Find amount of vertical movement of object

I am wanting to find the amount of movement up or down for an object (regardless of movement in other axis) and if its above a set amount, or below a set amount. certain things happen. Unfortunately it does not seem to work as intended. The variable "move_distance" is ALWAYS a positive number, which makes absolutely no sense.

 max_fog_height_increase is 0.1
 
 max_fog_height_decrease is 0.1


 Vector3 old_position = fog_clone_active_pool[i].transform.position;
 if (flow_on_terrain)
   {
   //this works. effects object position in world
   fog_clone_active_pool[i].transform.position += GetTerrainNormal(fog_clone_active_pool[i].transform.position) * flow_speed;
   }
 if (effected_by_wind)
   {
   //this works. effects object position in world
   fog_clone_active_pool[i].transform.position += wind_master.transform.TransformDirection(Vector3.forward) * wind_velocity;
   }
 if (fog_clone_active_pool[i].transform.position != old_position)
   {
   float move_distance = fog_clone_active_pool[i].transform.position.y - old_position.y;
   if (move_distance > max_fog_height_increase && fog_clone_active_pool[i].transform.position.y > old_position.y)
     {Debug.Log("Clone moved too high");
     }
   else
     {
     if (last_move_vector.y < -max_fog_height_decrease) 
       {
       Debug.Log("clone fell too far");
       }
     }
   }