The flightcrew could have easily avoided both the upset aircraft and the upset passenger had they recognized that an afternoon arrival at that particular airport would always be met by low-level turbulence as aircraft passed the final approach fix, just inside the shoreline.
Despite the vastness of the continents over which we fly, an exceptionally large number of airports are positioned along or very close to a body of water. These are not just airports serving some maritime city. Communities of all sizes have historically developed near a water source. It is estimated that over half of the world's population lives within a few miles of either an ocean or large lake.
Therefore, the airports that serve these communities are also located near those coasts. In fact, until recently, people didn't value coastal real estate as buildable land because it was low-lying, comprised of soft sediment and prone to flooding. Those large, flat areas were considered ideal for the local airfield rather than for homes or factories. Unfortunately, the same geography that made these places good physical locations for airports also made them more of a challenge, weatherwise, for pilots operating in and out of them.
Pilots who fly frequently into coastal airports know very well that the conditions they are likely to encounter on arrival or departure often are quite different from the conditions that are forecast for the region based on the larger weather patterns. Simply having a large amount of land sitting right next to a large amount of water is enough to affect winds, temperature, humidity and even pressure over very small distances within the coastal zone. In turn, modification of these atmospheric elements can create adverse flying conditions such as turbulence, clouds or fog, storms, windshear and icing.
Land vs water
San Francisco's Golden Gate Bridge peaks above a thick advection fog as warm air flows over the cold waters of San Francisco Bay. Such fogs are common along many coastlines and can drop visibility to near zero for days at a time.
Dominant causes for the often significant local variation in weather conditions are 2-fold. First, the change in topography between a relatively smooth-surfaced body of water and the comparatively rougher adjacent land results in changes in the flow of air between the 2 surfaces.
In addition, that transition from land to water is not normally a nice straight line of gradual change in roughness. Coastlines are often marked by high cliffs, bays and sounds, spits or barrier islands to name just a few irregularities that can alter wind patterns and set up turbulence and shear.
The 2nd major factor that influences local coastal weather is the difference in the ability of land and water to handle energy. Water has a very great capacity to absorb and store the energy it receives from the Sun. It stores it as latent heat energy, which is released and transformed into sensible heat energy when there is a large enough energy deficit. Because it is fluid, it also can quickly move that latent heat away from the surface, keeping the water surface cooler than it otherwise would be.
On the other hand, soil, concrete and other terrestrial surfaces do not have that capacity because they tend to lack water. These surfaces still absorb, but can't readily store, the solar energy as latent heat. Instead, they quickly transform it into kinetic (sensible) heat and reradiate it to the atmosphere.
This difference sets up a daily circulation pattern in many coastal locations. During the day, the Sun imparts energy to both the land and water along the coast. But while the water remains about the same temperature by storing that energy, the land surface cannot hold onto the energy. Consequently, it quickly returns through conduction to the overlying air as sensible heat.
Sea breeze circulation
Once the surface air becomes heated—normally by late morning to mid-day—it becomes buoyant and rises. As it does so, it creates a shore-based region of low pressure, and cooler air from over the water flows ashore toward the low. This surface flow establishes a circulation pattern over the coastline, known as the sea breeze.
In most cases, the sea breeze will be present only as a steady onshore wind that extends further and further inland as the afternoon progresses. However, if there is a weak to moderate temperature inversion aloft—just a few degrees—it can prevent the heated surface air from ascending until it has accumulated enough energy to break through and rise explosively.