The amount of energy the surface receives from the Sun (see 1 in Figure 1.01) varies considerably over the course of one day. The surface receives heat from the Sun only from sunrise to sunset, while it is continually losing heat because of longwave radiation, conduction, evaporation, or advection (see 6-9 in Fig. 1.01). As a result, the temperature of the sea surface can vary by as much as 0.5°C over a 24-hour period (Cornillon and Stramma, 1985). Certain conditions must exist in order to observe these daily SST differences; namely, clear skies (maximizes nighttime cooling) and low wind speeds (minimizes vertical mixing). These small temperature differences are usually difficult to distinguish in satellite data without using more sophisticated data processing techniques than we’ll use here.
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