The primary contributor of skin color is the pigment known as melanin. The quantity of melanin in the skin varies based on genetic factors and is also dependent on the degree of sun exposure. Melanin is produced by specialized pigment cells known as melanocytes.
Average new tires used on cars typically start with 10/32 to 11/32 of original tread depth. Tires are considered worn out. In most States when they reach 2/32 of remaining tread depth. Ans 2 - Here in Canada (and I'm sure in many parts of the US ) we get a lot of rain.
Wet-weather braking and snow traction also decrease as tires wear. Tires are considered bald when one or more of their grooves reaches 2/32 of an inch deep, compared with about 10/32 of an inch for new tires (tread wear is usually measured in 1/32-inch increments).