Here’s a quick Photoshop tip to add some fake snow to this photo of some cranes…

snow1

Create a new layer and fill it with black then set it’s blending mode to “Screen”. The original layer will then show through again as black is transparent when using the Screen blend mode.
Go to Filter > Noise > Add Noise and set the Amount to around 20% with Distribution set to Gaussian and Monochromatic checked. This adds lots of white speckles to your image but it doesn’t yet look like snow.

snow2

Next go to Filter > Pixelate > Crystallize and select a cell size of somewhere between 3 and 10 and click OK (the size you choose depends on how big you want your snowflakes and the relative size of your image).

snow3

Looking a bit more like a blizzard now but the snow layer is a bit soft so let’s sharpen it a little using Unsharp Mask (Filter > Sharpen > Unsharp Mask). The settings for this again depend on your image. In this case I’ve used Amount: 100%, Radius: 2px and Threshold: 0. Here’s the finished snowy scene.

snow4

It probably won’t pass for real snow but it still might be a useful effect if you need to conjur up a wintery feel. You can use a similar process to add rain to a photo. Try adding some vertical or slightly off-vertical motion blur to give the look of rain falling.