Summer Sun and Your Hair

I'm sort of recycling part of a previous post. Recycling is good though, right? ©Science-y Hair Blog 2013
Full sunlight on your hair can be bad news if you get too much.

• When hair is exposed to 200 hours of ultraviolet radiation from sunlight, the cuticle edges begin to fuse and as a result the total cuticle surface “shrinks.” As it shrinks, tiny openings are created where cuticle once was and these are known as "porosities." It's like putting a piece of plastic on a hot stove or in the oven - it shrinks as it heats and as it is assaulted by ultraviolet light.
• After 400 hours of ultraviolet light / sun exposure, porosity continues to increase with further cuticle damage.
• After 1200 hours, the cuticle becomes rigid, brittle, and may crack under stress, leading to even further increases in porosity. ©Science-y Hair Blog 2013

If your hair grows the "average" of up to 6 inches per year (15 cm) and you get sunlight for even half an hour per day, during half of the days of the year, the ends of your hair most certainly have sun damage unless you wear it very short. 

When hair is more porous, it loses moisture and becomes dehydrated more easily. It is more susceptible to damage. To protect your hair from the sun, wear a hat or use products with "UV protectants." I feel that the "UV protectant" is only half the story. When my hair is baking in the hot sun, it is being dehydrated by the high heat. All those lovely proteins and oils in the top layers of my hair are doing unknown chemical reactions with the heat and UV light. A different effect, but it's one that makes my fine hair feel drier and more fragile in summer than it does in winter when the air is cold and dry.
©Science-y Hair Blog 2013
A healthy (undamaged) or continuous cuticle holds water in, preventing dehydration. A sun-damaged cuticle has a harder time doing this. To manage sun-damaged hair, use protein-enriched or humectant-enriched conditioners or protein treatments to temporarily patch up the gaps in the cuticle. Leave-in conditioners or small amounts of oil used on damp hair to seal in moisture, slow dehydration of the hair and keep it supple. A porous cuticle patched and lightly coated with conditioners, proteins or oils also holds moisture within the hair longer. 
©Science-y Hair Blog 2013
Don't forget - if you're going out without a hat, a little sunscreen on your scalp near the part in your hair will save you from a part-burn that hurts for days and can lead to skin cancer down the road. 

Source: Chemical and Physical Behavior of Human Hair 

Robbins, 1994. 3^rd Ed. Springer-Verlag, New York