Understanding the "for" and "i" syntax

hey guys, new here

one thing I deffinately don’t know how to do is actionscript but I would like to learn, It seems extremely complicated. I have flash 8, but unfortunately only a flash MX bible (help book thats really big)

well my first question was answered so instead of making a new thread Im editing this one.
anyways heres my problem, I have a function that is used in a movie, it makes snow fall, But I do not want that snow to fall after about 100 frames later on from the point it starts snowing, soo basically how do I do this

also cant use buttons cause this is a movie

if you need a reference to what ive got then heres the code im using

init = function () { 
width = 550; 
// pixels 
height = 400; 
// pixels 
max_snowsize = 10; 
// pixels 
snowflakes = 50; 
// quantity 
for (i=0; i<snowflakes; i++) { 
t = attachMovie("snow", "snow"+i, i); 
t._alpha = 20+Math.random()*60; 
t._x = -(width/2)+Math.random()*(1.5*width); 
t._y = -(height/2)+Math.random()*(1.5*height); 
t._xscale = t._yscale=50+Math.random()*(max_snowsize*10); 
t.k = 2+Math.random()*2; 
t.wind = -1.5+Math.random()*(1.4*3); 
t.onEnterFrame = mover; 
} 
}; 
mover = function() { 
this._y += this.k; 
this._x += this.wind; 
if (this._y>height+10) { 
this._y = -20; 
} 
if (this._x>width+20) { 
this._x = -(width/2)+Math.random()*(1.5*width); 
this._y = -20; 
} else if (this._x<-20) { 
this._x = -(width/2)+Math.random()*(1.5*width); 
this._y = -20; 
} 
} 
init();

this is what makes the snow fall, how do I make it stop 100 frames later for example