Wet Bulb and Heat Index

 One of the things I learned from living one week without central air conditioning during a heat wave in Texas is the critical importance of humidity.  

While we often talk only about temperature, humidity is just as important as temperature in determining comfort and safety.  They really need to be combined in one index for that purpose.   The two most common ones are Wet Bulb Temperature and Heat Index.  Those are two different ways of combining temperature and humidity into one number.

 (There are endless other similar indexes you can find at Wikipedia, sometimes adding in more information like sunlight and wind speed, of these the WBGT is most commonly specified for occupational safety...but rarely measured directly).

The Wet Bulb temperature is the temperature that would be read from a "wet bulb" covered in saturated wet cloth that is evaporating water.  At low humidities, evaporation is increased and the wet bulb is far cooler than the normal ("dry bulb")  Only at 100% humidity is the wet bulb temperature the same as the dry bulb temperature  Once you grasp the basic idea, it is a very powerful tool, because it is easy to understand that when the temperature and humidity are high enough, your body can get no cooling effect from perspiration, and you cannot survive that situation for very long.  And this relates to our body temperature of 97-99F.  To carry off the heat our body produces at rest, the Wet Bulb temperature has to be 95F or less for even the most healthy people.  Recent studies show a better number is 86F for most people including young and elderly.  These are often referred to in Celsius: 35C for very healthy people, and 30C for most people.  These maximums assume you are in the shade and getting as much air cooling as possible (possibly with a powered fan helping, and light if any clothing).

Using this information, and a Wet Bulb calculator it is easy to calculate the maximum safe humidity at any temperature (which the highest point where the Wet Bulb temperature is still below 86F).

Below 86F....all humidities are "safe" with sufficient air cooling and shade

86F        99%

90F        84%

95F        66%

100F      54%

105F      43%

110F      35%

115F      28%

120F      22%

Above 122F is dangerous regardless of humidity or air movement

Because the outside humidity virtually always drops as the temperature rises, dangerous Wet Bulb temperatures are very rare in areas that humans inhabit.  However, this is changing because of global heating and dangerous outside Wet Bulb temperatures are beginning to appear more often.  Inside there may be trapped humidity combined with solar heating, leading to more dangerous and uncomfortable wet bulb temperatures than outside unless you have air conditioning.

The Heat Index expresses the combination of temperature and humidity more intuitively for most people, as it relates everything to a typical and comfortable temperature/humidity combination, rather than the 100% humidity extreme case which is rarely observed.  So the heat index goes higher than the dry bulb temperature when the humidity is notionally "high" and therefore matching our intuition that high humidity is making it "hotter." So, to show how this works, look at the Heat Index for 100F:

Temp    Humidity    Heat Index    Wet Bulb Temp

100F    30%              102                76F

100F    35%              106                78F

100F    40%              109                80F

100F    45%              113                82F

100F    50%              118                84F

100F    54%              123                85.7F

Here is a calculator for Heat Index.

Here is an article on it.