Snowing again! (particle time smooth?)

Hi please feel free to hack this to bits. I have been asked to provide a controllable snow projection for our "seasonal" show. Keys 1-5 control the amount of snow. Press 5 to start the snow.

As you will see in the comp I am not able to smoothly control the speed of the particle systems. I thought I could just use the smooth patch but that just sends it flying up to the new speed.

I am quite happy with the look and behaviour of the snow but wish that when there is light snowfall (1) that it falls more slowly and smoothly increases up to heavy snow (5) without anything weird happening.

So I guess I am asking how do I change the speed of the particle system gradually with no weird effects?

The show is next week.

Merry snowtime! Carl