Depending on your what your intended result is there is a couple of ways to handle the particle system. I’ll give you two to get your started.
First you will need to make sure that you have your material setup to take advantage of this as well.
Here is a basic material I’ve set up using a smoke texture from the Starter Content.
You will want to make sure that you’ve got a particle color node that is setup. You will need to multiply the alpha of that and your cloud texture to get the opacity.
To setup the particle you will need to plugin the texture into the particle module. Then you will need to select “Color Over Life” and set some values for the Alpha over life. This will control our fade in/out.
Change the Distribution to “DistributionFloatConstantCurve1”
Next we will add some points that is controlling how long it will take to fade in and out.
For the first setup, this is assuming that you want it to fade in and out over the course of its life. We have three points here.
Point 0 - Full opacity
Point 1 - Fade in
Point 2 - Full opacity
You control these values in the “In Val” which is the Time to fade in over (in this example 1 second) and “Out Val” which is the alpha value.
In this next example this is assuming you do not want it to fade out.
So everything is the same as above with the exception that we are using two points over the life of the particle.
It will fade in and then stay at no opacity over the particle life.
Give this a shot and if you have any questions feel free to ask!