When using a pair of min/max thermometers for daily temperature observations, the time of day at which the readings for the previous 24 hours are observed, and the thermometers are reset, will often cause a time of observation bias (TOB). If readings are taken near the times of daily highs, or daily lows, those highs, and lows, often affect the readings of two days. Annual averages of the effects of TOB on recorded temperatures can be more than 1° F (0.56° C) at many locations, and near 2° F (1.11° C) at some. (This review of TOB is limited to temperature observations using min/max thermometer pairs, and/or electronic min/max thermometer sets which yield comparable results. Temperature observations using other kinds of thermometers may also have some kinds of TOB, but they are outside the scope of this review.)
In the "United States Historical Climatology Network" (USHCN), one kind of temperature adjustment is a TOB adjustment relative to midnight for observations made at times other than midnight. The occurence of TOB, and adjustments for it, are particularly important factors if the time of observation at a weather observation station changes.
In order to gain a better perspective of this bias, hourly temperature data of 190 US locations were used to calculate estimates of TOB relative to midnight, as well as estimates of some other items that seemed interesting.
The approach used is to choose several hypothetical "times of observation", and to calculate what high, and low, temperatures a 24 hour min/max thermometer set would have "observed" at those times based on the hourly temperature records. These estimates cannot be precisely accurate partly because hourly observations will miss highs, or lows, that occur between the times of those observations, but hourly observations can provide at least an approximation of TOB.
Since some people prefer their temperatures in degrees Celsius, and others in degrees Fahrenheit, this discussion continues in either. Select your preference: Celsius , Fahrenheit .