More so than ever in my career I am paying attention to the dew points. But I'm beginning to realize that even though they mean a lot to me, the average person has no idea what they are. "Dew points? Who cares. What's the humidity?" you might be thinking. And why not - when it's humid outside we say "Boy it's humid" not "Boy it's dewey". Or, "Boy it's pointy". You get my drift.
Dew point and relative humidity are closely related, but aren't the same thing. The offical definition of dew point is: the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water. Whoa. There are too many big words in there. How about this: the dew point is a measure of the amount of water in the air. Yea, that's better.
The relative humidity is a ratio between the amount of water in the air to the maximum amount of water possible at a certain temperature. So the equation (uh oh, here's the math) is essentially: (Amount of water in air)/(Max water possible) x 100%.
So now let's get into why one is better to use than the other. In a desert, where the air is very dry, you could have 100% relative humidity on any given day. The amount of water in the air is going to be low, but considering that there isn't much water in the air anyway the ratio could stand at 1 to 1. Now look at Louisiana, where the relative humidity could be 70%. But considering that there is usually a lot of max water possible in the air, even 70% could be a very humid day.
Wow, I got confused just typing that, which is why meteorologists don't love using humidity levels to describe the weather. The relative humidity is very... well relative. Relative to the place you live, the time of day, the time of year etc. But you know what isn't? Dew points. Since dew points are constant a 70 degree dewpoint will be muggy in Louisiana and muggy in New York. It's a very constant way to desribe how it feels outside.
Sometimes you might hear a person complain that it is 90 degrees with a 90% humidity. That person is somewhat of an exaggerator (and usually my sister). The only time that would happen (while it's not raining) would be if the dewpoint was around 88 degrees. And lord help us if that ever happens. Now, if that person were to complain it is 90 degrees with 75 degree dew points, I would respond, "Yuck, I know".
The rule of thumb for dew points is this: under 60 degrees you'll be comfortable, 60-70 degree dew points are going to feel sticky, anything above 70 degrees feels like you got hit in the face with a wet towel. And we will routinely get above 70 in the next few months.
Don't you just love summer?
