In practise, there are 4 factors that effect Carb-icing:
1) Air Temperature
2) Dew Point
3) Carbs
4) How much your crank the throttle!
Points 1 and 2 combine to indicate the humidity; and matters such as air-pressure only become relevant inside the Carb, and really we're talking about the
change in air pressure that causes the air's temperature to drop.
Why do I get so attached to this subject... I always seem to end up writing a few paragraphs about it to no avail!
Oh well... my understanding of carb-icing is from an aviation point of view. However I am led to believe that due to the construction of the SV's carbs, it can affect the system in ways other than the classical induction icing usually found on carb funnels.