http://thecodeplayer.com/walkthrough/html5-canvas-snow-effect

html

<canvas id="canvas"></canvas>
css

/*Some basic styles*/
* { margin: 0; padding: 0;}
#canvas {
display: block;
/*Fill canvas with black by default*/
background: #000;
}
js

window.onload = function(){
	var canvas = document.getElementById("canvas");
	var ctx = canvas.getContext("2d");
	
	//Lets make the canvas occupy the full page
	var W = window.innerWidth, H = window.innerHeight;
	canvas.width = W;
	canvas.height = H;
	
	//Lets make some particles
	var particles = [];
	for(var i = 0; i < 25; i++)
	{
		particles.push(new particle());
	}
	
	function particle()
	{
		//location on the canvas
		this.location = {x: Math.random()*W, y: Math.random()*H};
		//radius - lets make this 0
		this.radius = 0;
		//speed
		this.speed = 3;
		//steering angle in degrees range = 0 to 360
		this.angle = Math.random()*360;
		//colors
		var r = Math.round(Math.random()*255);
		var g = Math.round(Math.random()*255);
		var b = Math.round(Math.random()*255);
		var a = Math.random();
		this.rgba = "rgba("+r+", "+g+", "+b+", "+a+")";
	}
	
	//Lets draw the particles
	function draw()
	{
		//re-paint the BG
		//Lets fill the canvas black
		//reduce opacity of bg fill.
		//blending time
		ctx.globalCompositeOperation = "source-over";
		ctx.fillStyle = "rgba(0, 0, 0, 0.02)";
		ctx.fillRect(0, 0, W, H);
		ctx.globalCompositeOperation = "lighter";
		
		for(var i = 0; i < particles.length; i++)
		{
			var p = particles[i];
			ctx.fillStyle = "white";
			ctx.fillRect(p.location.x, p.location.y, p.radius, p.radius);
			
			//Lets move the particles
			//So we basically created a set of particles moving in random direction
			//at the same speed
			//Time to add ribbon effect
			for(var n = 0; n < particles.length; n++)
			{
				var p2 = particles[n];
				//calculating distance of particle with all other particles
				var yd = p2.location.y - p.location.y;
				var xd = p2.location.x - p.location.x;
				var distance = Math.sqrt(xd*xd + yd*yd);
				//draw a line between both particles if they are in 200px range
				if(distance < 200)
				{
					ctx.beginPath();
					ctx.lineWidth = 1;
					ctx.moveTo(p.location.x, p.location.y);
					ctx.lineTo(p2.location.x, p2.location.y);
					ctx.strokeStyle = p.rgba;
					ctx.stroke();
					//The ribbons appear now.
				}
			}
			
			//We are using simple vectors here
			//New x = old x + speed * cos(angle)
			p.location.x = p.location.x + p.speed*Math.cos(p.angle*Math.PI/180);
			//New y = old y + speed * sin(angle)
			p.location.y = p.location.y + p.speed*Math.sin(p.angle*Math.PI/180);
			//You can read about vectors here:
			//http://physics.about.com/od/mathematics/a/VectorMath.htm
			
			if(p.location.x < 0) p.location.x = W;
			if(p.location.x > W) p.location.x = 0;
			if(p.location.y < 0) p.location.y = H;
			if(p.location.y > H) p.location.y = 0;
		}
	}
	
	setInterval(draw, 30);
}