Regular haircut
Human scalp hair grows on average about one eighth inch per week or one half inch per month. Most clipper guards are numbered in eighths of an inch. The number of the guard denotes the number of weeks of hair growth left on the scalp when a clipper with a certain numbered guard is guided over the head with the guard in contact with the scalp. A #1 guard leaves 1/8 inch (3 mm), one week's growth of hair, on the scalp; a #2 guard leaves 2/8" (6.3 mm), two weeks' hair growth, on the scalp; a #3 guard leaves 3/8" (9 mm), three weeks' hair growth on the scalp; and so on.
Hair
Jablonski asserts head hair was evolutionarily advantageous for pre-humans to retain because it protected the scalp as they walked upright in the intense African (equatorial) UV light. While some might argue that, by this logic, humans should also express hairy shoulders because these body parts would putatively be exposed to similar conditions, the protection of the head, the seat of the brain that enabled humanity to become one of the most successful species on the planet (and which also is very vulnerable at birth) was arguably a more urgent issue (axillary hair in the underarms and groin were also retained as signs of sexual maturity). Sometime during the gradual process by which "Homo erectus" began a transition from furry skin to the naked skin expressed by Homo sapiens, hair texture putatively gradually changed from straight hair (the condition of most mammals, including humanity's closest cousins—chimpanzees) to Afro-textured hair or 'kinky' (i.e. tightly coiled). This argument assumes that curly hair better impedes the passage of UV light into the body relative to straight hair (thus curly or coiled hair would be particularly advantageous for light-skinned hominids living at the equator).