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UK sea temp. bucket and sea thermometer (photo by G. Allen, Met Office)
DWD sea surface temperature measurement instrument (photo Deutscher Wetterdienst)
A sea-bucket is lowered over the side of the ship, a sample of seawater is hauled on board and a thermometer is then used to obtain its temperature. The sample should be taken from the leeward side of the ship, and well forward of all outlets. The thermometer should be read as soon as possible after it has attained the temperature of the water sample. When not in use the bucket should be hung in a shady place to drain.
A sea-bucket should be designed to ensure that sea water can circulate through it during collection and that the heat exchange due to radiation and evaporation is minimum. The associated thermometer should have a quick response and be easy to read and should preferably be fixed permanently in the bucket. If the thermometer must be withdrawn for reading, it should have a small heat capacity and be provided with a cistern around the bulb of volume sufficient that the temperature of the water withdrawn with it does not vary appreciably during the reading. The design of bucket should be deemed adequate for the purpose by the Member recruiting the ship for observations.
Sea-buckets of good design (not simple buckets of canvas or other construction) can be expected to agree well over an extensive rang of conditions. However, they are less convenient to use than instruments attached to the ship and their use is sometimes restricted by weather conditions.