It’s not the heat, it’s the humidity. But its not the relative humidity.(*)
As a raw number, relative humidity doesn’t tell you anything. The video above shows how the same amount of water produces different relative humidities depending on air temperature.
For example, early yesterday morning in my backyard it was 80 degrees with relative humidity 79%. Last Tuesday it was 66 degrees with relative humidity 83%.
So didn’t yesterday’s 79% humidity feel better than 83% last Tuesday? No! Yesterday’s 80 degrees held a lot more water.
Dewpoint (the temperature at which the air is so saturated that it rains or produces dew) is the helpful number that tells us that. If you know the temperature and relative humidity you can calculate the dewpoint here.
The National Weather Service in Chicago made a chart to describe how we feel at various dewpoints. I’ve marked it in red to show my own heat-averse opinion. (Click on the screenshot to see their dewpoint video that includes this chart.)
So here’s what was really going on this week and why it felt so hot yesterday even though the temperature never reached 90 degrees. Notice that the relative humidity was at its lowest yesterday afternoon.
|Date/Time||Temperature||Relative Humidity||Dewpoint||Comfort Range|
|Tuesday Aug 9, 7am||66oF||
|61oF||Rather humid, almost comfortable|
|Friday Aug 12, 7am||80oF||
|Friday Aug 12 afternoon, 2pm||88oF||
Find out the dewpoint before you go outdoors and you’ll know whether you want to brave it!
(*) p.s. See the comments!
(video from Richard Clements on YouTube. screenshot from NWS Chicago video. Click on the screenshot to see the video)