We consider the uniform motion of a drop of one fluid moving in anothe
r immiscible, unbounded, fluid. We examine the condition of normal str
ess balance across the interface. Consistent application of this condi
tion to linear order in the velocity requires the introduction of a ne
w parameter, Delta, which takes into account deformations of the drop
from a spherical shape. Imposing a condition for the critical size of
these deformations allows us to predict the maximum size of the drop f
or a given terminal velocity. For the case of raindrops this gives a m
aximum size of approximately 2 millimeters.