Cloudmap for use on Terrains and using the Expanse Creative sky. Assign script under your terrain, create your particle system and assign to the script.

CloudMap.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class CloudMap : MonoBehaviour
{
    private Terrain m_terrain;
    [Tooltip("The Global particle System for rain/snow on the terrain according to the cloud map")]
    public ParticleSystem m_globalParticles;
    [Tooltip("Lower values will emit from denser clouds")]
    [Range(0, 1)]
    public float m_visibilityThreshold = 0.5f;
    [Tooltip("The higher the resolution, the more samples taken so will have an impact on performance ")]
    public CloudMapResolution m_cloudMapResolution = CloudMapResolution._256;
    int m_cloudMapSize;
    [Tooltip("Height of the global particle system relative to the highest point on the terrain")]
    public float m_globalParticlesHeight = 1000f;
    [Tooltip("Cloudmap update in seconds. 0 means only sample will be taken on start")]
    public float m_cloudMapUpdate = 5;
    private List<float> m_visibility = new List<float>();
    private List<float> m_density = new List<float>();
    private float max_height;
    // Start is called before the first frame update
    void Start()
    {
        m_terrain = gameObject.GetComponent<Terrain>();
        max_height = m_terrain.terrainData.size.y;
        switch (m_cloudMapResolution)
        {
            case CloudMapResolution._64:
                m_cloudMapSize = 64;
                break;
            case CloudMapResolution._128:
                m_cloudMapSize = 128;
                break;
            case CloudMapResolution._256:
                m_cloudMapSize = 256;
                break;
            case CloudMapResolution._512:
                m_cloudMapSize = 512;
                break;
        }
        if (m_globalParticles != null)
        {
            var shape = m_globalParticles.shape;
            shape.enabled = true;
            shape.scale = new Vector3(m_terrain.terrainData.size.x, m_terrain.terrainData.size.z, 1);
            m_globalParticles.gameObject.transform.localScale = m_terrain.gameObject.transform.localScale;
            m_globalParticles.gameObject.transform.position = new Vector3(m_terrain.gameObject.transform.position.x + (m_terrain.terrainData.size.x / 2), m_globalParticlesHeight + max_height, m_terrain.gameObject.transform.position.z + (m_terrain.terrainData.size.z / 2));
            
        }
        if (m_cloudMapUpdate > 0) InvokeRepeating("CreateRTDensity", 0.5f, m_cloudMapUpdate);
        else Invoke("CreateRTDensity", 0.5f); //if update == 0, the cloudmap will only be generated once on start
    }

    public enum CloudMapResolution
    {
        _64, _128, _256, _512
    }
    //Brads visibility Query
    public void CreateRTDensity()
    {
        var height = gameObject.transform.position.y;
        int r = (int)m_terrain.terrainData.size.x / m_cloudMapSize;
        for (int y = 0; y < m_cloudMapSize; y++)
        {
            for (int x = 0; x < m_cloudMapSize; x++)
            {
                Expanse.GameplayQueries.QueryInfo q;
                q.startWS = new Vector3(x * r, height, y * r);  // Visibility start point is the probe's position.

                q.endWS = new Vector3(x * r, height + 10000, y * r);                  // Visibility query end point is the target's position.
                q.density = 0;                              // Initialize to null result, to be safe.
                q.visibility = 1;                           // Initialize to null result, to be safe.

                // Dispatch it using the static GameplayQueries class.
                Expanse.GameplayQueries.Dispatch(q, OnQueryCompletion);

            }
        }
        Invoke("CreateTex", 0.2f);
    }

    void OnQueryCompletion(Expanse.GameplayQueries.QueryInfo q)
    {
        m_visibility.Add(q.visibility);
        m_density.Add(q.density);//not used but no further impact on performance
    }

    //Texture is created by sampling the terrain size and taking readings at intervals set by the reloution. 
    //The higher the resolution, the more samples taken so will have an impact on performance 
    void CreateTex()
    {
        print(m_visibility.Count);
        var c = 0;
        Texture2D tex = new Texture2D(m_cloudMapSize, m_cloudMapSize);
        for (int y = 0; y < tex.height; y++)
        {
            for (int x = 0; x < tex.width; x++)
            {
                if (m_visibility[x + c] > m_visibilityThreshold)
                    tex.SetPixel(x, y, Color.black);
                else
                    tex.SetPixel(x, y, Color.white);
            }
            c = y * m_cloudMapSize;
        }
        tex.Apply();
        //add cloudmap to Global particle system
        if (m_globalParticles != null)
        {
            var partShape = m_globalParticles.shape;
            partShape.texture = tex;
            partShape.textureClipChannel = ParticleSystemShapeTextureChannel.Red;
            partShape.textureClipThreshold = 0.9f;
            m_globalParticles.gameObject.SetActive(true);
        }
        m_visibility.Clear();
        m_density.Clear();

    }
}