...Unless the background is a gradient itself, then Rsiel's good approach may not give you the results you want. In that case, you may want to create a gradient mask:
1) Create a rectangle that covers your cloud art entirely.
2) Fill that rectangle with a basic black-to-white gradient (no stroke). Make the white at the top, black at the bottom.
3) Select both the cloud art and the rectangle.
4) In the Transparency palette, click the "Make Mask" button. This should create a gradient on your cloud, which goes from your original color to 100% transparency.
This method basically shows everything that's covered by white, and hides everything covered by black. It allows you to use this art on top of any art: solid color, gradient, or photo, with no worry about the background color.
If you want to show more (or less) of the cloud, go back to the Transparency palette (with the cloud selected), click "Release", and adjust the gradient to show more white or black, depending on how much you want to show/hide.
Hope this helps, good luck!