One of the issues that affects the efficiency of data centers is the climate in which the data center is located. A well-run data center in Phoenix could draw more power than a poorly run data center in Toronto. When looking at measures of data center efficiency, it's important to consider these factors.
There's a nifty utility available from UCLA called Climate Consultant (see
http://www2.aud.ucla.edu/energy-design-tools/). This software allows you to draw
psychrometric charts, plot average/min/max temperatures, etc. for any location for which you have the proper type of weather file. For example, the graphic at the right shows a psychrometric chart for Hillsboro, Oregon.
Not only is this utility free, you can get weather data for many locations for free. The U.S. Department of Energy (DoE) maintains weather data for over a thousand locations around the world, including most airports at http://apps1.eere.energy.gov/buildings/energyplus/cfm/weather_data.cfm. For example, http://apps1.eere.energy.gov/buildings/energyplus/cfm/weather_data3.cfm/region=4_north_and_central_america_wmo_region_4/country=1_usa/cname=USA#OR provides data for several sites in Oregon. Just save the .EPW file to your local hard disk, and you're good to go.
P.S. Thanks to Pasi Vaananen for the pointer to this tool.