You can use dedicated snow filters or particle systems to create snow that can be stylized or realistic, or something less successful. There are filters built into After Effects like CC Snow, CC Particle World, and Particle Playground -- or you can use 3rd-party filters like Trapcode Particular (which is AE 3D camera-aware), Boris FX Snow, Cycore FX HD Snowfall, DigiEffects Delirium Snow, and GenArts Monsters GT.
If you use an older snow filter like CC Snow, for more realism you might try multiple layers or filters, as well as lessons learned in tutorials for other filters. By the way CC Snow worked in Premiere CS5, so that should still be an option.
Not that templates or stock footage need be expensive. You could also (gasp) shoot your own footage – or even try Projector Snow, which earned AE guru Brian Maffitt mention in Wired and everywhere else too.
There are many, many tutorials for snow-related effects that can be used straight from the free projects or mimicked easily, even if you haven't seen snow in years. First, to help with a forum question, here's Michael Park on how to make snow stick (see also Maltaannon, pictured left, and Angie Taylor (in Happy Holidays After Effects Presets) on Particle Playground’s Persistent Property Mappers; Particle World should work too, with caveats):
Here are a few embedded video tutorials (bottom) and links to more:
* An After Effects Christmas from the Genesis Project uses the CC Snow effect.
* Church Media Design had its own take in Episode 121: Let it snow inside After Effects, which at 12:30:00 shows how to create looping snow inside AE with CC Particle World (video embedded below). They also have a few free looping backgrounds.
* How to Create Snow in After Effects CS 5.5 #1.05 by Mike Gentilini, Jr. uses Trapcode Particular (video embedded below).
* Looping a Particle System from Aharon Rabinowitz and Red Giant & Adobe TV uses Trapcode Particular and Starglow to create a stylized holiday snowstorm (video embedded below).
* After Effects and Particular: Free snowflakes background from Motionworks.
* Snow Globe in After Effects CS4 from J Schuh and Layers Magazine uses Particle Playground.
* Creating a Snow Globe in After Effects by Rob Mize.
* Making the Snow Globe by Michele Yamazaki uses snow stock footage.
* Frosty Breath and Glass Snow Orbs from Video Copilot.
* Freeze Your Logo With A Snowy Transition from Eddie Bogdanov uses Trapcode Particular (you can work around using Cinema 4D).
* Creating a 3d Winter Scene with Photoshop and Particular by Michael Park -- doesn't really have snow.
* Particular Basic Training Session FOUR: Sticky Snow by Michael Park -- really does have snow!
* Creating Realistic Snowfall in AE CS3 from DVPA uses the Foam filter built into AE.
* Realistic Snow Tutorial in After Effects by James Polits uses the built-in Particle World plug-in, and shares the project.
* After Effects: Snow!, by Robert's Productions, uses the built-in Particle World to quickly render snow for compositing into Premiere or Sony Vegas.
* Particle World Transitions: Part 1 by Rob Mize could generate ideas for sticky snow, or accumulating or melting snow.
* Quick Tip - How To Achieve Realistic Snow Fall by Chandan Kumar in AEtuts used Trapcode Particular.
* Red Giant Particular Tutorial - 27FX by James Crawley creates snow.
* How to Create Snow in Adobe After Effects (Trapcode Particular) by TutorialGrid
* Integrating weather into a composting can be tricky if you're ambitious. While not on snow itself, keep in mind Climate: Air, Water, Smoke, Clouds in After Effects, a sample chapter of After Effects VFX Studio Techniques by Mark Christiansen. See also Daniel Broadway on tips for compositing fog or mist into a scene.
* Harry Frank's Holidays for Particular is a $69 Guru Preset Pack from Red Giant People that features 175 seasonal elements like snow, stars, ice, light and ornaments. There are numerous other one-off templates and presets available over the web if you have the time to search and preview.
* Happy Holidays After Effects Presets by Angie Taylor includes animated Christmas lights, a 3D snowstorm with CC Particle World, and sticky snow with Particle Playground's Persistent Property Mappers. All done without extra 3rd party plug-ins.
* There are a number of free project templates and pre-rendered movies, including ones by Bluefx.net (+ snowstorm), Freeworshiploops, Engageworship, etc.
* Do-it-yourself methods are often easy and more realistic, as Chris Meyer notes in Let It Snow Eye candy: A quick shot at photographing falling snow. Similar thoughts come from Snow Particles by Andrew Kramer, who used his free practical effects flour particles. William J. Meyer showed something similar in 2008 in WiP: Let it Snow, Let it Snow, Let it Flour.
That may seem like quite enough, but for additional holiday resources and maybe additional tutorials, see The Ultimate Christmas Roundup! by Topher Welsh from 2009.
Finally, here's the tutorials noted above by Aharon Rabinowitz, Mike Gentilini, and Brad Zimmerman:
*** Let it snow in After Effects starts at 12:30:00 ***
Update: Eran Stern shared Jack of All Trades, Master of Expressions, a cartoon Shining view of Christmas where "Eran shows how to create a jumpy and squishy animation using the power of expressions."
Red Giant TV Episode 35, Creating a Christmas Tree with Trapcode Particular, by Michael Park, is another useful piece with several elements. And there's Adobe After Effects Tutorial - Trapcode Particular 2 Snow (Star Wars) by TheMrMenagerie2.
Frosty Window (with project files) by John Dickinson, at Artbeats, was inspired by the animated movie Frozen.
Disney refined digital snowmaking for that 2013 movie; see Disney's Frozen - A Material Point Method For Snow Simulation [PDF tech background]:
In Making snowflakes in After Effects tutorial, Mikey Borup uses an animated Shape and CC Particle World, with particle texture time set to birth, to make unique snowflakes fast.
After Effects Top Tip: Creating Atmospheric Effects to Add to Yout Visual Effects Shots from Digital-Tutors adds snow and blowing fog to a gothic scene:
Main image Copyright P Flannagan. This work is licensed under the Creative Commons Attribution-Share Alike 2.0 Generic Licence. Thumbnail image Copyright Jerzy Drozda Jr. (aka Maltaannon).