A CodePen by Tristan. Lightrays v2 - Added some colour. Update: Added colour shift for sun-rise/set.

script.js

var canvas, c, w, h,
    twoPI = Math.PI * 2,
    mX, mY,
    resize = true,
    mousemove = true,
    per = { x: 0, y: 0 },
    mtn, trackmouse = false;

window.onload = function(){
  canvas = document.createElement('canvas')
    w = canvas.width = window.innerWidth - 40;
  h = canvas.height = window.innerHeight - 40;
  c = canvas.getContext('2d');
  document.body.appendChild(canvas);
  
  !resize || window.addEventListener('resize', function(e){ w = canvas.width = window.innerWidth - 40; h = canvas.height = window.innerHeight - 40; });
  !mousemove || canvas.addEventListener('mousemove', function(e){
    trackmouse = true;
    mX = e.pageX-20; mY = e.pageY-20;
    per = { x: mX, y: mY };
  });
  
  mX = w/2;
  mY = w/2;
  
  per = { x: w/2, y: h/2, step: 1 }
    mtn = new Mountains(300,"10");
  window.setInterval(animate,17);
}
  
  function newGradient(gradient){
    var grad;
    switch(gradient.type){
      case "radial":
        grad = c.createRadialGradient(gradient.x1, gradient.y1, gradient.r1, gradient.x1, gradient.y1, gradient.r2);
        break;
      case "linear":
        grad = c.createLinearGradient(gradient.x1, gradient.y1, gradient.x2, gradient.y2);
        break;
    }
    
    for(var i = 0; i < gradient.stops.length; i++){
      grad.addColorStop(gradient.stops[i].s, gradient.stops[i].c);
    }
    
    return grad;
  }

function animate(){
  if(!trackmouse){
    per.x = mX = w/2 + Math.round(Math.cos(per.step)*w/2);
    per.y = mY = h/2 + Math.round(Math.sin(per.step)*h/2);
    per.step += 0.03;
    if(per.step > twoPI)
      per.step = 0;
  }
  
  c.save();
  c.globalCompositeOperation = "source-over";
  c.fillStyle = newGradient(
    {
      type: "radial",
      x1: mX,
      y1: mY,
      r1: 0,
      r2: w,
      stops: [
        { s: 0, c: "rgba("+ (h-mY) +","+ (h-mY) +","+ (h-mY) +",0.5)" },
        { s: 0.05, c: "rgba("+ (h-mY) +","+ (h-mY-128) +",128,0.5)" },
        { s: 1, c: "rgba(0,"+ (h-mY-128) +","+ (h-mY) +",0.5)" }
        
      ]
    }
  );
  c.fillRect(0,0,w,h);
  mtn.draw();
}

function Mountains(peaks,seed){
  var points = [];
  this.init = function(){
    var step = w / peaks,
        y = 0;
    
    points.push({x: 0, y: y});
    for(var i = 1; i <= peaks; i++){
      y = y + (Math.random() * 20)-10;
      points.push({x: i * step, y: y});
    }
  };
  this.draw = function(){
    c.save();
    //c.fillStyle = "rgba(20,20,20,1)";
    c.fillStyle = newGradient({type:"linear", x1: 0, y1: 0, x2: 0, y2: h, stops: [{s:1, c:"rgba(0,0,0,1)"},{s:0, c:"rgba(80,80,80,1)"}]});
    c.beginPath();
    c.moveTo(points[0].x, h/2-points[0].y);
    for(var p = 1; p < points.length; p++){
      c.lineTo(points[p].x, h/2 - points[p].y);
    }
    c.lineTo(w,h);
    c.lineTo(0,h);
    c.closePath();
    c.fill();
    c.restore();
    
    c.globalCompositeOperation = "lighter";  						
    c.fillStyle = "rgba("+(h-mY)+","+(h-mY)+","+(h-mY)+",0.03)";
    for(var p = 0; p < points.length-1; p++){
      var va1 = Math.atan2(h/2-points[p].y - per.y, points[p].x - per.x),
          va2 = Math.atan2(h/2-points[p+1].y - per.y, points[p+1].x - per.x);
      
      c.beginPath();
      c.moveTo(points[p  ].x, h/2-points[p].y);
      c.lineTo(points[p+1].x, h/2-points[p+1].y);
      c.lineTo(points[p+1].x + Math.cos(va2)*w, h/2-points[p+1].y + Math.sin(va2)*w);
      c.lineTo(points[p  ].x + Math.cos(va1)*w, h/2-points[p].y + Math.sin(va1)*w);
      c.closePath();
      c.fill();
    }
  };
  
  this.init();
  
}

style.css

*{margin:0;padding:0;-moz-box-sizing:border-box;-webkit-box-sizing:border-box}
body{font-size:75%;background:#222;font-family:arial;padding:20px 0 0 20px}
canvas{border:1px solid #181818;display:block;box-shadow:0 0 10px #111;background:rgba(20,20,20,1)}