3 Mechanisms of heat transfer

Heat is transferred in three ways:

1. Conduction. Heat passes through a solid body or between two bodies in contact, from the region of higher temperature to lower temperature, without the particles themselves moving from their places (particles vibrate and pass energy to their neighbours; in metals free electrons carry energy too). Some materials are good conductors (metals: silver, copper, aluminium), others are poor conductors (insulators) such as cork, wood, plastics, still air and wool. In the atmosphere: the ground heats the thin layer of air touching it by conduction (air is a very poor conductor, so this reaches only a few centimetres).
2. Convection. Heat is transferred in fluids (liquids and gases) by convection currents. The heated part of the fluid expands, its density becomes less than that of the cooler parts, so it rises and the cooler, denser fluid moves down to replace it, forming a circulation. Convection is the main way the lower atmosphere is heated and the cause of winds, sea breezes and clouds. Thermal flight: birds such as eagles, vultures and storks exploit rising warm-air currents (thermals) to climb and remain in the air for long periods without flapping their wings, saving energy — the bird floats on rising warm air currents and maintains its altitude.
3. Radiation. Heat transfer as electromagnetic radiation (mainly infrared). Thermal radiation travels in all directions without the need for a medium, so it passes through a vacuum and also through gases. This is how the energy of the Sun reaches the Earth through empty space.

Combined picture (answers the "Inquiry" diagram): Sun → Earth's surface by radiation; surface → thin layer of air by conduction; warm air rises, cooler air sinks — convection currents — carrying heat through the troposphere; and the warmed surface radiates infrared which is partly absorbed by greenhouse gases and re-radiated (Module 12).