Nocturnal cooling of earth and atmosphere

Sun is the primary source of energy for all living things on earth. Sunlight is needed for photosynthesis by the leaves of the plants, which in turn produces food in the presence of few other ingredients. The heat from the sun evaporates water, forming clouds and rain, creating a water cycle. The land surface gets heated up due to sunlight, causing the atmosphere to become warmer and more liveable for most creatures. A fraction of the energy gained by various parts of the earth’s surface is transferred to the atmosphere.

Air in the atmosphere is a mixture of gases, predominantly oxygen and nitrogen, with other trace gases. Air is often loaded with a dynamic distribution of dust or solid suspended particles and water in liquid, vapour, and solid phases. Air which does not contain a significant amount of solid and liquid particles is called dry air. Certain gases in dry air can potentially absorb the energy transferred back by the earth’s surface, making the atmosphere we live in habitable. If the earth and its atmosphere retain the heat from the sun continuously, the temperature will rise till it becomes unbearable. Certain gases in the atmosphere, especially carbon dioxide, have this property. We call this greenhouse gas.

But what happens in the absence of sunlight? The darker parts of the earth no longer get heated up. The heat from the earth’s interior (a few meters below the earth’s surface) will reach the earth’s surface. The surface, in turn, will try to transfer the heat accumulated during the daytime to colder surroundings, which are the surrounding atmosphere and the deep dark space. The heat reaching from the interior to the surface is called conduction. The surface, in turn, transfers heat to the surrounding air in the form of convection, and a part of the energy will be radiated by the earth’s surface.

The radiated component is quite complex. Part of the energy released by the surface gets absorbed by greenhouse gases in the atmosphere. These are carbon dioxide, water vapour, clouds, methane, etc. The remaining part will escape the atmosphere to reach the colder space. Under clear dry sky conditions, heat convection to the surroundings becomes negligible. The energy stored from the deeper soil layers will conduct to the surface, which in turn gets radiated to the cold space. The balance between this conduction and radiation helps us to understand how quickly the earth’s surface gets colder at night, also known as nocturnal cooling.

Under suitable environmental conditions, people worldwide have used nocturnal cooling for the mass production of ice, even when the surrounding atmosphere is warmer! Tetsu Tamura, a Japanese meteorologist, in his published work [1], describes how soil conduction and clear sky radiation contribute to the faster rate of surface cooling. This work was published more than a century ago and is considered a classic example of a conduction–radiation problem in heat transfer.

[1] S. Tetsu Tamura, (1905), Mathematical Theory of the Nocturnal Cooling of the Atmosphere, Monthly Weather Review, Vol. 33, pp. 138-147.