Day length turns out to be one of the critical calculations for calculating potential evapotranspiration in the Climatic Water Deficit Toolbox .In order to improve the ease of use of the Climatic Water Deficit Toolbox I've decided to automate the calculation of day length for any given latitude for any given Julian day. The result is the Day Length Tools for ArcGIS . There is also a version of the tool that calculates day length for all months using the 15th day of each month. I can imagine that there is potential for this tool beyond just climatic water deficit. For example, maybe this tool could be used by people studying plant phenology to map areas where day length exceeds a certain number of days.
One question that people may have is "where can I get a latitude raster?". I created one in ArcGIS using lines of latitude and then performing a second order trend analysis using the Trend tool in Spatial Analyst (ArcToolbox - Spatial Analyst Tools - Interplotation - Trend).
