CONVECTION Convection involves the large-scale flow of matter. It is the kind of temperature change that affects the weather. There is a large amount of flow of air from the hot tropics on Earth to the poles with the flow being easterly in the northern hemisphere. In addition, the flow of water in the cooling system of a car will cool the pistons of the car. Blood vessels will dilate near the surface of the body to allow heat to dissipate by the flow of the blood so that sweating can occur. Cold air results in shrinkage of these vessels in order to reduce loss of heat from the body. Convection also helps in the loss of heat through breathing. Convection is more complex than conduction. Hot air rises because its density decreases with increased temperature. Natural convection happens with the ocean currents and with large-scale atmospheric circulation with a transfer of energy from one part of the globe to another. While heat is transferred via conduction or radiation on a stove, the pot of water it heats will heat via convection and the rising of hot water from the bottom surface of the pot. Cold wind is more chilling to the body than still air that is cold because convection combines with conduction in the body in order to increase the rate at which energy is transferred from the body to the surrounding air. This is what the wind-chill factor is all about as it gives an idea of what the air temperature would feel like if the wind were not blowing. The wind chill depends on the wind speed and on the actual temperature outside. Interestingly, although air can transfer heat rapidly via convection, it is a poor conductor of heat and is actually a good insulator. The addition of wall insulation will prevent airflow so that the loss or gain of heat is decreased. The same is true of using double-paned glass with an airspace between them. In that case, the air between the windows acts as an insulator. The air spaces in animal fur has multiple convection currents that insulate an animal from temperature changes.
177
