a derivative work from climagick's "Let It Snow In Your Terminal" (http://climagic.org/coolstuff/let-it-snow.html), see also https://gist.github.com/4344001

xmas.rb

require 'set'

rows,columns = `stty size`.scan(/\d+/).map{|x| x.to_i}

# ruby 1.8.x compatible way to say "\u2743"
Flake = ["2743".to_i(16)].pack("U*")
# Shapes from far away to still closer
Snow = ['.','*',Flake]

# flake state descriptor
FlakeState = Struct.new(:row, :speed, :shape)

# this will map column to a flake [row, speed]
snow_progress = {}

# available columns (initially all columns are available)
avail = Set.new 1..columns

# clear terminal
puts "\033[2J"

# terminal positioning
def char_at(char,row,col) ; "\033[#{row};#{col}H#{char}" ; end

# infinite blizzard loop
loop do
  # pick a random available column
  column = avail.to_a[ rand( avail.size ) ]
  # remove it from available columns
  avail.delete column
  # give a snowflake some random speed
  speed = rand( Snow.size )+1
  # chose a shape based on speed
  shape = Snow[speed-1]
  # initialize this particular snowflake
  snow_progress[ column ] = FlakeState.new 0,speed,shape
  # loop over all active snowflakes (clone the hash so we can edit it as we go)
  snow_progress.clone.each do |column, flake|
    # clear previous snowflake image
    print char_at(' ',flake.row,column) if flake.row > 0
    # update snowflake model
    flake.row += flake.speed
    # if still falling
    if flake.row < rows
      # draw its shape
      print char_at(flake.shape,flake.row,column)
    else
      # on the ground always draw the Flake
      print char_at(Flake,rows,column)
      # make its column available again
      avail << column
      # and remove the snowflake from simulation
      snow_progress.delete column
    end
    # move the terminal cursor away
    print char_at('',0,0)
  end
  # ruby 1.8.x buffers stdout by default, flush the current animation frame
  $stdout.flush
  # ~17 FPS
  sleep 0.06
end