National Climatic Data Center's Climate Maps of the United States database offers polygon data for many popular climate variables. We would like to use this data for preliminary visualizations on bridge decay research -- the primary benefit to this data being its simplicity (it's already averaged by day/year over a 40 year period), national continuity, and file format (shapefile).
However we encountered two issues with the data.
Issue #1: it's based on a funky datum "GCS_NAD_1927_CGQ77", which addresses some issues in Quebec with NAD 1927.
We were able to side-step this issue by just re-defining the projection as something more common ... in our case we just went right to WGS 1984, since we wanted to view in Google Earth, and since a rough visualization was our immediate purpose. I suppose NAD 1927 would have been fine, since we weren't doing anything with Quebec. BTW, there are no readily available transformations of the GCS_NAD_1927_CGQ77 datum.
Issue #2: the data is not continuous or even numerical
Values are expressed as "0 - 5 in", so we have no way of redefining what values are grouped in what colors, and since it's polygon data we also lose the ability to redefine spatial grouping based on new intervals. That's not a big problem for this first rough visualization, but we will need to find more fine-grained data for subsequent analysis. We'll probably end up taking point data from other NCDC sources and doing our own interpolation and temporal reduction.