Hydrophobic and anti-rain surface treatments are increasingly used to treat various glass articles such as windscreens, windows, headlamps, wing mirrors, optical lenses, sunglasses, etc. To evaluate the efficiency and durability of these treatments, we determine the smallest volume or critical sliding volume, V-c, of a water drop able to slide down spontaneously under gravity after having been deposited on a vertical treated, glass surface. The property of water repellency is considered to be better when V-c is smaller. In this paper, a new simplified theory to describe the capillary force retaining the water drops on an inclined plane is proposed and verified practically. The experimental method allows us to compare the efficiency and durability of a commercial anti-rain and of a Corning Inc. proprietary hydrophobic surface treatment for glass, both being based on silicone derivative chemistry. As defined in this paper, the critical sliding volume appears to be a practical parameter which may be used to characterize quantitatively the hydrophobicity of a solid surface.