First, you need to pick a photo that will look natural once you add the snow to the shot. This tutorial works best with a shot that has a small covering of snow on the ground.
Step 1: Open your image
Open your image up in Photoshop then create a new layer (Layer>New Layer) and rename it ‘snow’.
Step 2: Rectangular Marquee Tool
Using the Rectangular Marquee tool, draw a box about 1/4 to 1/2 the size of the main image over the image. You should have now have a dotted line making a rectangular box in the middle of your photo.
Go to Edit>Fill and fill the shape with black. This is the background we’ll be creating our snow on.
Step 3: Add Noise
To create the snow go to Filter>Noise>Add Noise and set the Distribution to Gaussian and tick Monochromatic. The only other thing to control is the size of snow flakes and that's done by dragging the Amount slider left or right. A percentage between 40 and 100 will give anything from a light flurry of snow to a heavy downfall.
Step 4: Adjust the amount of Noise
Next go to Image>Adjustments>Threshold to increase the amount of white dots appear in the image. Threshold turns everything black or white and the slider controls how much is black and how much is white. Drag it to the left to increase the level of white and to the right to reduce it. On our image we dragged it to the right slightly to reduce the amount of snow.
Step 5: Scale the snow
Now go to Edit>Transform>Scale and drag the corners of the box so that it fills the whole image area. You should now have a black canvas with white dots of noise all over. This "digital snow" is now appearing as larger flakes and spread more over the picture so it looks less like noise and more like snow falling from the sky.
Step 6: Remove the black background
Set the snow Layer blend mode to Screen so that the black disappears and the image below can be seen again. Now it's starting to look like a winter blizzard but to make it look more natural a little movement is needed.
Step 7: Add movement to the snow
With the snow layer still active go to Filter>Blur>Motion Blur and from the dialogue box that appears set the angle in the direction you want the snow to fall. Then adjust the Distance slider so it creates the motion blur. Try not to overdo this as it can turn your snow into what looks like streaks of rain. If you make sure the Preview box is ticked you can the changes as you adjust the slider.
Step 8: Adjust the snow
If you think there’s a little too much snow falling from the sky take the Paint Brush Tool, select a large sized brush, reduce the opacity to around 50% and dab the brush over some parts of the image. This will vary the snow pattern and make it look more natural.
And that's all there is to it. As with all layered images, you can save the layers if you save the file as a Tiff or PSD file and come back and edit them again at a later stage. If you are sure you won't make any more changes flatten the layers to reduce the file size.
Image before and after: