Prof. Cumulus Isobar was a travelling con-artist and snake-oil salesman in the west during the 19th century. At some point circa 1880, he traveled to the mining-town of Tumbleweed which was encountering a harsh drought due to the angered spirit of Big Thunder Mountain.
Cumulus offered his abilities as a, "Rain-maker" to try and exploit the town only for the spirit of the mountain to send a flash-flood overnight. This flood lead to Isobar being marooned in the town of Tumbleweed.
Cumulus is in several ways inspired by unused gags created for the unbuilt Western River Expedition attraction developed by Disney-legend Marc Davis. One of these would have had an indigenous tribe practicing a rain-dance outside the town of Dry Gulch causing it to be ironically flooded. The other gag reminiscent of Isobar is the snake-oil salesman Doc Cogwheel who would have had a carriage where he sold, "Magic elixirs" to the townsfolk of Dry Gulch. Imagineer Tony Baxter has cited the film The Seven Faces of Dr. Lao as inspiration for the Rain Wagon gag Professor Isobar was part of.
Big Thunder Mountain Railroad
Show guides for the attraction described Cumulus Isobar as the following:
"Although the little settlement suffered from bouts of arid weather the arrival of Professor Cumulus Isobar, "Rain Maker Extraordinaire and Purveyor of Magical Elixirs" always guaranteed that his customers would no go dry for long. Sometimes it even chanced to rain while he was in town, enhancing his reputation, and usually resulting in a flash flood, if the ground refused to absorb the rainfall because it had baked too long in the sun."
As of the 2012 queue updates, a letter from a customer named Eberhardt Z can be found where he is upset that a, "Miracle cure" for his bunions has failed and hopes that he can deliver a proper remedy, while expressing a desire to see the bridge that Isobar sold him. Isobar himself appears in the flooded town of Tumbleweed, at the back of his wagon bailing out water.
Cumulus Isobar appears in the first issue demonstrating his supposed rainmaking talents with a cannon. The clouds he creates are enough to fool the onlookers into purchasing his various wares, though Abigail Bullion is smart enough to not take him seriously.
His appearance in the comic is much more kept up in contrast with his animatronic figure and bears a closer resemblance to Doc Terminus.