Final answer:
Hot air balloons ascend because heated air inside the balloon is less dense than the cooler outside air, creating a buoyant force. Without engines, balloons use varying wind currents at different altitudes to navigate horizontally.
Step-by-step explanation:
Hot air balloons are able to move smoothly in the air based on the principle that hot air is less dense than cool surrounding air. When the air inside the balloon is heated, the molecules speed up, collide with the walls of the balloon more forcefully, increasing the internal pressure and causing the balloon to expand. The hotter air inside the balloon becomes less dense than the cooler air outside, creating a buoyant force which makes the balloon rise. Although balloons do not have an engine or other system to move horizontally, pilots can steer them to a limited extent by adjusting the altitude to find wind currents moving in different directions and at various speeds. This is how balloons can travel long distances, such as flying around the world in 11 days.