Air flow coming out of a fan feels much stronger than air flow coming in. Why? [duplicate]

Possible Duplicate:
Why is exhaling more forceful than inhaling?

If I put my hand behind my computer's system block, I can feel a very strong flux of air coming out, as it is being pushed by the fans. However, if I put my hand in front of the box, where the air is coming in, I can barely feel the air flow. Why is that?

I understand that this is probably because the air is coming into the box from a greater solid angle than the air coming out, so the incoming air has a lower velocity. But what is the reason for such asymmetry? Can it be explained in simple terms, without solving the Navier-Stokes equations?

Just to clarify: it is definitely not due to the difference in the area through which the air can come in and out: my computer has holes all over the back side, so no nozzles there :) In addition, all regular floor fans has the same property: the air flow coming out feels stronger than the air flow coming in.