What are the standards for sorting/descending arrow directions? Grid is opposite Thunderbird. Everyone seems to agree on what ascending/descending is:

Ascending: The next item is higher value than the last
Descending: The next item is lower value than the last

However, the arrows don't always point the same direction. When sorting ascending
In Thunderbird, the arrow points down
In Evolution, the arrow points down
In Grid the arrow points up
In ExtJS's grid the arrow points up
In Google Documents the arrow points up

So far all web applications are opposite desktop applications (or at least open source email apps). Does anyone know the definite standard?