This paper reports the results of an experimental investigation on the disintegration of cylindrical liquid jets. The aim of this work is to shed more light on the physics responsible for the break-up of low speed liquid jets through an analysis of the stability curve and mainly of the development of its first maximum i.e. the critical point. The experimental work consisted in measuring break-up length of liquid jets under different ambient pressures. Contrary to what Weber's theory stipulates, it is found here that the development of the critical point is not always due to the action of the aerodynamic forces and may be due to the liquid flow itself. This conclusion leads to the definition of a new jet parameter that allows a better classification of jet behavior as far as the disintegration process is concerned. This parameter indicates whether the initial internal flow configuration will have an influence on the break-up of the jet before the appearance of the aerodynamic forces. Furthermore, in order to correctly predict the critical velocity in any situation, a new modification of Weber's theory is suggested and discussed.