I recently had a haircut, and have noticed that water in my air dries up faster when I short hair. I assume that hair loses water due to evaporation. We have been taught that evaporation is faster when the surface area is increase (which is the reason why behind drinking tea off a saucer)
But, when we have longer hair, the surface area of the hair is supposed to be more. So, why does hair dry faster when it is shorter and has lower surface area? Just genuinely curious to know this.
Actually, evaporation increases with increase in surface area, i.e actually your hair should dry faster when long. But, we have not considered that longer the hair more water gets accumulated and the hair traps the water from evaporating. Hence, conceptually water from shorter hair should take more time to evaporate. But then why does it take less time? It is because water does not accumulate or get trapped in short hair and the head is lot more open for proper evaporation. Also when the surface area of hair increases i.e longer hair, the surface area of the head becomes relatively small but when the surface area of hair decreases i.e small hair, the surface area of head becomes relatively large and hence contributes in faster evaporation.
You are confusing the total rate of evaporation with the time it takes for the water to completely evaporate. The rate of evaporation increases with the surface, true. But this don't tell which system will have all the water evaporated first. The ocean has much large surface area than the puddle left by the last rain. But the ocean is still there after the puddle is long gone. Isn't it? Besides, there are other factors that affect the rate of evaporation like the circulation of air which removes the vapors from the surface allowing more evaporation. If you take a wet cloth and roll it it will dry much slower than if it is all exposed to the air. So, short hair has less water to evaporate, a better circulation of air and less overlaping of threads of hair. You should consider that the surface expsoed to air in case of long, wet hair is not the sum of all surfaces of individual hairs due to clumping toghether in thicker threads.
