We can measure air temperature easily, but calculating humidity isn’t so simple — and it’s even harder to understand. Most know the term “relative humidity,” but fewer are familiar with the dew point. That quantity is more valuable in understanding how uncomfortable a hot, humid summer day is.
Relative humidity can be misleading because it is, well, relative … relative to the temperature that is. It is a measure of the moisture in air compared with the amount of moisture that can exist in air at a certain temperature. It becomes misleading in the summer because the warmer air is, the more moisture can exist in that air. For example, the past few days the relative humidity during the afternoons — the most uncomfortable part of the day — was about 40 percent. It cooled off at night, while the moisture content remained fairly constant. The result was a morning relative humidity of about 80 percent. The relative humidity was higher, but it was more comfortable because it was cooler.
The dew point is not a relative quantity. Rather, it measures how much moisture is in the air at any given time. The higher the dew point, the more moisture there is in the air and the harder it is for our bodies to cool. It starts to get uncomfortable when dew points rise into the 60s (where they have been the past few days) and is very uncomfortable when they rise above 70. The muggiest days late last week had dew points in the low to even mid-70s. Thursday we will combine temperatures in the 90s with dew points in the low 70s. That will make it feel (to our bodied trying to cool) like it will be 100 or higher — that’s a quantity we call the heat index.