There's a line in the "Girl" story of O Henry saying, "Going to be something doing in the humidity line to-night." What does this mean? I didn't get the "humidity line" part.
... A puff of red-hot air flavoured with lemon peelings, soft-coal smoke and train oil came in through the half-open windows.
Robbins, fifty, something of an overweight beau, and addicted to first nights and hotel palm-rooms, pretended to be envious of his partner’s commuter’s joys.
“Going to be something doing in the humidity line to-night,” he said. “You out-of-town chaps will be the people, with your katydids and moonlight and long drinks and things out on the front porch.”
Hartley, twenty-nine, serious, thin, good-looking, nervous, sighed and frowned a little.
“Yes,” said he, “we always have cool nights in Floralhurst, especially in the winter.”