Efficient thermalization of overlapping supernovae within star-forming galaxies may produce a supernova-heated fluid that drives galactic winds .
For fiducial assumptions about the timescale for cloud shredding from high-resolution simulations ( which neglect magnetic fields ) we show that cool clouds with temperature from T _ { c } \sim 10 ^ { 2 } -10 ^ { 4 } K seen in emission and absorption in galactic winds can not be accelerated to observed velocities by the ram pressure of a hot wind .
Taking into account both the radial structure of the hot flow and gravity , we show that this conclusion holds over a wide range of galaxy , cloud , and hot wind properties .
This finding calls into question the prevailing picture whereby the cool atomic gas seen in galactic winds is entrained and accelerated by the hot flow .
Given these difficulties with ram pressure acceleration , we discuss alternative models for the origin of high velocity cool gas outflows .
Another possibility is that magnetic fields in cool clouds are sufficiently important that they prolong the cloud ’ s life .
For T _ { c } = 10 ^ { 3 } K and 10 ^ { 4 } K clouds , we show that if conductive evaporation can be neglected , the cloud shredding timescale must be \sim 15 and 5 times longer , respectively , than the values from hydrodynamical simulations in order for cool cloud velocities to reach those seen in observations .