Klimaform magic accuracy. This is mainly exploratory as I wanted a sense of the effect size, not to obtain a precise value.

SCH/BLM, 114 INT, 420 elemental magic skill (Dark Arts active), used Thunder Magian accuracy staff (affinity+6) to make sure "magic hit rate" above 50%. Rairin Obi when Klimaform + Thunderstorm active.

Thunder I, Arboreal Bastions in Yorcia Weald, no weather (supervised data collection to ensure this), neutral day (Iceday)

Raw data upon request.

Code:

Arboreal Bastion #1
Thunder/Klimaform
Full: 96/134 (.716418)
1/2: 32/134
1/4: 5/134
1/8: 1/134
Thunder/no Klimaform/no weather
Full: 100/150 (.666666)
1/2: 31/150
1/4: 11/150
1/8: 8/150

95% CI for difference in full damage rate (-.05755791, .15706040).

Code:

Arboreal Bastion #2
Thunder/Klimaform
Full: 178/227 (.784141)
1/2: 39/227
1/4: 8/227
1/8: 2/227
Thunder/no Klimaform/no weather
Full: 136/229 (.593886)
1/2: 60/229
1/4: 19/229
1/8: 14/229

95% CI for difference in full damage rate (.1071265, .2733826). (I also checked goodness of fit to the known multinomial model for resists for all the count data. Nothing unusual)

If you pool the above results the overall 95% CI is (.08240814, .22587096). This compares to adjusted 95% CI (.08811906, .2258532) if you control statistically for a (potential) difference in meva between Arboreal Bastions (ran GLM in R):

Code:

Call:
glm(formula = h ~ m + k, family = binomial(link = "identity"),
data = junk)
Deviance Residuals:
Min 1Q Median 3Q Max
-1.7141 -1.3777 0.7233 0.9896 1.0346
Coefficients:
Estimate Std. Error z value Pr(>|z|)
(Intercept) 0.58556 0.03726 15.717 < 2e-16 ***
m 0.02730 0.03672 0.744 0.457
k 0.15699 0.03514 4.468 7.9e-06 ***
---
Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
(Dispersion parameter for binomial family taken to be 1)
Null deviance: 859.25 on 689 degrees of freedom
Residual deviance: 839.66 on 687 degrees of freedom
AIC: 845.66
Number of Fisher Scoring iterations: 4

So the point estimate of Klimaform magic accuracy (with weather) is close to +15 but cannot rule out +10 or +20 taking the data as a whole. Probably +15 if you want to cherry pick (I don't mind)

Also Klimaform overwrites Focalization status but Focalization does not overwrite Klimaform. This doesn't necessarily mean 1/5 Focalization has a lower magic accuracy bonus than Klimaform, but it makes me wonder, given that 5/5 Focalization gives an additional +20 magic accuracy, whether greater magic accuracy buffs necessarily overwrite lower magic accuracy buffs (unless overwriting doesn't mean anything). I will do 5/5 Focalization and look into this (edit: Klimaform still overwrites 5/5 Focalization without weather active)

And as I recall Ascetic's Tonic overwrites Klimaform but not vice versa but I don't remember exactly

Obviously if weather itself provides an accuracy bonus (not sure about this?), cannot determine additional magic accuracy of Klimaform from this data alone.