just as an interesting, yet related aside, the speed at which a tyre will commence aquaplaning is 7 times the square root of the tyre pressure. useful stuff eh? There are also different types of aquaplaning as well, one in which the water forms a "mini bow wave" in front of the approaching tyre, thus slowing it down, and degrading grip, and the other where water is compressed between the tyre surface and the friction surface of the runway, and degrading efficiency. I would imagine that this is where a grooved runway will assist in offering a reduction in the water film, leading to extra grip, and therefore extra retarding effect from the brakes.
The JAR definitions of Snow, and other contaminants is equally arcane!
Tailwinds