I also need to take a step back here as I did some more testing today.
If the wt_data_inc file is not being processed by Cumulus it also brings the same error.
So firstly, make sure the following files are being processed in Cumulus by copying them across to your main Cumulus directory and then:
For Cumulus 1.9.x version go to Configuration --> Internet and click on the Files tab and add the entries like so (just an example)
Local Filename: C:\Cumulus\wt_data_inc.php
Remote Filename: public/website/weather/wt_data_inc.php
Make sure the "Process" and the "FTP" buttons are checked.
The same will apply if you are using Cumulus MX except the settings are found under the "Settings" --> Extra Web Files menu.
Files that need processing are wt_data_inc.php and wt_month_inc.php
Let me know how you get on. As I say you may need other changes if you are using CumulusMX.
Andy