as in the title, just build the cryometric monitors and was wondering what "-40% % maximum season predictability" does.

In the lower right corner of the screen you will see an area that specifies a number of turns (IE: 10-20). This amount of turns is the "approximate" amount of time until the next season change. If you are in summer then winter will occur anywhere from 10-20 turns from now. The '-40% predictability' will shorten that range so that it is clearer when winter will be. (IE: instead of 10-20, it might show 11-16). If you get the couple other buildings/researches that reduce this then generally you will know within 2 turns or so when the season will change.