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Status: 05/26/2023 12:09 p.m

Why is it often cloudy on the beach in the afternoon?

When the high-pressure weather is calm, there is a typical weather pattern on the bathing beach. In the morning it is cloudless and there is little wind. During the course of the day, a wind blows in from the sea and mostly harmless cumulus clouds form in the hinterland. In the evening the wind dies down and the clouds dissolve. At night, a wind can be observed that blows in the opposite direction, i.e. from land to sea, and clouds often form on the sea near the coast.

This is the land-sea wind system, a thermally driven local wind system that only occurs in coastal regions. This wind system is driven by the different levels of warming in the layers of air over land and sea. This in turn is caused by the very different heat capacities of water and, for example, sand.

While about 0.8 kilojoules are required to heat one kilogram of sand, water requires more than 4 kilojoules, i.e. about five times as much energy. As a result, the layers of air above the land surface warm up more quickly due to the solar radiation in the morning than above the sea. The warm air expands, becomes lighter and rises. This creates a thermal low on land close to the ground and the characteristic cumulus clouds (Latin: cumulus) form along the coastal strip from midday onwards.

Since there is no vacuum in the atmosphere, air flows from the sea onto land to replace the air that has risen. This is the so-called sea wind, which is often perceived as pleasant in warm and sunny weather, but whose cooling effect makes one underestimate the harsh UV radiation of the day.

Air sinks on the sea to compensate, which is why a thermal high pressure area develops there and it is therefore often cloudless there during the day. If the radiative forcing decreases in the evening, this local wind system also falls asleep again, the sea breeze decreases and the cloud dissipates. At night the conditions are reversed: the land surface cools down faster than the sea surface and the land wind blows towards the sea.

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