Modern hot air balloons were developed around 1960 and are used primarily for recreation. Most hot air balloon launches are made during the cooler hours of the day, at dawn or two to three hours before sunset. At these times of day, the winds are typically light making for easier launch and landing of the balloon. Hot air balloons consists of a bag, called an envelope, which contains heated air, suspended beneath it is a gondola which carries passengers and (usually) a source of heat, in most cases an open flame. By heating the air inside the envelope, it makes the balloon buoyant since it has a lower density than the colder air outside the envelope. What makes ballooning so attractive to most people is the exceptional quiet, the lack of a feeling of movement, and the bird's-eye view. Since the balloon moves with the direction of the winds, the passengers feel absolutely no wind, except for brief periods during the flight when the balloon climbs or descends into air currents of different direction or speed.