It Ain't the Heat........

        Half an hour of pulling weeds in 89 degree heat is enough. I’m not really sweaty, though, as I would be most days at 89 Degrees. Thankfully, the dewpoint was just 74 at the time I finished. Also, uncharacteristically, the humidity was a mere 68%.  It was in the 80s yesterday

        It's (the relative humidity) been running in the low 80s over the past several weeks. That's when the weather report says "temperature is 92, feels like 113!  Our actual predicted high is 104 this afternoon. I Hope the humidity and dewpoint stay low. Why does it matter?  Because it's all about the "feels like!"

        Factoring in dew point and relative humidity at the time, an outside 89 degrees “felt like” 98, which is 8 degrees higher than the actual temperature, but just about body temperature.  That’s why I wasn’t sweaty, just dirty, when I came inside. Tuesday, on the other hand, on the golf course finishing hole the temp was 92 with a dew point of 80. The “feels like” temperature, that “heat index” they refer to on the TV weather report, was about 110 F.

      “Dew point” is the true indicator of the total amount of moisture in the air. Most people find dew points of less than 60 degrees comfortable. When the dew point rises above 65 degrees we begin feeling that the air is “sticky.”  As the dew points climb through the 70s the humidity bothers more and more people who are outdoors.

         As I said, that whole "feels like" thingy is really the heat index, which is, sometimes called the "apparent temperature." It is a measure of how hot it really "feels" when relative humidity is factored with the actual air temperature. ... As an example, if the air temperature is 96°F and the relative humidity is 65%, the heat index (how hot it feels) is 121°F.!    This is also the origin of the "It's a dry heat" saying, because if the temperature is the same 96 degrees, but the humidity is a desert like 25%, the heat index is 94 F. At the same temperature, a 40% increase in relative humidity equates to an apparent temperature 23 degrees higher! Let’s take the ultimate comparison: Phoenix averages less than 40% relative humidity in summer. Orlando averages more than 40% higher than that.

      Deplaning in Phoenix several years ago the outside temp was 113 F. That's hot, but your clothes didn't immediately stick to you because, although it was hot, the heat index was lowered by the low dew point and/or humidity. Right now, , the temperature at Sky Harbor airport in Phoenix is 112 F, but the relative humidity is 11% and the dewpoint is 45.7 degrees. The heat index ("feels like") is 107 degrees. Meanwhile, at the “Magic Kingdom” (Disney World) here, it is 96  F with a relative humidity of 84% and a dew point of 79 degrees. The heat index (drum roll) is between 115 F (using dew point), and 144 (using humidity), which is why there is a high humidity advisory for Disney right now! The calculated heat index usually varies by a degree or two depending upon whether we use relative humidity or dewpoint, but they are usually close. Very high temperatures and humidity will skew the data a bit, as it has today at Disney.

 Using Villages current data at 1 pm local, about 2 ½ hours after I began this, with a lunch break, the temp is 94, humidity is 74% and the heat index is 106 and on the way up. The table below uses only relative humidity and temperature to derive heat index. Results using dew point would be similar except at the extremes. 



Note that the average Southwestern humidity is at the extreme low end of the chart, and the current Phoenix relative humidity (11%) is actually off the chart low.

        That’s what the “dry heat thingy really means. If we hit the predicted 104 degrees and the humidity remains stable at 70% and/or the dewpoint holds at 70, the heat index here will “feel like” between 111 and 113, depending which variable I use for the calculation. Although right now it is 18 degrees hotter in Phoenix than at Disney World, it actually feels cooler. So the next time some sage intones “It’s not the heat, it’s the humidity,” just nod, smile and say, “But what about the dew point?” Cue the crickets.