A Model to Estimate the Time of Observation Bias Associated with Monthly Mean Maximum, Minimum and Mean Temperatures for the United States

Recent Changes in Frost Days and the Frost-Free Season in the United States

“…la) are consistent with the patterns of mean annual temperature changes shown in Karl et al (1996). Their analysis showed substantial warming for most of the country, including the western, north-central, and northeastern states, but a general cooling for the southeastern United States.…”

“…He found a tendency toward fewer cold minimum temperature threshold exceedences, more warm minimum temperature threshold exceedences, and also fewer warm maximum temperature exceedences. These findings are consistent with a number of studies (Easterling et al 1997;Karl et al 1993) that find evidence of more warming being found in minimum (nighttime) temperatures than in maximum (daytime) temperatures. More recently, Schwartz and Reiter (2000) found phenological evidence of a move to an earlier spring in parts of North American, and Cayan et al (2001) also found a change toward earlier spring in the western United States using both phenological and streamflow data.…”

“…I then used each station time series to create an average regional time series using the nine U.S. regions defined in Karl and Knight (1998). Last, I calculated simple linear trends for each regional time series, with the results shown on the maps in Fig.…”

“…However, the warming has not been constant, and, for instance, Karl et al (2000) showed that starting in 1880 there was a slight cooling trend until about 1910, then strong warming (0.12°C decade" 4 ) until early 1940s, and another period of slight cooling until the mid-1970s; since the mid-1970s, however, global temperatures have risen rapidly (0.2°C decade -1 ). Global and U.S. temperatures for 1998 were the warmest since 1880 and may have been the warmest for any year in the last millennium (Mann et al 1999).…”

The reliability of global and hemispheric surface temperature records

“…It is, however, possible to identify such changes and correct for them, provided dates of the changes are known. Another example from the USA is the change in observation time of daily maximum and minimum temperatures from late afternoon to early morning, which has been referred to as "time of observation bias" (TOB) and corrected for by Karl et al (1986). The effect is noticeable because morning readings tend to be slightly cooler than those taken in the late afternoon.…”