A commented version of https://twitter.com/zozuar/status/1443012484189888515 by https://twitter.com/User2E32

mountains-and-clouds.glsl

// r = probably screen resolution, o = output color, g = ray depth into the scene
// FC.xy = which pixel we are on
float i,e,f,s,g,k=.01;
for(o++;i++<100;){// go upto 100 steps with the ray
    s=2.;
    vec3 p = vec3((FC.xy-r/s)/r.y*g, g-s); // defining ray direction
    p.yz *= rotate2D(-.8); // rotate the camera/ray downwards
    p.z += t;// fly forward over time
    e=f=p.y
    // compute the terrain height and fog density at this xz coordinate
    for(;s<200;s*=1.0/0.6){
        // the position also serves as a source for the seed: higher octaves just use a more-quickly changing noise function
        p.xz *= rotate2D(s);
        // generate two random numbers: fog density, and terrain height
        e+=abs(dot(sin(p*s)/s,p-p+.4));// fog density, because it contains y
        f+=abs(dot(sin(p.xz*s*.6)/s, vec2(1.0)));// terrain height
    }
    o += (f > k*k ? e : -exp(-f*f))*o*k;// when the terrain is hit, add terrain color, otherwise add fog color
    g += min(f,max(.03,e))*.3 // go deeper, but at max go to the terrain
}