Creating animated light snowfall

Start by opening an image of your choice.

Create a new layer. Select brush tool. Press F5 to open brush settings. Apply brush settings as shown.

Apply it on the document in random areas to create perfect snowfall.

Press Ctrl+T and move this layer downwards.

Apply the same brush on the the upper areas again.

Open animation window. For the first frame move the snow layer so that it's bottom align with the file's bottom.

Duplicate frame. For this frame move the snow layer so that it's top align with the file's top.

Click tween button from animation window. Apply settings as shown.

Press Ctrl+Alt+Shift+S to save an optimized animated GIF.