A lead fall is a fall of a leading climber, in the case he is above his last piece of protection. The severity of a is estimated by the impact force that is generated during the arrest of the fall. The greater the impact force, the more serious the fall is.
Lead falls are, of course, more serious and dangerous than toprope falls, because the energy absorbsion requires greater breaking forces. The breaking forces are greater the longer the fall is, and the shorter the rope used to break it. Although not acurate, a usefull method to estimate how potentially dangerous a fall is Using the Fall Factor. The FF rises with the impact force and so the higher the FF, the more severe the fall is.