Temperature records from the longer-term stations in the region indicate that the NV has experienced warming over the last several decades. At several of the stations, the warming that is detected is stronger during the nighttime than in the daytime, and it has occurred preferentially during the year--primarily during January through August. Relatively high rates of warming in NV are found in six to nine decades of temperature records within the NV from the Napa State Hospital and St. Helena cooperative observer (COOP) stations. Similar warming trends are found at other cooperative stations surrounding NV.

The trends of minimum and maximum temperature at the COOP (COOP) stations, which amount to a warming of mean temperature that is approximately .03 F/yr since 1931, are essentially the same trends that have been reported in previous studies of regional temperature in the Napa region, including the study by Jones and Goodrich (2008). However, COOP stations have undergone several location changes and have had instrument changes. The present installations and locations do not give confidence for stable, un-altered temperature records. The Napa State Hospital installation is very close to a building and an air conditioner outlet. St Helena is mounted on the roof of a building. That there may be an excessively high warming in the Napa and St Helena COOP records is suggested by comparisons between the temperatures from the COOP stations with those from other stations in the region. These include the temperature recorded at and above the earth’s surface by the Oakland radiosonde (upper air sounding) record. California Irrigation Management Information System (CIMIS) weather records and several sets of vineyard temperature records mimic the variability shown by the longer records.

The U.S. Historical Climate Network (HCN) cooperative station temperature record from the National Climatic Data Center provides a record that is adjusted, in attempt to eliminate spurious trends. The adjusted trends in maximum and minimum temperature are lower than the trends from the COOP data, however this change is problematic because of the lack of a long stable record in the immediate region that could be used as a reference series. Petaluma, which appears to be the record having minimal amounts of adjustment, has itself gone through moves and is in a dubious site as it is also close to a structure. Thus it is possible that even adjusted Napa State Hospital COOP trend may still be affected by some, unknown amount of artificial trend, although the amount of that error could be by way of either too little or too much warming trend.