What makes a bad hair day? Ok, there are many factors that can cause a bad hair day, but somewhere at the top of the list is our little frenemy known as humidity. Humidity seems to be good for our skin by keeping it from drying out, but it makes our hair go flat or frizz. Hair lengthens when the humidity increases and then contracts when the humidity decreases. This causes your hair to “fall” and lose its style. Over the past year or so I have been checking the dew point (almost) daily and taking note of my hair triumphs and tragedies. I have come up with my own little guideline for acceptable dew points when it comes to my hair.
First, I want to talk about dew point vs. relative humidity. The dew point is an indicator of how much moisture is in the air. A high dew point means that the air has high water vapor content.
So what is relative humidity? It is very deceiving is what it is. Let’s say we have two locations, A and B. Location A is 90 °F with a dew point of 75 °F which makes its relative humidity 62%. Location B is 50 °F with a dew point of 45 °F which makes its relative humidity 83%. (Standard SLP) Even though location B has a higher relative humidity, it is going to feel much more humid at location A. The warmer the air temperature, the more moisture the air can “hold.” Colder air simply cannot “hold” as much moisture as warmer air can. (Air doesn’t really “hold” water, but whatever. It is a good explanation.) When the temperature and the dew point are the same, the air is saturated with moisture and the relative humidity is 100%. But, just because the relative humidity is 100% does not necessarily mean that it feels disgustingly sticky outside.
I often joke that I can tell what the dew point is just by what my hair does when I go outside. If the dew point is in the 70s then there is absolutely no hope for my hair. If the dew point is in the upper 60s, it is a ponytail day. In my experience, the best hair days are those when the dew point is less than 55. I realize that a dew point in the upper 50s to low 60s is still very comfortable and not what most folks would consider “humid.” But, this seems to be the threshold for my hair’s style. This is why I chose 55 as the lower limit for my “bad hair-days” analysis. Everyone’s hair is different, so perhaps you other gals (and dudes…) have higher thresholds. Below is a guideline that I have made up for me.