Several factors affect the movement of air currents, most notably temperature, barometric pressure, and the Coriolis force caused by the Earth’s rotation from west to east. The Coriolis effect displaces circulating air to the left in the southern hemisphere and the right in the northern hemisphere.
Localized coastal winds change direction between night and day because of differences in the temperatures and air pressures between land and sea. However, coastal winds can also change direction due to seasonal winds blowing in from the sea or down from mountains and other elevated areas.
Coastal winds are more localized than prevailing winds because they are created by temperature and pressure differentials between the land and sea on that particular coastline. Depending on the structure of the coastline and its geographical features, land and sea breezes can be weaker or stronger.
Wind Direction Changes Caused By Temperature Differentials
Air over land generally heats up faster than air over water. It also cools down faster than air over water. Wind is the movement of air currents in and around coastal areas generated by changes in temperature. Since water and land heat up and cool down differently, the land is warmer by day, and the sea is warmer at night.
Experiments with convection in science or geography would have taught you that warm air rises while cool air sinks because it is denser. The temperature differential between land and sea alternates, causing the direction of coastal winds to change depending on if it is day or night.
In the day, warm air over the land rises, allowing the cool ocean air to stream in below it. As night falls, the sun has warmed the sea air all day, so the water temperature is higher than the land temperature. The warm air over the sea rises, allowing the cooler air from the land to flow in beneath it.
Therefore the cooler air blows from the land out across the water at night, while cooler air flows in from the sea towards the land in the day.
Wind Direction Changes Caused By Pressure Differentials
Air currents flow from areas of high pressure to low pressure. This is called a pressure gradient. Alternating temperature gradients between the land and the sea cause the wind to blow northward during the day and southward at night.
Because the air above the land is warmer than the air over the ocean during the day, it rises and expands, causing a low-pressure region to develop over the land. Since the air over the sea is cooler and denser, it is at a higher pressure than the air over the land and flows towards the low-pressure region.
At night, the opposite happens, and the air pressure over the sea is lower, causing the high-pressure air from the land to flow towards it.
The Coriolis force causes the wind to blow anticlockwise around a low-pressure zone and clockwise in a high-pressure zone in the northern hemisphere. In the southern hemisphere, the opposite applies.
The area of convergence between land and sea air can cause a weather front to develop as the colder air from the sea meets the warm land air. If the air currents are strong enough, large cumulus clouds form in the sea breeze front and result in thunderstorms over the land.
Similarly, land breezes blowing out to sea at night can cause thunderstorms over the sea. Land breezes are generally not as strong as sea breezes. This is because the cooling land impairs the air’s vertical motion, weakening the circulation of the land breeze. Vegetation, buildings, and geographical features like hills and sand dunes can also inhibit the flow of air currents from the land out to sea.
Katabatic winds may affect wind direction in coastal areas if there are mountains or high plateaus inland. These winds are caused by air cooling in elevated inland regions. The air then begins descending downhill, warming as it flows and gathering speed. If coastal valleys confine katabatic winds, they can reach gale force as they hit the coastline.
Katabatic winds generally bring dense cold air from high places down slopes and are propelled by gravitational forces. Coastal areas are usually low-lying and present few obstructions to these land-generated winds.
The berg wind of South Africa is an example of a katabatic wind. It blows from the central inland plateau down into the Great Escarpment and out into the sea. The berg wind is typically hot and dry by the time it reaches the coast.
Other examples of katabatic winds are the Santa Ana in California, the Barber in New Zealand, and the Oroshi in Japan. Not all katabatic winds are warm. The Barber, for instance, is cold and biting. Katabatic winds are usually seasonal in most places.
Thus wind in coastal areas may change direction if a strong katabatic wind blows from inland to the sea.
Other Coastal Winds
Prevailing winds usually blow along the east-west axis across the planet rather than the north-south axis. Where they meet is called a convergence zone. Prevailing winds blow across the Earth’s surface predominantly from one particular direction. They are usually easterly at lower latitudes and westerly in middle latitudes.
The sea and land breezes caused by temperature and pressure differentials are more localized than prevailing winds, but they can interact with each other. In the case of narrow land masses, sea breezes generated by opposing coastlines can collide, giving rise to thunderstorms.
Sand dunes are usually oriented perpendicular to prevailing winds at the coast. Prevailing winds are more persistent and longer-lasting than land and sea breezes. They are planetary winds, whereas land and sea breezes are periodic winds.
If prevailing winds converge, they can cause a low-pressure zone that draws in the dry, cooler air from high-pressure zones, creating convection cells. The Intertropical Convergence Zone, where the Southwesterly trade wind meets the Northeasterly trade wind, is an example of a convergence zone.
This convergence zone stretches across the planet over several coastal areas and is famous for weakening wind strength and direction to the point that there is virtually no wind. It produces light, shifting breezes called the doldrums that caused distress to many sailing ships in days gone by because they could get stuck for weeks.
Other periodic winds are seasonal, like the monsoon. They occur when the wind reverses direction due to the season. India experiences summer monsoons that bring torrential rain because the wind has absorbed so much water over its long journey across the ocean before making landfall. Japan and China, by contrast, experience winter monsoons, which are far more potent than summer monsoons.
Coastal winds can change direction due to significant storm systems moving inland from the sea. They can also be affected by strong, low-level air currents moving through gaps in mountains and cliffs towards the sea at certain times of the year.
Coastal zones often experience unique weather patterns because weather disturbances become trapped between the sea and the land. They can cause the reversal of wind direction and increases in wind intensity as the weather system moves along the coast. This can result in severe coastal storms, especially in winter.
Coastal winds change direction because of alternating temperature and pressure differentials between land and sea during the day and night. However, coastal winds can also change direction if seasonal winds blow down from mountains and other elevated areas down to the sea or blow in from the sea at certain times of the year. If a weather system gets trapped in a coastal area, it can also change the wind direction as it moves along the coast.