Not sure what you are looking for, but your particles should
a) only be driven by a negative gravity, not velocity (so they get faster towards the top
b) not live too long
c) of course have a proper b/w gradient over their short life
d) lots of lifespan/ timing variation
You should also not make them too opaque. Instead use lots of them ands see to it, that they give you a decent "depth". This way, the overlap will produce even more variations in brightness to drive your displacement. For some swirling, use a very high Spin frequency, but with a relatively small amplitude. As usual with any credible effects - don't overdo, keep everything subtle.
what do you mean b/w grad? do you mean colour over life, black to white? i Like then motion.
I didn't no the trick about not using velocity but gravity instead, guess that comes from being a 3d genius.
The shot i am trying to create is a POV, travelling down a straight road (in London) and then in the distance a heat wave/mirage effect appears that is of an african plain. Subtlety is the problem as the displacement effect is a bit to much like a ripple dissolve, but will give it a go with your tips
[harry rambaut]"what do you mean b/w grad? do you mean colour over life, black to white?"
Yepp, exactly. I think whhat you are looking for is more like a smear, so you may want to add a CC Vector Blur or even ReelSmart Motionblur before using your comp as a displacmenet map. I don't think that being a 3D guy is a problem, it's more a matter of understanding the physics a little and I think in such situations using any velocity is counterproductive (actually many thermal phenomenons such as smoke should mainly be driven by such forces, a common mistake made in the 3D world).