Use up or down arrow to represent "sort ascending" at table header

To represent "sort ascending" (a -> b -> c -> ...) , my first sense told me that, I should use ↓ to represent it. This is because if I lay my data

a
b
c

It seems like thing is moving naturally toward downwards.

However, for the table header under Windows, the thing I saw is, they are mostly using up arrow.

Hence, I was wondering, which symbol is more suitable? up arrow or down arrow?

From http://www.iconfinder.com/search/?q=ascending, I can see they mostly use up arrow.

But, from end user point of view, if I saw a up arrow at table header, I would thought the largest value (says z) nearest to the top table, and smallest value (says a) at the bottom of table.