Subject: Why does frost formation stop?

I understand that frost formation is due to:

1. Low partial pressure of vapor in air
2. Cold surface temperature < 0 deg C
3. This brings the vapor to below the triple point of water (4.5 mmHgA at 0 deg 
C) which causes deposition.

But what stops the frost layer (say on a cold windshield) from continually 
building up the level?  It may be because the surface ice temperature is not 
cold enough, or perhaps the weather does not stay at the right conditions long 
enough.

A secondary question: why does frost formation take place in low velocity 
(still air)?  

thanks

Re: Why does frost formation stop?