We always seem to drop the first name of “relative humidity” to just plain old “humidity”. That doesn’t sound like a big deal. People I know call me Zollitsch all the time. In fact my high school football coach would call me Zollitsch and then proceed to call me things I cannot mention in a family friendly blog. But that first word, “relative” is important to the term since it tells us why it won’t always indicate how humid it feels outside. Dew point on the other hand almost never fails.
Let’s say you have a box of air about 3ft. by 3ft. by 3ft. sitting on the floor next to you. If you spill a bit of water into the box the air inside would eventually make the water evaporate until it’s all gone. But how much water the air in the box can hold all depends on the temperature. At 60° that box can hold about 11 grams or 11 ml. of water. So adding 11 ml. to the box would raise the relative humidity to 100%. But what if we raise the temperature to 90°? At this temperature the same box of air can hold about 31 ml. of water. So adding 11 ml. of water would only bring the relative humidity to 35%. It’s the same amount of water in the same volume of air, but since the humidity is relative to the temperature (and pressure which we did not change) the relative humidity is very different.
So how do we measure if a day is going to be humid or not? We do it with dew point temperature which we shorten to dew point. The dew point is what the air temperature would have to be for relative humidity to be at 100%. So a dew point of 60° with an air temperature of 60° is at 100% relative humidity. But as you now know a dew point of 90° with an air temperature of 90° is also at 100% relative humidity but with a lot more water! Fortunately for us our dew point almost never exceeds 80° in Milwaukee. Since we can never have more than 100% humidity, dew point will never exceed the air temperature.
Here’s a quick guide to dew points and what the numbers mean to you.
Dew point below 55° = Very comfortable, you don’t even notice the humidity.
Dew point of 55° to 60° = Pretty comfortably but you notice a touch of humidity
Dew point of 60° to 65° = Starting to feel humid but not too bad.
Dew point of 65° to 70° = Now we’re feeling humid
Dew point above 70° = Wow it’s humid, let’s get inside, I need to towel off my forehead.
Everybody had a different tolerance for humidity. The above quasi-scientific scale would not apply to residents of Florida (of course their cold weather scale starts at 50°). So the next time you’re looking at a forecast trying to figure out if tomorrow will be humid or comfortable, skip the relative humidity and look at the dew point. At least until we can get the “frizzy hair scale” calibrated for humidity.