method for merging meshes

As long as they share one material, you might want to have a look at Mesh.CombineMeshes. The meshes are treated as one object, but it looks exactly the same. No changes to the vertices, but just batching the objects together as one, to reduce draw calls. Here's an example:

     // Parent all your similar meshes under one object
     public void CombineMeshes(GameObject parent)
     {
         var meshFilters = parent.GetComponentsInChildren<MeshFilter>();
         var combines = new CombineInstance[meshFilters.Length];
 
         for (int i = 0; i < combines.Length; i++)
         {
             var combine = new CombineInstance()
             {
                 mesh = meshFilters[i].sharedMesh,
                 transform = meshFilters[i].transform.localToWorldMatrix
             };
 
             combines[i] = combine;
         }
 
         var combinedMesh = new Mesh();
         combinedMesh.CombineMeshes(combines);
 
         parent.GetComponent<meshFilter>().Mesh = combinedMesh;
 
         // Now you can delete all the separate cloud meshes as
         // they are all combined into one mesh, above
     }