The Colorado Climate Center has worked closely with the National Oceanic and Atmospheric Administration (NOAA) since the early 1990s attempting to monitor and document the impact of the Automated Surface Observing System (ASOS) on long-term climate records. This paper is a compilation of the results and conclusions of nearly a decade of work. Some general conclusion of the study have been: 1) ASOS measures less precipitation than the rain gauge system it replaced, especially when the precipitation falls as snow, 2) ASOS temperatures are generally cooler than the previous temperature measurement system, 3) Reported wind speeds are slightly lower, 4) Dew point temperatures do not appear biased but occasionally depart from truth. All these changes posed difficulties to the National Climatic Data Center when new "climate normals" were computed in 2001.

Changing how we observe our climate does affect the resulting data. If we need to know and understand long-term climate conditions, we must avoid or thoroughly document the changes that occur as a result of the instruments we choose and the sites that we use. Once we begin down the path of automation, more changes become inevitable. For example, the National Weather Service is currently ready to deploy a new dew point measurement system and a new all weather precipitation gauge. Changes in wind measurements and freezing rain detection are also taking place. All will lead to more data discontinuities in our historic records. How the NWS plans to fund and conduct future climate data continuity analyses is not known at this time but is under discussion. The National Weather Service should be presenting their plans very soon. It is in the best interest of climatologists to become very familiar with this process and make sure it is successfully accomplished.