Forecasting 101: What the dewpoint?

Ever walk outside and say dam it is so muggy. Well maybe only us east coasters say this but humidity (or better known in the weather field as "Relatively Humidity") whether low or high is a big part of forecasting. 

Relatively humidity is just that relative to the actual air temperature. It is a percentage of how much water is in the air. I personally loathe humidity in more than one way. Firstly, because I do live in an area where the humidity is often high throughout the year which makes it feel much warmer or colder than the actual temperature is. Secondly, because it is a percentage, I don't agree that it accurately tells the story of what the weather is going to feel like. 

Let me be more clear. Ever hear someone say "Ugh it's 90% humidity". So lets do some rough math. Lets say the temperature outside is 80 degrees, 90% of 80 degrees is 72. So the rough dewpoint is 72. There's a much more in depth equation but this gets you in the ballpark. 

Ah ha there's one of my favorite words in weather, dewpoint! I found this great graphic from the National Weather Service. The Muggy Meter! So lets go back to our "90% humidity" problem. A 72 degrees dewpoint is muggy and will feel gross outside.

However, lets take the same problem. "Ugh it's 90% humidity", but this time the air temperature is 60 degrees. This means the rough dewpoint is 54 degrees, and truly there may be some dew on the grass but it won't be as oppressive as the 80 degree day with 90% humidity. 

This is why I like and really only refer to dewpoints as they are an actual reading rather than a percentage. Now why are dewpoints so important to forecasters. Dewpoint tells you how much water is in the air. This will clue you into whether it will be clear skies, foggy, or if there is a storm potential. 

It also sets the floor for the overnight low. If the dewpoint is 65 degrees the overnight low will not go below 65 degrees. And if the air temperature comes close to the dewpoint that's when you'll start to see fog.  

I could go on and on about dewpoint, but I think this is good enough for a Tuesday morning.