To superimpose falling snow on a scene in Photoshop Elements is fairly straight forward. However, I don’t think the effect works equally well on all images. Ones that include highlight & shadow areas appear to be the best but a little trial & error is probably recommended.
First, we need to resize the image (this is important for the snowflake size later). Image>Resize>Image Size & set the resolution to 72ppi. Make sure the “Constrain Proportions” & “Resample Image” boxes are ticked.
Create a new layer (Layer>New>Layer or the “New layer” button) then Edit>Fill Layer & set the “Contents” to black with 100% opacity.
Filter>Noise>Add Noise tick “Gaussian” at 100% & make sure “Monochromatic” is ticked also.
Filter>Blur>Gaussian Blur & set the radius to 1.0 pixels.
Enhance>Adjust Lighting>Levels set the slider input levels to 140 low & 142 high.
Change the blend mode to “Screen” & adjust the opacity to make the effect look convincing.
Now copy that layer (Ctrl + J) & put the blend mode back to “Normal”. Image>Transform>Free Transform, grab the bottom circle & rotate the whole image 180 degrees. Filter>Blur>Gaussian Blur & set the radius to 0.5 pixels. Change the blending mode back to “Screen” & reduce the opacity. Make sure the opacity of this layer is lower than the previous one. This will add some depth to the snow effect. You could repeat this previous step again (reducing the pixel radius further & lowering the opacity) as many times as you wish. It all depends upon the look you’re trying to replicate (blizzard white out or casual snow fall).
Depending on the original image you may need to alter the opacity of the two snow layers accordingly to make the effect realistic. The end result may also benefit from adjusting the “Hue/Saturation” of the original image after you’ve resized it to give it more of a winter feel (eg. lowering the hue & saturation, slightly increasing the lightness etc.) but this will vary from one project to the next.