Ultimate weather only changes textures. It still will use the default weather generator if that is all you have.  If you want a real time weather generator, Active Sky, or FSmeteo will put those high altitude clouds up there for you. Real enviroment Extreme is replacement textures with a great weather generator.

This is a quote from the flight sim review That can be found here http://www.flightsim.com/main/review/ultwxfx.htm

"This add-on definitely creates more realistic weather and lighting effects than the MSFS defaults. Its best weather results are available after add-on weather generators have been installed. The 3D lighting effects work only when A2A Shockwave 3D Lights Redux has been installed. For these reasons, I recommend Ultimate Weather FX as an enhancement to the MSFS weather generator and especially as enhancements to the other named add-ons.

Bill Stack"