Well there are quite a few ways of making snow and here is just one of them:
1. Create a new image and make it 2000 pixels by 1500 pixels, resolution 300, RGB, White.
2. Now change your background colour to a mid blue. The one I used is 25389f.
3. Create a new layer above your background, fill it with white and call the layer “Snow”. Filter>Noise>Add Noise 400, Gaussian, Monochromatic.
4. Then Filter>Artistic>Cut out and set it to 4,4,2 (You probably have never used this one before).
5. Now do Enhance>Adjust lighting>Levels and pull all the sliders together in the middle like this.
6. Next do Filter>Adjustments>Invert and it will start to look a little like snow.
7. Now get the zoom tool and zoom in very close, get the magic wand, make sure contiguous it NOT ticked and click on a black spot.
8. Then do Select>Feather 1 pixel, Edit>Delete then deselect. It should now be looking more like snow.
9. Next it needs a little blur so do Filter>Blur>Gaussian Blur use about 7.
10. Duplicate this layer, call it “Plain snow” then switch off its eyeball.
11. Go back to the “Snow” layer. Filter>Blur>Motion Blur and use an angle of about -50 and a distance of 28. That should give you a snow flurry effect like this
You can now drag this “Snow” layer or the “Plain snow” layer over your photographs to give you instant snow.
You can vary the effect by using the eraser with a soft brush, at a reduced opacity, to give a better effect (or use a mask to do a similar thing). Try using the move tool to stretch it out. Why not blur it some more or reduce the opacity.
Just experiment … and enjoy :)