Need to standardize instruments and the way they are used

This raised yet another problem. Unless all observers are taking readings at the same hour, it is extremely difficult to compare their records, even if they are using standard equipment, properly sited. If one person is in the habit of noting the temperature immediately after breakfast, for example, and her neighbor prefers to do so after lunch, but neither troubles to record the

Need to standardize instruments and the way they are used

time of day, merely noting the temperature, they will appear to record markedly different climates—despite living next door to each other.

Small variations in method can make a big difference. When weather stations changed from using alcohol maximum and minimum thermometers in a wooden housing to using thermistors in a plastic container, maximum temperatures apparently fell by about 0.7°F (0.4°C), minimum temperatures rose by approximately the same amount, and the daily temperature range decreased by 1.3°F (0.7°C). This was due entirely to the equipment being used.

Between 1940 and 1941, the sea-surface temperature appeared to rise by almost 0.9°F (0.5°C) and it was some time before the reason was discov-

Stevenson screen. A Stevenson screen is a container of standard construction that is used to house the thermometers used at a weather station. The screen is painted white, has double-louvered sides, and is sited in the open well clear of the ground.

ered. Prior to the 20th century, sea-surface temperatures were measured by lowering a bucket over the side of a ship to obtain a water sample, and then taking its temperature. Early records were made using wooden buckets. Then some ships began to use canvas ones and by the latter part of the 19th century most ships were using canvas buckets. These sometimes produced different readings, because water cools more quickly in a canvas bucket. The bucket was hauled onto the deck and there was a short delay while the thermometer was placed in the water and registered the temperature. During this delay the water either cooled in the cold air or warmed in the hot sunshine. Provided the type of bucket was noted, however, the data could be corrected, and by the 20th century only canvas buckets were being used.

In 1940 and 1941, the technique changed, because during wartime it was too dangerous to show the lights that were needed when lowering a bucket over the side at night. Instead, the temperature was measured by thermometers fixed inside the engine intake, where water is taken in to cool the engines, but the meteorological centers using the data were not informed. When scientists came to compile a long-term record of sea-surface temperature, they had to correct the data by +0.234°F (+0.13°C) from 1856, when wooden buckets were used, and by +0.756°F (+0.42°C) in the early 20th century when canvas buckets were used, in order to bring them into line with modern readings.

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