Is there a website where I can freely download monthly average temperatures per city? I am building a garden calendar app, that uses the average temperature for each month to create a calendar. Are there websites that allow me to easily download this data in json format?
Out here in the Western U.S. the Western Regional Climate Center offers such historical data for download.  If I recall, their downloads are in .csv format, not .json. However, converting data formats is relatively straightforward for GIS software, such as QGIS.