Many airports must regularly treat the runway to remove excess rubber that over time makes the landing zones slippery. When an aircraft lands, the tires do not spin. The time it takes for the tires to get up to speed is referred to as “spin up time.” During this time, the tires drag on the runway and are put under pressure by the weight of the aircraft. This can be seen in the faint smoke that comes from the tires when an aircraft lands and touches the runway surface. The friction that builds up causes the rubber to polymerize and harden into the runway surface.