Open a new window with:
Width:200pixels Height:100pixels
Resolution: 72 pixels/inch
Mode: RGB color Contents:White
Choose Gradient Tool (G) and apply to the image from top to bottom. Make sure your foreground and background colors set

to default. Press "D" to set to default. Hold down Shift when you apply.

Go to menu Filter > Render > Difference Clouds.
Go to menu Image > Adjust > Invert or (Ctrl + I)