It's a rainy day

Bash: 111 bytes - 20 = 91 points!

A contemplative gentle drizzle in your terminal. Adjust the numbers 819 and 41 for different height and width respectively.

e='printf \e';while :;do for i in {0..819};do((RANDOM<9))&&$e[1\;36m.||$e[1C;((i%41<1))&&$e'
';done;$e[20A;done

A pleasant bonus is the way the cursor pitter-patters across the rain area.

Edit: shortening from 140 bytes to 129 bytes thanks to @manatwork's suggestions. 2nd edit: shortening from 129 bytes to 111 bytes thanks to @manatwork's and @Tobia's suggestions, plus additional inspiration - see comments.

(Note: screenshot shows previous less-golfed version of the code, which is functionally identical)

Python, 312 bytes - 50 (wind) = 262

from pygame import*
R=__import__('random').randint
t,u=640,480;init();d=display;s=d.set_mode((t,u))
w=[255]*3;r=range(t)
a=[[R(0,t),R(0,u),R(3,6)]for i in r]
while time.wait(9):
 d.flip();event.get();s.fill(R(0,99)<1and w)
 for i in r:x,y,z=a[i];draw.line(s,w,(x,y),(x+z,y+2*z));a[i][0]=(x+z)%t;a[i][1]=(y+z*2)%u

Sample output (a 50-frame loop):

Actual playpack is significantly faster than gifs allow.

HTML / JS, 170 chars - 20 = 150 points

<canvas id=c></canvas><script>d=400;with(c)width=height=d,t=getContext('2d');t.fillStyle='blue';setInterval("t.fillRect(Math.random()*d,Math.random()*d,5,5)",50)</script>

(sidenote: golfed further by passing a string to setInterval, with, automatic ID variable names... it feels so wrong! shudders)

It just draws random blue rectangles.

HTML / JS, 309 chars - 20 - 50 = 239 points

Now with wind!

<canvas id=c></canvas><script>s=400;r=Math.random;with(c)width=height=s,t=getContext('2d');t.fillStyle='blue';o=[];setInterval("t.clearRect(0,0,s,s);for(i=0;++i<o.length;)d=o[i],t.fillRect(d[0],d[1],d[2],d[2]),d[0]+=1,d[1]+=2,d[2]-=1,d[2]<0?o.splice(i,1):0;if(r()<.6)o.push([r()*400,r()*400,20])",50)</script>