Cefas observers record coastal sea surface temperature using calibrated thermometers approximately 6 - 14 times per month, usually close to the time of high water. Other organisations record sea surface temperature ranging from daily values to monthly means. The Cefas instruments are calibrated at Lowestoft to an accuracy of ±0.1°C. The accuracy of other instruments is not known, but is thought to be at least to an accuracy of ±0.2°C. The ferry route observers record offshore sea surface temperature from the ships main seawater pipe using a calibrated thermometer 4 times a month. The temperatures are recorded to at least an accuracy of ±0.2°C. The seawater samples are taken from the sea water main pipe to the harbour pump about 1.5 metres inboard.
Quality assurance checks are applied to the data for each station by comparing the current dataset with either a 5 or 10 year running mean for each month. The data is first tested to see whether it is normally distributed ie, whether all the data are close to average. The standard deviation is calculated to see how tightly the data are clustered around the mean; three standard deviations are then calculated to account for 99% of the data. If the data is outside of this range (3 stdev) then the value is flagged and removed from subsequent analysis.
Yearly averages are only derived from those years which have a complete set of monthly values.