# Heat transfer by convection

Ever been by the beach when it was sunny? On a clear day, on the beach, there is always wind blowing from the sea. Why is there always a wind on the beach and why do the sea breeze (winds blowing from the sea to the land) occur during the day, the land breeze (winds blowing from land to sea) occur at night? Why in the rainy season can clouds come down to the slopes? Why does the wind feel cool? The answers to these questions relate to the specific heat of land and sea, expansion, density and heat transfer by convection!

By understanding well and correctly the subject matter, you can answer the questions above and other questions that you might ask later.

Heat transfer by conduction usually occurs in solid objects or from solid objects to liquid objects (and from liquid objects to solid objects) or from solid objects to gas objects (and from gas objects to solid objects). While heat transfer by convection usually occurs in liquid objects (e.,g. water) and gas objects (e.,g. air). Heat transfer by convection is heat transfer accompanied by the transfer of objects. So that you better understand heat transfer by convection, review a case, for example, heated water using fire. When water in a container is heated with fire, heat moves from fire (high temperature) to a container (low temperature) by conduction and radiation. Then heat moves from the container (higher temperature) to the water near the container (lower temperature) by conduction. The additional heat causes the water temperature near the container increased.

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Increasing water temperature causes the water to expand or the volume of water increases. Because of the volume of water increases, the density of water decreases. Water near the base of the container has a higher temperature than the water next to it. In other words, water near the bottom of the container has a larger volume and a smaller density, while the water above it has a lower volume and a higher density. The difference in density causes the water on the surface of the container, which has a higher density, moves down and the water near the base of the container, which has a smaller density, moves upwards. This process occurs continuously until all the water inside has the same temperature (If the air pressure is 1 atmosphere then the water in the container is evaporating or boiling at 100 oC).

The occurrence of land winds and sea breezes also involves heat transfer by conduction and convection. The specific heat of the land is smaller than the specific heat of the sea water. Hence the ground is hotter faster when it is exposed to the sun and also colder when the night comes. Land heat more quickly heats the air above (heat moves from land to air by conduction). The air temperature increases and the air expands. As a result, the air density decreases. On the contrary, the temperature of the sea water is colder so that the air above the sea level is also colder than the air on the land surface.

The air on the sea surface is cooler so that the density is higher. The difference in air density causes the air on the surface of the sea move towards the land, while the air on the ground moves upwards. The farther from the surface of the earth, the amount of air decreases because the gravitational force of the earth gets smaller. Because the amount of air decreases, the air pressure is also getting smaller.

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Hot air on the land that moves upward has cooled down because it is farther away from the surface of the earth, the air pressure decreases. The cold air then moves down, not towards the surface of the land but towards the surface of the ocean which has more freezing temperatures. This process occurs continuously so that there is the flow of air from the sea to land. In short, air near the surface of the sea moves to the ground, the air near the surface of the earth moves upward, the air above moves to the surface of the sea.

Why does smoke always move up? Smoke moves up because the temperature is higher than the temperature of the surrounding air. Because the smoke temperature is higher, the volume increases and the density decreases. The smaller mass of smoke causes less pressure, compared to the surrounding air pressure. The air around the smoke presses the smoke upwards.

Why in the rainy season does the cloud move down? During the rainy season, clouds contain a lot of water vapor so that the density of clouds increases. Clouds that contain a lot of water vapor and have a large density, move down towards the place where the air around the area has the same density as the cloud density. You can question and answer many things related to this subject if you have understood well and correctly the explanation above.

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