It is also relevant to the generation of major flows that water has a high specific heat, which in essence means that it takes a lot of heating to raise its temperature (4186 J to raise 1 kg by 1 °C, or by 1 kelvin). However, it is just as slow to lose it again, save that evaporation, turning water liquid into vapour, is very consumptive of accumulated heat (2.243 x 106 J kg-1). Nevertheless, the exchange of incoming and outgoing heat is a major component in the physical behaviour of the oceans. In fact, the patterns of motion are subject to a complex of drivers and the outcome is usually complicated. Empirical description of motion can only be probabilistic and, in any case, far beyond the scope of this book. In essence, the energy to drive the circulation comes from the Sun. Because of its relevance also to local variability in heat exchanges and its obvious links to the energy fluxes used in photosynthesis, a deeper consideration is given later to the solar irradiance fluxes. For the moment, it is necessary to accept that the proportion of the solar energy flux that penetrates the atmosphere to heat the surface of the sea or lake is first a function of the solar constant. This is the energy income to a notional surface held perpendicular to the solar electromagnetic flux, before there is any reflection, absorption or consumption in the Earth's atmosphere. Confusingly, it is not constant, as the elliptical orbit of the Earth around the Sun varies around the mean distance (149.6 x 106 km) fluctuates during the year within ±2.5 x 106 km. Besides, the heat radiated from the Sun also fluctuates. Nevertheless, there is a valuable reference (~1.36 kWm-2) against which the absorption, reflection and backscatter by dust, water vapour and other gases and, especially, clouds can be scaled. Even before those losses are deducted (see Fig. 2.2a), however, the heat flux per unit area
, I \ solar constant
, I \ solar constant
500 1000 1500 2000 2500 Wavelength / nm
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