Humidity is water vapor suspended in the air.
How humidity is measured:
Dewpoint – the temperature at which the air is saturated with as much water as it can hold, and so dew (or fog) forms. Dewpoint tells us the most about how dry the air is. Unless a new air mass moves in, the dewpoint can stay the same all day. This is what you want to watch to know just how moist or dry the air is.
Dewpoints below 50° F (10° C) are “dry.”
In the 50s (10-15° C) is “comfortable”
60-65° (15- 18° C) is “muggy”
65-70° (18-21° C) is “humid”
70° (21° C) and greater is “oppressive” – rainforest-y - you feel you need to grow gills
This is important for hair because dewpoint tells you how much moisture is in the air. Except for hair which has a very strong curl pattern, low moisture in the air tends to lead to flatter, less defined wavy or curly hair.
When researchers measure the effects of air moisture on hair, they use relative humidity. Partly because that’s what can be manipulated in the laboratory. Relative humidity changes throughout the day as the temperature changes because it’s a measure of how much moisture there is in the air relative to how much moisture the air could possibly hold. The warmer the air, the more moisture it can hold because air “expands” as it heats and that means you can cram in more water molecules.
Let's say you have a box in a laboratory at 74°F (this experiment was real). Add hair which has previously had all the water removed. Set the relative humidity at 29% and the hair will then take on moisture from the air so that contains 6% moisture. This is very dry – your mouth and nose and lips would feel parched. So here's how it stacks up:
At 29% relative humidity, hair holds 6% water
At 29% relative humidity, hair holds 6% water
At 40% relative humidity, the hair holds closer to 8% water
At 50%, the hair holds about 10% water – 40-50% is the “comfort range”
At 65%, hair holds around 13% water – at 65% relative humidity, the air starts to feel “wet”
At 70%, hair holds close to 14% moisture©Science-y Hair Blog 2013
Note the temperature never changed during the experiment. In real life the temperature fluctuates throughout the day and the dewpoint tends to stay the same. But in winter in the high latitudes, away from the humid coastal areas, it’s pretty safe to say that you won’t encounter relative humidity above 50%! If you have a thermometer or humidistat in your home to measure humidity indoors or outdoors, it is measuring relative humidity. I used this experiment to demonstrate how much less water your hair can hold when the air is dry. ©Science-y Hair Blog 2013
When your hair has a lot of moisture from the air around it, it stretches more and bounces back less readily. Like worn-out elastic. Stretching damages hair. If you pull on a piece of taffy or chewing gum, you can see how the outer surface breaks. This is also what happens to your hair (more or less).
Even though hair can stretch more when it has absorbed more moisture, it takes more force to make it stretch in high humidity than in low humidity. In dry air a smaller stretch (or pull, or tug with a comb or snag on a button) will damage hair than in humid air.©Science-y Hair Blog 2013
Temperature also makes a difference. At higher temperatures, it takes more force to cause hair to break and at lower temperatures, less force is required to break or otherwise damage your hair.
©Science-y Hair Blog 2013
So when it is cold and the air is dry, your hair breaks more readily than when it is warm and humid.
©Science-y Hair Blog 2013
How to prevent hair breakage in cold, dry weather?
You need to plasticize your hair! “Plastic” means pliable and flexible. Oils, fatty alcohols, silicones, cationic surfactants (in other words, hair conditioners) can give your hair the desired flexible quality and help prevent tangles. Humectants may or may not be helpful for hair. If you like humectants, you may lean towards those that are polymers or "film formers" (aloe vera, flaxseed gel, vegetable “gums” like guar, xanthan, carrageenan, panthenol) rather than humectants like glycerin. Conditioners may prevent some loss of moisture to the air from your hair and definitely help prevent breakage due to tangles and rubbing on clothes, scarves and hats.©Science-y Hair Blog 2013
An ounce of prevention is worth a pound of deep conditioners:
Truly the best protection for your hair in low humidity and cold weather is to protect it with a warm hat, a scarf or hooded scarf. These hold warmth and moisture around your hair so it is not exposed to extremes.©Science-y Hair Blog 2013
Other interesting humidity tidbits: When hair is surrounded by ample humidity, as it is taking on water, it swells and also lengthens slightly. As your hair is absorbing moisture, a whole bunch of things can happen depending on your curl pattern, the amount of water vapor in the air, the temperature, and how porous or damaged your hair is. If it’s not too overpoweringly humid, your waves and curls may be more defined and more bouncy as they are hydrated by the water in the air. As the humidity creeps up, your hair may absorb far more water and begin to lose its curl pattern, getting big or puffy or frizzy (undefined) or become limp. When your hair swells with water, the resulting disruption of the cuticle (the scales are “lifted up”) causes hair to look less shiny. In this state, it is more prone to tangles. Very porous/damaged hair can potentially hold more water than not-porous or little-damaged hair. Damaged or porous hair is more prone to frizz in high humidity than healthy hair. But in general, wavy and curly hair gets bigger, frizzier, and less defined in very high humidity.
©Science-y Hair Blog 2013
For those of us with wavy hair or loose curls, low humidity can dramatically change our hair’s curl pattern. The hair just doesn’t have the moisture it needs to express a defined wave pattern. Lock in moisture after cleansing with rinse-out and leave-in conditioners or oils, use a humidifier in your home.
Source: Chemical and Physical Behavior of Human Hair
Robbins, 1994. 3rd Ed. Springer-Verlag, New York
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