The diurnal cycle is a fundamental mode of the climate system associated with solar forcing. Sea surface temperature (SST) is directly affected by the diurnal cycle of the sun's radiation. This thesis presents a detailed study of the diurnal variability of SST using a variety of data sources from satellites, moored buoys and a 1-D mixed layer model. 10 years of daily day- and night time satellite data are analysed to provide the first global long-term assessment of diurnal warming patterns. In situ buoy and model data are used to investigate the variability of diurnal warming. A 1-D mixed layer numerical model is used to investigate the sensitivity of the evolution of the diurnal warm layer to the presence of chlorophyll. Under low wind conditions, the distribution of solar energy is largely determined by the optical properties of the water column. Chlorophyll has little impact on the heating within the top few centimetres of the ocean, but below 0.7m, the presence of chlorophyll increases the diurnal heating rate and consequently temperature. High resolution buoy data are analysed to investigate the sensitivity of the evolution of the diurnal warm layer to fluctuations in the meteorological conditions. Over three hundred individual diurnal cycles of SST, insolution and wind speed are analysed. the results reveal that the evolution of the warm layer is strongly dependent on the time variability of the forcing fields. The shallower the warm layer, the greater its sensitivity to meteorological fluctuations. Under low winds, large differences in SST can occur between the surface and 1m. Based on these results, a new parameterisation is developed to estimate to diurnal variation of SST at the surface and at 1m, taking into consideration the time variability of the forcing fields.