Cloud System: Ideas to Create

Hi everybody, I'm trying to create a cloud system on my own. Basically, I want the clouds I have (prefab particle systems) to spawn at a certain height with a certain movement direction (wind). There is 5 types of clouds, and each type spawns at different heights, as well as spawns depending on the temperature and other specifications.

So I get the basic idea to do it, but it'd be nice to hear if someone has tricks. Like how to determine the spawn pattern, with B/W maps or directly with a map sized array, etc. Also how to do percentages for probability of spawn and get random numbers to have variations. Which Unity functions can help me, how to optimize cloud numbers, how to optimize the code, anything you know about spawning things randomly based on a pattern.

This is not really a question more like ask for info, but I guess questions will follow...

Thanks in advance. V.Gillot

PS: The map is 1000x1000 units big and around 50 units high. It represents ireland on a scale of about 2-5 Km per pixel on the height map(don't know exactly). It's big.