Move to random position based off user's input of terrain size?

We need a way for our rain to randomly move around the parameters of the user’s terrain size, which is inputted by the user.

How can we make it so that the rain will randomly move across the terrain based on time.

Example (which will be hard to explain): The rain ellipsoid is set at 500x500 and the terrain size is 5000x5000 the rain is moving randomly based off the user’s input of the terrain size 5000x5000. Since the rain is 500x500 moving at a few degrees a second it can only cover a parameter of 500x500 at a time. So it can move itself across the terrain over time to 2500x2500 then over more time 1000x1000 ect. So pick a point on the terrain and move towards it.

So we need the rain to move across the x and z axis only within the parameters of the given input. The rain has to move all as one object which makes it confusing for having random positions for 2 axis’.

Here’s what we got so far it doesn’t work properly. It just rapidly moves all over the terrain like it’s glitching, but it is pick random points. Also it always sets the Y axis to 14 for some reason.

For X it’s 0 to 5000
For Z it’s 0 to 5000

Covering all possible terrain areas.

var parameter1 = 0;
var parameter2  = 0;
var parameter3  = 0;
var parameter4  = 0;

parameter1 = Time.time;
parameter2 = Time.time;
parameter3 = Time.time;
parameter4 = Time.time;

 transform.position = Vector3(Random.Range(parameter1, parameter2), 809.3739,  
     Random.Range(parameter3, parameter4));

You can create a random point and move to it at constant speed with MoveTowards; when the point is reached, create a new random point and repeat the loop:

var speed: float = 20.0; // rain absolute speed
var xMin: float = 0.0;  // set the terrain limits
var xMax: float = 5000.0;
var zMin: float = 0.0;
var zMax: float = 5000.0;

private var point: Vector3;

function Start(){
  // force a new random point in the first Update:
  point = transform.position;

function Update(){
  if (transform.position == point){ // if current random point reached...
    point.x = Random.Range(xMin, xMax); // draw new x, z coordinates, but
    point.z = Random.Range(zMin, zMax); // don't change the original y
  // anyway, move towards the current random point:
  transform.position = Vector3.MoveTowards(transform.position, point, speed*Time.deltaTime);

MoveTowards(a, b, dist) returns a point at dist distance from a in direction to b; the point returned is clamped to b, thus the rain will never pass this point.

NOTE: If you don’t want to rain out of the terrain, subtract the ellipsoid half-size from the limits: if it’s 500x500, set xMin=250, xMax=4750, zMin=250, zMax=4750, for instance.

I’m guessing this is in an Update function?

parameter1 = Time.time;
parameter2 = Time.time;
parameter3 = Time.time;
parameter4 = Time.time;

That’s going to set all 4 parameter variables to the same exact value. So not very random.
Why not use something like Random.Range(0f,5000f)? Or Random.value * 5000?

Are you trying to pick random spots for this rain patch to move, and have it move over time? I would look into using Mathf.Lerp (see scripting reference). Pick a new random position every so often (but not every frame!) and set a ‘target’ position. Then let it Lerp toward that target.