Temperature varies quite much.
Something like
April-May approx. +5 to +10 Celcius
June approx. +10 to +15 C
July-August approx. +10 to + 25 C
September approx. +5 to +10
October approx. 0 to +5 C
and during the winter break the temperature could be something like -35 C.
These are just guidelines.
I'll check the undersoil heating thing and put the list at my website
http://www.sportfinland.cjb.net
as the Veikkausliiga season is looming behind the corner and I will again make the season previews and then ratings and picks to games of course.