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Actually, most modern hot air balloons are designed with a hole in the top. This hole is called a parachute vent (or deflation port). The vent is covered by a flap, which pilots can open and close. Pilots use the vent to release hot air from the balloon. This is done to slow the balloon's ascent, start a descent, or increase the rate of descent. Some hot air balloons even have turning vents, which are vents on the side of the balloon that are used to rotate the balloon.