"The June day was at its sultriest hour."
Sultry is related to French "saltir" to jump. Lightning jumps. So sultry weather is hot humid weather, when thunder and lightning are common.
"In sultry weather they clanked together and sparks shot xxxxxxxx"

But this from MW is quite, quite different, wwh, in word history:

"Etymology: obsolete English sulter to swelter, alteration of English swelter"

Dear WW: it is quite possible that the etymology I learned many years ago was incorrect. Yours does seem to fit quite well. Thanks, Bill