The temperature is a measure of the concentration of one kind of energy, called sensible heat. It has this name because it can be sensed by touch or by a thermometer. Sensible heat is a measure of the speed of the molecules of the object being observed; the molecules of a hot object move around fast. It does not include the chemical energy involved in photosynthesis (Note 1.B), nor radiation energy (Chapter 2), nor the energy used in evaporation (Chapter 4), because these forms of energy do not register on a thermometer.
In solids such as the ground, sensible heat is transmitted from a hot to a cold part by conduction: the heat is transferred by contact from one molecule to the next. The conducting material does not itself move. But heat is mainly transferred by convection and advection, in fluids like air and water. 'Convection' involves the stirring of heat away by either local turbulence or buoyancy, as in cooling an object by putting it into a bucket of water which is swished around. 'Free convection' in the atmosphere and oceans is due to buoyancy and therefore vertical, as in the case of smoke eddying from a cigarette. 'Forced convection' is due to wind. Convection is the chief means by which the ground heats the air above (Section 1.6), the rate depending on the difference between the surface and air temperatures (Note 3.A).
The convection of heat is a kind of 'advection', the transport of something in a moving fluid. The advection of heat, for instance, involves a stream of warmed fluid carrying heat away to a distant cooler place. Advection is generally horizontal, although important cases of vertical advection in the atmosphere and oceans are discussed in Chapters 7, 12 and 13.
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