Wonky Weather - Graphs!

A very science-y post. Fun!©Science-y Hair Blog 2013

Dewpoint and relative humidity and how they effect hair has been swirling around my mind this spring as we have had some really variable weather. I live where winters are usually cold and dry, summer is usually hot and humid and spring and fall are combinations of everything. Dewpoint and relative humidity are not easy to understand. They're in the realm of physics - things like “saturation air pressure” that we experience every day and know what they are from experience. But when we try to measure them and predict and explain their effects on other complex systems like hair – Man! It’s not easy.

Here’s what is wonky: Dewpoints getting into the “moderate” range, but dry air. You think your hair should be bouncy and defined, but instead it wants to be limp. But the dewpoint is above 40°F, you think – so what gives? When you go outside, your lips and eyes and mouth dry out and your waves and curls lose their spring or feel rough.

In transitional seasons – it helps to pay attention to relative humidity. For example, the dewpoint is 44°F and the temperature is 80°F. Relative humidity? 28% That’s dry. At this temperature, a cubic meter of air can accommodate up to 30 grams of water (about 1/8^thcup).  But it’s not. It’s got about 28% of that.  If it cools down to the dewpoint (or pretty close), we’ll have dew on the grass and a higher relative humidity, but given the moderate dewpoint, it’s still not super-duper juicy air.©Science-y Hair Blog 2013

When the air is super-juicy, I don’t need a weather forecast to tell me that. When I take the dog out in the morning, my hair immediately feels heavier and bouncier as it begins soaking up moisture. That and my shoes are soaking wet.

I’m going to share my love of graphs for some more dewpoint and relative humidity visual aids. Graphs are worth many thousands of words.

Graph #1: Hydration Levels of Hair

This graph is the amount of water which can be absorbed by hair at various relative humidities (RH).  On the left (upright axis) is the RH and along the bottom the numbers indicate the percent of water your hair can hold at these amounts of 

relative humidity. This only goes gown to 40% relative humidity.  You know it gets drier than that!

My point here is to show you how much LESS water there is in your hair at low RH, or how much more moisturized your

hair is at higher RH values.

Graph #2: Water in air at various temperatures

This graph shows the maximum amount of water in grams per kilogram of air, that air can hold at various temperatures

(when it is saturated with water). Please note the temperatures are in °F. 

Grams of water is on the upright axis and temperature in degrees F is on the horizontal axis. The goal of this graph is to 

show you the huge variation in how much water can be in the air at different temperatures. It's not always as wet as it can 

be! One gram of water is just under one quarter (1/4) teaspoon.  This is the basis of dewpoint. Dewpoint is a set point whereas relative humidity is a moving target. But relative humidity is always relative to whatever the dewpoint is in your region.

Graph #3: What happens indoors

Let's say you are indoors at about 68°F (20°C). You look at a hygrometeror or humidity meter and it shows the indoor

relative humidity is 50%. Find 50% on the bottom (horizontal) axis of the graph, follow it up and you see that it

corresponds to an "indoor dewpoint" of just over 50°F. An indoor relative humidity of 30%, if the indoor temperature

stays the same corresponds to an "indoor dewpoint" of about 35°F. When the temperature stays the same, dewpoint and

RH give you similar information.

Graph #4: Dewpoint vs. Relative Humidity

For units, refer to Graph #2
Blue Bar: How many grams water a kilogram of air can possibly hold at the temps (in °F) shown along the bottom of the graph. But in reality, the air isn't always full of water and the temperature and relative humidity are frequently changing in relation to each other.
Red bar: How much water is in the air when the relative humidity (RH) is 25%.
Green bar: How much water is in the air when the RH is 50%.

So even if your dewpoint is up there at 60°F, when the RH is down around 25% - it feels dry.

Glad you stuck with me! I hope that gave some images to your experiences. Or drove you crazy, but in a good way. Skin and hair are not losing moisture to the air around them when the ambient relative humidity is around 60% or above. I have seen two sources for this figure, but I don't think they considered that it can be 60% relative humidity when it's 20°F outside.

So above 60%-70% RH, hair can begin to frizz because it's taking on moisture. As it does that, it A) better expresses its wave/curl pattern and B) said wave/curl pattern wants to take up more space because the fibers don't align neatly. Shorter hairs reach for the sky! If your waves or curls are not very springy or your hair is very fine, when the dewpoint and RH are both very high, your hair may get limp as the weight of the moisture overwhelms the fibers.

Here is another post about humidity.
And here.

Any physics or climatology teacher (or climatologist) would and should feel faint when they see how I have boiled this down because I'm leaving out some technical points. But hey - science belongs to all of us. And we're talking about how the humidity as "juicy or dry air" effects hair - a proteinaceous fiber system. So we need to cut out some of the details because what we really want to do is be able to apply this information to our daily lives.