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Thermal Energy Transfer: Question 8

Syllabus 2.3.2

Structured Extended 7 marks

On a sunny afternoon at a coastal town, the sand on the beach becomes much hotter than the sea water nearby. A noticeable breeze is blowing from the sea towards the land.

(a) Explain, in terms of air density and convection, why this breeze blows from the sea towards the land during the afternoon. [4]

(b) After sunset, the sand on the beach cools until it becomes cooler than the sea water. State what happens to the direction of the breeze once this has happened, and explain your reasoning in terms of air density and convection. [3]

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Worked solution

Part (a): Why the breeze blows from sea to land by day

Follow the air above each surface step by step:

  1. The hot sand warms the layer of air directly above it by conduction.
  2. This warmed air expands, so it becomes less dense than the air above the cooler sea.
  3. Being less dense, this warm air rises above the land.
  4. Cooler, denser air above the sea flows in at ground level to replace the air that has risen over the land.

This inflow of air, moving from the sea towards the land at ground level, is exactly what is felt as the afternoon sea breeze. It is driven entirely by the density difference between the air above the hot sand and the air above the cooler sea, a convection current on the scale of the whole coastline.

Part (b): Why the breeze reverses after sunset

After sunset, the situation is reversed: the sand has now cooled below the temperature of the sea water.

  • The air above the sea is now the relatively warmer air, so it is the air that expands, becomes less dense, and rises above the sea.
  • Cooler, denser air above the land now sinks and flows outward at ground level, towards the sea, to replace the air rising above the sea.

This flow of air from the land towards the sea is felt as a land breeze. The direction of the breeze has completely reversed compared with the afternoon, simply because the relative temperatures (and so densities) of the air above the land and the air above the sea have swapped over.

Final answers

  • (a) Air above the hot sand warms, expands and becomes less dense, so it rises; cooler, denser air above the sea flows in at ground level to replace it, felt as a sea breeze.
  • (b) The breeze reverses to blow from land to sea (a land breeze), because air above the now relatively warmer sea rises, while cooler, denser air above the land sinks and flows out towards the sea to replace it.