[esp-r] what is the defination of the site's longitude?

[Leepaul]

Dear all,
 
I downloaded a weather file from ENERGY+ website, and transfered it as the binary file ESP-r needs. when I opened that binary weather file with ESP-r, and tried to output the "integration radiation" data, the esp-r reminded me"the longitude difference is more than +-15 degree", I noted that the site data of my downloaed file are: longitude -32.8, and somwhere is 32.8W. So  what does "longitude -32.8, or 32.8W" mean? and why computer tell me"the longitude difference is more than +-15 degree"? 
 
thank you for your answer.
 
kind regards,
Paul
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