Derivation of "Calamity"

Forcellini, *s.v. Calamitas*, says:— "Proprie significat imminutionem clademque calamorum segetis, quæ grandine vel impetuoso aliquo turbine aut alia quapiam de causa fit." He then quotes Servius, *Ad Georg*, i. 151:— "Robigo genus est vitii, quo culmi pereunt, quod a rusticanis calamitas dicitur." Then follows the note of Donatus on Ter. *Eun*. i. 1. 34. It appears to me, if "*calamitas*" were derived from *calamus*, it would mean something very different from what it does. Another suggestion is, that the first syllable is the same as the root of *cad-o*, to fall; *l* and *d*, everybody knows…