  Bag breakup of drops has been a subject of interest for almost over a
century. Several issues such as theoretical estimation of the regime boundary
marking the onset of such breakup, bag growth rates, drop size distribution,
and the effect of Weber number, $We$, and Ohnesorge number, $Oh$, on these
quantities remains unaddressed. The current study aims to clarify aspects of
the atomization process through experiments and theory. We examine bag breakup
of a single drop of various inviscid and low viscosity fluids as it deforms in
the presence of a continuous horizontal air jet. The We boundary at which bag
breakup begins is theoretically determined and the expression obtained, $We =
12(1 + \frac{2}{3} Oh^2)$, is found to match well with existing experimental
data. An exponential growth in the radial extent of the deformed drop and the
streamline dimension of the bag is predicted by the theoretical model and
confirmed by experimental findings. These quantities are observed to strongly
depend on $We$. However, their dependence on $Oh$ is weak for the range of $Oh$
considered in this study. Subsequent to drop deformation, bag formation and
expansion is the bursting process. This is marked by the disintegration of the
bag owing to instability of the Rayleigh-Taylor type, followed by collapse of
the liquid rim bounding this bag by Plateau-Rayleigh instability. The sizes of
the drops thus produced are measured using Phase Doppler Anemometry (PDA) which
is in contrast to shadowgraphs used in earlier studies. A discernible shift in
the peak of the drop size distribution for viscous drops is seen which
indicates a preponderance of drops of higher diameters vis-\`a-vis fragment
size distribution for inviscid drops. Furthermore, an estimate of the Sauter
mean diameter ($D_{32}$) is presented which is somewhat lower than earlier
predictions.
