Plain English Meaning

The speed at which a tire stops touching the runway and starts skimming on top of standing water, like a water ski. Once you reach that speed on a wet runway, braking and steering become very poor.

Full Definition

The minimum groundspeed at which a tire, rolling on a wet runway, lifts off the surface and rides on a thin film of water rather than maintaining contact with the pavement. For dynamic hydroplaning, this speed is approximately nine times the square root of the tire pressure in psi (e.g., a tire inflated to 36 psi will hydroplane at about 54 knots).

Context Anchor

Seen in wet-runway landing and discussions, especially when why braking and steering may be poor on a runway with standing water.