Eek!
Another simple sounding question that can get really hard, really fast...
I just looked it up and I got 23,000-40,000 feet (from one source) as a direct answer to your question. That would be a little above 500 MB up to about 200 MB.
I also got (from two other sources) "middle and upper troposphere"--which I would interpret as 500 MB to about 150-200 MB (the tropopause is at different heights depending on a bunch of things).
WV imagery does not look at one particular level, you see...
I think if we got into some of how water vapor imagery works...if you want...you might find that to be interesting.
If we do, we could end up with a journey into satellite-derived winds
http://cimss.ssec.wisc.edu/tropic/real- ... winds.html
...be careful what you ask for...
Let me know.