So what is this humidity all about?


HOUSTON – Our current heatwave has everyone diving for the cover of A/C. While heat is pretty easy to understand, humidity can get a little tricky.

I’ve had a couple of viewer question about it. Let’s start with this one via Facebook from Jason:

Frank- You talk a lot about Humidity. What exactly is Relative Humidity? Relative to what? Can you send me a link that can clearly explain this? Or add this to your list of topics to address in your blog? I know it’s the amount of moisture in the air but does 90% humidity really mean the air is 90% water? (I would think I would not be able to breathe if that was the case.) Just curious. No hurry. — Jason

That’s a great question and the key here is “relative.” The amount of moisture in the air is relative to the temperature of the air. Warmer air can hold more moisture than cold air. Warmer air molecules are moving faster (creating heat) and spreading apart (expansion) allowing water molecules to fill the gaps. The hotter the air, the higher the humidity can go. For instance, 88° can hold two times more moisture than 70°. In fact, for every 18 degrees in Fahrenheit the temperature increases, the moisture can double!

The takeaway is that if the air is holding 90% moisture and it’s NOT cloudy or raining, then it must be really hot!

So how LOW can the humidity go?

The other question came via my email.

Just curious about the humidity. I was checking the weather in Las Vegas yesterday and the humidity was 2%, air temp 109, dew point 4. So just how low can it go? — Chris

Honestly, it can go pretty low. The lowest recorded is 0.03% in Antarctica, but you’ll notice there is still SOME humidity there. We live in a water-vapor atmosphere and so there will always be traces of humidity unless you create a controlled environment (some labs do this so that the air will be absolutely free of vapor). While we are on this, that Las Vegas dew point temperature of 4° is simply the temperature to which the 109° must drop in order to get some kind of dew (i.e., condensation) to form. Dew point temperature is an even better way to determine how much moisture is in the air than relative humidity.

Thanks for the questions, Jason and Chris!


About the Author:

KPRC 2's chief meteorologist with three decades of experience forecasting Houston's weather.