Altos includes a cloud shadow rendering system as well. You can enable it from the Cloud definition.
Cloud Shadows don't write to Unity's shadowmap since it doesn't support transparency, which is really important for good-looking cloud shadows.
But, you can integrate cloud shadows with transparent objects! This is easiest when using custom shaders or unlit shader graphs. Lit shader graphs don't work very well since you don't have a way to override the lighting.
Go to Altos/ShaderLibrary/CloudShadows.hlsl for function implementations
In your includes, include the CloudShadow file:
# include "Packages/com.occasoftware.altos/ShaderLibrary/CloudShadows.hlsl"
In your shader, get the cloud shadow attenuation:
float cloudShadowAttenuation = GetCloudShadowAttenuation(postionWS);
Use the cloud shadows to attenuate your lighting, e.g.,
float lightAttenuation = light.shadowAttenuation * cloudShadowAttenuation
Use the included GetCloudShadows subgraph