Tempered glass, is made by heating annealed glass uniformly. The annealed glass is then cooled rapidly by blowing air uniformly onto both surfaces at the same time. This is known as air quenching. Rapid cooling increases the compression forces on the surface and the tension forces inside the glass. Tempered glass is about four times stronger than a lite of annealed glass of the same size and thickness. The only characteristic of the annealed glass affected by tempering is its bending or tensile strength. Tempering increases the tensile strength of glass. This makes tempered glass better able to resist the forces caused by heat, wind and impact.