The structure for these is documented on the wunderground wiki. create a URL (web address), one for each data record to be uploaded.I prefer SI units but Wunderground uses American conventions. This is easy for Cumulus since the data is stored in a text file with commas separating the values and the Cumulus help file tells you what each column is. get the stored data into a spreadsheet.I guess a similar process works for weather upload sites other than Wunderground too.

The steps should be applicable to any weather station software that stores the data as a “CSV” or formatted text file, with a little bit of fiddling.

It seems there is no “off the shelf” way of doing this so here is a how-to-do it. A few months ago I bought a weather station but only lately registered with Wunderground and was slightly dismayed that the software I use ( Cumulus, which is heaps better than what the station supplier included) does not post historical data it only uploads to Wunderground what it downloads from the station when Cumulus starts up.

Wunderground historical weather