The variable

Only about 17% of solar radiation is absorbed directly by the atmosphere as it passes through it (Lockwood 1974). In fact the atmosphere is heated primarily as follows: solar radiation heats the ground and the ground's heat is transferred to the air, firstly by molecular diffusion across the laminar boundary layer (a layer only a millimetre or so thick, which clings to most surfaces); beyond this, in the turbulent boundary layer, transfer is by turbulence, which is much more effective at transferring heat than is diffusion. Heat is also, thereafter, transferred by convection, bubbles of warmer air rising into the cooler air above. This transfer of warmed air away from the surface is the sensible heat flux. While it is difficult to measure the rate of energy transfer (see eddy correlation, Chapter 6), the resultant changes in air temperature are important and more easily measured.

A proportion of the heat from the warmed ground is also transferred downwards as the soil heat flux, the rate of transfer being influenced by the amount of water in the soil and also by the pore and particle sizes and the presence of vegetative material. Many biological processes are influenced by soil temperature, from the activity of micro-organisms to the germination of seeds and plant growth, and observations of soil temperature are made at depths down to a metre or more.

The importance of the oceans in weather and climatological processes is now firmly established, and in this the sea-surface temperature is of particular significance. While the measurement of sea temperature is made with identical thermometers to those used for the air, how the observations are made is dealt with in Chapter 13 on the oceans.