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Time of Observation Bias

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The TOB (Time of Observation Bias) is a problem that arises when temperature recording practices change at a surface station. Generally, at least in the past, temperature recordings were made with a pair of min/max thermometers that record the coldest and warmest temperature only. These thermometers would be examined every day at approximately the same time, according to schedule. If the temperature recording time changes, a bias is likely to be introduced.

The reason for this is difficult to understand without an example, so one will be provided:

Suppose that you record temperatures at 10AM every day, and suppose that you record the following information:

Day 1 Recording 50 60
Day 2 Recording 50 60
Day 3 Recording 50 80
Day 4 Recording 50 60
Average 50 65

Also suppose that the Max temperature occurs at 3PM every day (so that the Day 2 max of 80 degrees was from 3PM of Day 1). What would be the effect of changing the daily recording time to 3PM instead of 10AM prior to Day 1?

Day 1 Recording 50 60
Day 2 Recording 50 80
Day 3 Recording 50 80
Day 4 Recording 50 60
Average 50 70

Since the Day 2 max temperature of 80 degrees occurs at 3PM, it shows up as the max when temperatures are recorded right then. As soon as the temperatures are recorded and the thermometers reset, the Max thermometer (now recording for Day 3) reads 80 degrees because it is maybe 3:05PM at this point and the temperatures are still near their daily high. Hence, moving temperature recordings closer to the time of the daily max will bias the average temperature upwards, and this is what the TOB adjustments attempts to take into account.

[Add information about various TOB adjustments in use]

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