I've done this a few times, thought not time lapse, more of a rolling in cloud [never ending story style] Script called for ILM clouds, so we made it an element that POST dropped in. you'll need:
*Fish tank, 20 gallon (depending on how big you wanna go)
*Purified water/ no minerals added / the clearer the better
*Milk, I've found that a 2% works best...
Fill the fish tank 1/2 or 1/3 full of water that has been 100% saturated with the salt [saline layer]
GENTLY float freshwater on top of our saline layer, ever so gently, do not bump the tank, do not stir the liquid, I've used surgical tubing to control the output, putting the tube directly against the glass of the tank above the saline layer so it trickles down the side. Let it settle.
Using a similar surgical tube to insert the milk (while running camera) near the top of the fresh water layer. The milk will sink to the saline layer and then spread out like a rolling in cloud. Milk apparently heavier than water but not salt water....probably easier to show than explain, hope it was clear.
Hope it helped,
Cinematographer | Gaffer
Los Angeles, CA
Dylan - you might be interested in some of the toys that Fourth Phase in New York has. They have offices all over the US and also in the UK, but I don’t ; know if they are up in your neck of the woods.
There are a couple of big scene machine manufacturers that I have worked with... the Pani projectors, which are Austrian, and the pigi projectors which I believe are French and are represented by ETC.
I have worked with HMI Pani projectors up to 6k HMI, but I think they now have a 12k unit also...The pigi projectors are Xenon and I have not worked with them.
All of these machines can take continuous scrolls and some of them can take two scrollers on one machine, for interesting and more complicated effects. The vendor has a service where they can take your digital files or camera ready artwork and make up the scrolls... In your case, you might want to make cloud scrolls or maybe something higher contrast and more graphic to roll over your foreground...the possibilities are almost endless.
I have used these to project a moving skyline onto a rear projection screen suspended over a taxicab to provide moving reflections on the windshield for scenes we were shooting with VistaVision rear projection backgrounds playing behind the vehicle.
Irrespective of where you end up sourcing the gear, a good first call would be to 4th Phase (formerly Production Arts Lighting) I do not know who to point you towards these days...the last person I spoke with there was Art Lavis, but that was some years ago.