Why relative humidity,?

[Rex R]

Why are we still using R.H. when dew point temps are easier to understand?

 

[jderimig]

Dew point is a measure of the absolute amount of water in the air, relative humidity correlates better with the ability of your body to cool itself using sweat or how well you can dry something.

 

[Hobie1dog]

John is one smart guy

 

[neil_w]

Both are useful, neither tells the whole story by itself.

 

[Sooner Boomer]

Both are useful, neither tells the whole story by itself.

To tell the "whole story", you need to measure the WBGT, Wet Bulb Globe Temperature. This is a measurement of the apparent temperature used to estimate the effect of temperature, humidity, wind speed (wind chill), and light (usually sunlight) on the human body. It is measured by using a wet bulb thermometer, a dry bulb thermometer, and a thermometer suspended in a 6" copper sphere painted black.

 

[bobby_hamill]

I got one of these about 10 years ago from Army surplus.
It did not have the tripod with it.

 

[lakeroadster]

Water Grains

 

[Sooner Boomer]

So, "why relative humidity"? It's a way to easily talk about the comfort level with a single number. Humans are pretty happy when the relative humidity is 30-40%. Much higher, and it starts to feel muggy, esp. on hot days. Lower than 30% and skin and sinuses start to feel dried out. These are generalizations, your milage may vary.

 

[manixFan]

I like to think of dew point as when the air acts, and relative humidity as how the air feels. If I am using spray paint here in Texas, I have found using the dew point is a far better predictor of success than relative humidity. If the dew point is below 60º, I'm fine (other conditions pending of course), but much above that and I'm likely to have moisture condense out of the air into the paint stream as I spray, or something to that effect.

Likewise, if it's 80º and 40% RH, it's a good day to work in the yard. But if it's 80º and 90% RH, not so much. It's also a way to help judge what a new destination will feel like - looking at the RH helps me figure out what it's going to feel like when I'm there and how to plan to dress.

At least that how I think of it.


Tony

 

[prfesser]

Dew point and relative humidity are directly related. Used to have students do an exercise where they determine relative humidity from the dew point and the current temperature. The dew point is the temperature at which the air is saturated with water vapor--measured as vapor pressure, in mm of mercury, just like atmospheric pressure. Find how much water vapor the air could hold at the current temperature, and divide, then multiply by 100 to make it a percentage.

Of course one needs either a table of vapor pressures, or (more advanced) calculate vapor pressure at the current temperature, using some equation I've forgotten...

As others have pointed out, most people have a "feel" for relative humidity values which is no doubt why they're used more often. But whatever is useful is good.

Best,
Terry