An analysis of experimental data on the critical conditions of drop breakup by aerodynamic forces is given. Possible causes of divergence among data on the critical Weber number, We(c), obtained by different authors is discussed It is shown, that the most important of these causes is incorrect technique for determining We(c) (at the point of breakup), and disregarding the influence of the rate of change of the flow action upon the drop. Thorough experimental investigation of peculiarities of drop fragmentation under critical and over-critical conditions was carried out. Temporal characteristics of the breakup process and the mean size of fragments being formed is measured. Generalizing formulas for We(c), the induction time, and the period of drop natural oscillations are obtained.