  Using ultra-violet absorption-lines, we analyze the systematic properties of
the warm ionized phase of starburst-driven winds in a sample of 39 low-redshift
objects that spans broad ranges in starburst and galaxy properties. Total
column densities for the outflows are $\sim$10$^{21}$ cm$^{-2}$. The outflow
velocity (v$_{out}$) correlates only weakly with the galaxy stellar mass
(M$_*$), or circular velocity (v$_{cir}$), but strongly with both SFR and
SFR/area. The normalized outflow velocity (v$_{out}/v_{cir}$) correlates well
with both SFR/area and SFR/M$_*$. The estimated outflow rates of warm ionized
gas ($\dot{M}$) are $\sim$ 1 to 4 times the SFR, and the ratio $\dot{M}/SFR$
does not correlate with v$_{out}$.
  We show that a model of a population of clouds accelerated by the combined
forces of gravity and the momentum flux from the starburst matches the data. We
find a threshold value for the ratio of the momentum flux supplied by the
starburst to the critical momentum flux needed for the wind to overcome gravity
acting on the clouds ($R_{crit}$). For $R_{crit} >$ 10 (strong-outflows) the
outflow momentum flux is similar to the total momentum flux from the starburst
and the outflow velocity exceeds the galaxy escape velocity. Neither is the
case for the weak-outflows ($R_{crit} <$ 10). For the weak-outflows, the data
severely disagree with many prescriptions in numerical simulations or
semi-analytic models of galaxy evolution. The agreement is better for the
strong-outflows, and we advocate the use of $R_{crit}$ to guide future
prescriptions.
