A hot air balloon is an aircraft that contains a balloon, which is filled with hot air, produced by a burner, with a basket underneath for carrying passengers.
In 1893, Doc Brown wanted to travel to 2035 to pick up the parts needed to finish the Jules Verne Train. He came up with an idea for a time parachute. Since the stresses of space-time and the temperature variation would wreak havoc on the cast-iron frame of his steam time car, he needed another method to use for the return trip.
He hooked up the flux capacitor and time circuits of the steam time car to a parachute that was stored inside its storage compartment, along with a hot air balloon. He then planned to don a diving suit to protect his body from the rigors of space-time and the temperature variation.
A person would ascend in a balloon to a distance of half a mile, and then would be released from the balloon. The person would fall at a rate of 35 feet per second per second, and would reach 88 mph between four and five seconds. Since the time circuits and flux capacitor are connected to the parachute, the person would travel through time. The time traveler would then pull the ripcord of the parachute once they traveled through time, and arrive safely on the ground in that time period.
The first successful test of the time parachute took place in 1986 by Doc Brown, wearing the diving suit, and Marty McFly, wearing a radiation suit. They traveled from 1986 in the Lone Pine Mall parking lot, and crashed through the ceiling of the Lone Pine Fli-Drome in 2035.
In 1991, Doc Brown flew a hot air balloon over Hill Valley. While demonstrating the possible activities that could be performed while flying in a hot air balloon, Doc fell out of the balloon and through the roof of his lab. He then accessed his video encyclopedia, wherein Bill Nye explained how a hot air balloon works.