Open the photograph which you wish to add snow effect in Photoshop. In this lesson I am going to use a photograph as below, I took last winter.
Create a new layer and rename it "Snow" by double click on its default layer name.
Fill the new layer with white by going "Edit - Fill" and fill the new layer with white color.
Add noise effect. Go to "Filter - Noise - Add Noise", set noise amount 100150%.
Transform the new layer by Click "Ctrl + T" to seriously change the new layer to both 150% of width ( W ) and height ( H ), then double click the image to apply the change.
Change the new layer's mode to "Screen" in the "LAYER" panel.
Select "Image - Adjustment - Threshold", set Threshold amount to 200250, it rely on how heavy the snow you would like.
Select "Filter - Blur - Gaussian Blur", set "Radius" amount 0.52 pixels.
Select "Filter - Blur - Motion Blur", set the "Angle" amount -7575 and the "Distance" amount 110 pixels. The same, it is dependent on how heavy the snow you need.
Then, right click the new layer and select "Flatten Image" to combine all layers. Now you are able to save the photograph.
The last effect is as below; it's completely straightforward to make snow effect to a photograph.