Sure, your Uncle Ed might be all wet, but that's not what the word "relative" in "relative humidity" is all about.
Relative humidity is simply the ratio of the current absolute humidity as compared to the maximum amount of water vapor that the air is capable of holding at any given temperature.
That means when the relative humidity is 100% there is no more room in the air to hold additional water.
When you use our Relative Humidity calculator, you will be asked to enter the current air temperature and the dew point temperature. Then just click the "Find Relative Humidity" button and you're good to go. We're using the official equation that's supplied by the National Weather Service.
The answer pops up quickly but that poor little calculator is expending a lot of energy to get the job done.
First, it converts the air and dew point temperatures from Fahrenheit to Celsius using this formula:
Tc=5.0/9.0*(Tf-32.0) for the air temperature and Tdc=5.0/9.0*(Tdf-32.0) for the dew point temperature.
Next, assuming a "standard atmospheric pressure", the calculator determines the "saturation vapor pressure"(Es) and the "actual vapor pressure" (E) in millibars using these equations:
Es=6.11*10.0**(7.5*Tc/(237.7+Tc))
E=6.11*10.0**(7.5*Tdc/(237.7+Tdc))
Armed with that information, it calculates the relative humidity (RH) using this equation:
RH=(E/Es)*100
Which is simply dividing vapor pressure by saturation vapor pressure and multiplying by 100 to return a percent.
That's simple enough, I guess, but what does all of this really mean? Why do we care about relative humidity and what's its effect on us anyway?
Let's find that answer by first examining absolute humidity and use that as a basis for understanding relative humidity.
Humidity is the amount of water vapor that is present in the air at any given moment. Note that I said "water vapor" and not simply "water". Water may be falling from the sky onto your head at this very moment (fix your roof!), but that's not humidity.
Although there are different types of humidity measurements, the one that you generally hear the TV weather folks talking about is called "relative humidity". Another term that the weather gurus will throw at you from time to time is "dew point" which is sort of a relative to relative humidity. Since they never seem to explain what the dew point actually is, we'll cover that here as well.
You often hear people moaning "It's not the heat that's killing me, it's the humidity!". While we tend to overuse the term "killing me" in our daily language, these people might actually be on to something. The human body perspires as a way to regulate body temperature. The perspiration, which is actually water, is supposed to evaporate when it reaches the outside of the skin. This evaporation process creates a cooling effect through some magic of physics that was explained on a day that I was absent from school.
What I do know is that in order for water to evaporate, it needs to go somewhere. That "somewhere" is with all the other water vapor that's being absorbed and held by the air. Well, if the air is already saturated with water then there is no place for the perspiration to evaporate to. This can cause our bodies to overheat resulting in heat stroke, heat cramps, heat syncope (fainting) and soaked underarms. Everything but the soaked underarms can be fatal depending upon the circumstances.
Warmer air can hold more vapor than cold air can. That's why it's always more humid in the summer than in the winter.
Absolute humidity is simply the total mass of water vapor in the air, divided by the total mass of dry air at any given air temperature. The higher the air temperature is, the more water it can absorb.
When the air becomes saturated there is no more room for evaporation to occur so water droplets start forming everywhere (condensation) and, eventually, it rains. That brings us to the discussion of dew point.
Since those TV weather people throw the term around so loosely every day, I thought that it would be poetic justice to have one of them actually explain it to us. Here's what our friend Denis Phillips, Chief Meteorologist at ABC Action News, has to say about the subject:
"Blaine, the Dew Point is simply the temperature at which the humidity in the air begins to condense and form dew or moisture. This 'condensing humidity' is what ends up as beads of sweat on our foreheads and accounts for the general dampness on humid days. A dew point of 70F or over is considered uncomfortable because that means that it's going to be a hot and humid day if the temperature reaches or exceeds 70F and the relative humidity is high."Now, not only do you know why your underarms are wet in August, but you finally got a real live meteorologist to explain dew point for you! Not bad for using a free calculator web site is it?
In addition to being used for weather related calculations it is also used with HVAC system design.