These winds are defined as the complete cycle of diurnal local winds occurring on sea coasts due to differences in surface temperature of land and sea. There is a complete diurnal reversal wind direction of these coastal winds.
That is why they are also referred to as diurnal monsoon, since both of these wind systems are caused by the unequal heating of land and water surfaces.
The only difference between land and sea breezes on the one hand, and monsoon winds on the other, is that while the former is caused by the diurnal variation of pressure, the latter is generated by its seasonal variation.
Now, it is evident that the daily temperature contrast between land and sea and the pressure pattern produced thereby generate land and sea breezes.
During the daytime, more so in summer, land is heated more than the adjacent body of water. As a result, warmed air over the land expands producing an area of low pressure. The isobaric surfaces bend upward as a result of which the cooler air starts moving across the coast line from sea to land.
This is the sea breeze. But at a certain height from the earth’s surface, the movement of wind is directed from coastal land to the sea.
Thus, a convectional circulation system set up. However, at night, because of nocturnal radiation land is colder that the adjacent sea and the pressure gradient is directed from land to sea, there is a gentle flow from land to sea. This offshore wind is called the land breeze.
It may be pointed out that the land-sea breeze system is very shallow. Average depth of this wind system varies from 1,000 to 2,000 meters in tropical regions. However, over the lakes the depth of land and sea breezes in much less.
Sea breeze modifies the temperatures of the coastal areas. Sometimes because of the sea breezes the coastal regions record a drop of 5°C to 10°C in their temperatures.
According to Trewartha, the cooling effect of these breezes reaches a maximum of 15 km to 50 km inland in the middle latitudes, and 50 to 65 km in the tropical regions.
But under favourable conditions the cooling effect of the sea breezes can be felt up to a distance of about 100 km inland in the tropics.
Sea breezes usually begin a little before noon, say, between 10 a.m. and 11 a.m., and reach their maximum intensity in the mid-afternoon when their velocity ranges from 10 to 20 kilometers per hour.
Sea breezes are also generated along the shores of the large lakes, but they are relatively much weaker in strength. In summer, the large cities situated near the lakes feel the impact of lake-effect where the areas near the shore are cooler than the outlying areas.
Chicago, because of its location near a lake, offers a typical example of experiencing the lake-effect where lake side areas are cooler than the much warmer outlying areas in summer.
In the morning and evening, when the difference in temperature between land and water is at its minimum, the sea breezes die down. The intensity of cool sea breeze is directly proportional to the temperature contrast over land and water bodies.
There is a marked regional variation in the velocities of land and sea breezes. In the temperate region the wind velocity is limited to 25 to 40 km per hour, while in tropical regions the land and sea breezes sometimes become stormy.
Wherever the coastal regions are flanked by cold currents, the pressure gradient becomes very steep which results in very strong land and sea breezes. Undoubtedly, the cool sea breezes have an important moderating influence on the climate of a narrow coastal belt.
It is because of these sea breezes that in summer, when the weather is clear, the maximum diurnal temperature is not allowed to rise very high. That is why the coastal areas have always lower temperatures than those situated away from the coasts.
Land breeze is just the reverse of the sea breeze. It is an offshore night wind which develops because of the cooling of land by nocturnal radiation. During night the land becomes colder than the adjoining water, so the air over land becomes colder and denser than that over water.
Now, there is higher pressure on land and lower pressure on water. Thus, the pressure gradient is reversed. Because of this difference in pressure, the surface air moves from land to sea. This off shore wind is called the land breeze.
Since temperature contrasts during night time are relatively less than daytime temperature differences, the land breezes are not as strong as the sea breezes. Besides, the horizontal and vertical extent of these off shore winds is also less than that of sea breezes.
These land breezes reach their greatest intensity shortly before sunrise, but as the sun ascends higher in the sky, these winds become progressively weaker and eventually die out.
Land-and-sea breezes are experienced on the subtropical and tropical islands in fully developed form. Under cloudless sky the islands in the Pacific Ocean and West Indies have the maximum development of sea breezes.
The formation of atoll clouds over numerous islands is a phenomenon of daily occurrence. These typical clouds are commonly associated with atolls; hence they are referred to as atoll clouds.
The land-and sea-breeze fogs are produced in the coastal regions in nearly all seasons by these local winds. This type of fog may be transported over coastal land by sea breeze in mid afternoon. When the sea breeze stops during night time the fog recedes to the sea.
Land and sea breezes, like any other local winds, have a significant influence on environment. These small-scale winds prove helpful in coastal navigation, particularly in case of small crafts. Besides, when the sea breeze moves far inland over some tropical islands or other localities, they bring great relief to the inhabitants of those areas who aptly call the local winds ‘the doctor’.
The intensity of land and sea breezes is largely controlled by the location and time of the year. Warm tropical areas, where intense solar heating persists throughout the year, experience stronger and more frequent sea breezes than the middle latitude locations.
Tropical coastlines adjacent to the cool ocean currents are the most favoured locations for the development of most intense sea breezes.
In the temperate zone the sea breezes are most common during summer months, but land breezes are often missing because land does not cool below the ocean temperature.
Land and sea breezes do not develop in the higher middle latitudes simply because frequent migration of pressure systems generally dominates the circulation.