Step Five ...
This one looks easy, but was actually quite difficult. Gradations like the two yellow ones already done - which fade out to nothing over a fairly wide area - are not so tough to do, but this brown one - a wide flat area which suddenly fades out to nothing at one end - is more of a challenge. Once the two pages are loaded into your browser, click back and forth between Step 4 and Step 5 to see the result; none of the brown actually touches the bottom of the plume. This should serve to give the effect of being illuminated from below, as you will see later ... (much later!)