Learn how to simulate snow in your photograph in this video tutorial by photoshopuniverse. For this you'll need a photo depicting winter scenery. Add a new layer, fill it in with black, go to filer noise and add noise as shown using the given amounts. Go to filter effects and add blur to the layer. Go to images and adjust the levels to reduce the snow-flakes as shown. And add some motion blur to it. Change the blend mode to screen, duplicate the layer, rotate the new layer, apply the filter crystallize and add a motion blur as instructed. Merge the new layers, duplicate the layer again and change the opacity of the newest layer to suit your needs.