There's gotta be a dip in the line somewhere that let a puddle of condensate form.

If you want an automatic anti-freeze up system that detects a blockage and dispenses alcohol into the line, look here:
http://www.koenderswindmills.com/Koenders_Windmills_Freeze.html

IIRC they are located in Canada.

www.hoosierpondpros.com

[Linked Image from lh6.ggpht.com]
http://www.pondboss.com/subscribe.asp?c=4
3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).