When referring to the mugginess of the air, meteorologists will point to dew point rather than relative humidity. Relative humidity is indeed a measure of how much moisture is in the air relative to the temperature. A 100% relative humidity(RH) at an air temperature of 40 degrees will not feel humid to us but a 100% RH at 75 degrees would feel very muggy.
That can be pretty confusing to understand so instead we use dew points as more of a static scale in determining whether or not it feels humid outside. The dew point is the temperature that air needs to be cooled to in order to condense or turn into water (like dew on the grass). This means that once the temperature equals the dew point there is 100% RH.
In Iowa, we are used to the humidity in part thanks to a lot of evapotranspiration from crops in the summer. Therefore at KWWL we generally call dew points under 60 degrees comfortable no matter what the air temperature is. With a dew point of 60 to 70 degrees you will feel a noticeable amount of humidity in the air. Anything over 70 gets quite uncomfortable.
Here is where things get tricky. Say we have a dew point of 70 degrees which will feel uncomfortably humid. If the air temperature is at 70 degrees we have 100 percent relative humidity. While it is humid, an air temperature of 70 isn’t too bad. If the air temperature is 90 with a dew point of 70, the relative humidity would be about 50% BUT a 70 degree dew point feels humid AND an air temperature of 90 is hot. Hot temperatures with humid air (regardless of the RH) will make our bodies perceive the temperature as higher than what it actually is. That is the heat index.
Higher humidity makes it feel hotter because our sweat is not as effective at cooling us off. Evaporation is a cooling process that doesn’t take place as efficiently in a humid environment.
**Warning scary equation below**
To calculate the heat index, you can use the equation of:
HI = -42.379 + 2.04901523*T + 10.14333127*RH – .22475541*T*RH – .00683783*T*T – .05481717*RH*RH + .00122874*T*T*RH + .00085282*T*RH*RH – .00000199*T*T*RH*RH
where T is temperature(F) and RH(%) is relative humidity. Note that to get the heat index you need the relative humidity. To find the RH given temperature and dew point you would have to jump through a few more equations. Luckily, computers do this for us. The heat index value that this equation produces will give you the heat index in the shade; if you are in direct sunlight you can add up to 15 degrees to the heat index.