How to put snowfall in a circle

[Inger Hindhed30546274tqd3]

Hi everyone,
I'm pretty new to AE and I'm still having trouble to figure out the logic of AE - though it's getting better 🙂 
Right now I need to put the snowfall effect into a circle. How do I do that?

(So far I tried to make an Adjustment layer with a mask with Shape ellipse - but the snowfall shows in a rectangle!). If someone could just point me in the right direction to make this, I would be grateful 🙂

[Andrew Yoole · Accepted solution]

Create a new layer above the snowfall layer.  It can be a shape layer with an ellipse, or a solid layer with an ellipse-shaped mask applied.

Then set the Track Matte setting on your snowfall layer to use the new ellipse layer as a Track Matte.

Alternately, you could precompose the snowfall layer, selecting "Move all attributes into the new comp".  Then simply apply an ellipse-shaped mask to the precomp layer in your main comp.

[Inger Hindhed30546274tqd3]

Yes, of course! Why didn't I think of that? 🙂 I hope at some point I will get so familiar with AE, that I'll be able to solve this kind of problem-solving myself 🙂
Thank you so much, Andrew! 🙂