Certainly cold weather makes my Parkinson's worse.
But I don't think a cold climate is in itself a major determinant in the prevalence of PD.
See the prevalence map in
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2865395/
"Geographic and Ethnic Variation in Parkinson Disease: A Population-Based Study of US Medicare Beneficiaries"
Allison Wright Willisa, Bradley A. Evanoffb, Min Lianb, Susan R. Criswella, Brad A. Racette
Neuroepidemiology 2010;34:143-151 (DOI: 10.1159/000275491)
I reckon the correlation is with wind direction and pollution. See the recent thread on American prevelance.
John