We have a lot of sunshine on the horizon as a ridge of high pressure has positioned itself just to the east of the Reno area, bringing in a dry and stable flow. High temperatures on Saturday and Sunday will range from the low to mid-90s with generally light winds, before a dry cold front Sunday night helps to drop the highs into the upper-80s on Monday and Tuesday. We may get a few clouds out of the front Monday, but those should clear out by Tuesday.

humid

Yesterday I mentioned that while we have been hot, at least it’s been a “dry heat.” How does that affect the Heat Index? I may have given some of it away by mentioning our relative dryness, but the Heat Index is sort of the summer’s equivalent to the wind chill in the winter. Just as wind makes cold temperatures feel even colder, high humidity makes hot temperatures feel even hotter. The Heat Index is an attempt to take humidity into account. The exact formula used to calculate Heat Index would make most of our heads spin, so I won’t bother putting it down.  But let’s try an example:

Let’s say the temperature here in Reno and in Houston is 90 degrees F. In Reno, a dewpoint of 30 degrees is fairly common, which gives us a relative humidity of 12%. The Heat Index for these conditions is 86 degrees… in other words; it “feels” like 86 degrees. But in Houston, you can easily have a dewpoint of 75 degrees (61% humidity), and that gives you a Heat Index (HI) of 100 degrees. An 80 degree dewpoint (72% RH) causes the HI to soar to 107. So the amount of moisture in the air can make more than a 20 degree difference in comfort level.

Heat Index Graph

But just why does the humidity make such a big difference in how hot it feels? That all has to do with how our bodies cool themselves.

When we get hot, our brains (marvelous organs they be) send a signal to our sweat glands to open up and wet down our skin. As the sweat evaporates, it draws heat out of our skin in order to provide the energy necessary to turn liquid into vapor. For the real geeky of you out there, it’s called the heat of evaporation. The faster the sweat (water) evaporates, the faster we cool down. In dry air, water evaporates much quicker than when the air is moist, and the cooling process is much more efficient. When it is muggy, we keep sweating, but since very little of it is evaporating, we don’t get cooler.

We just get smellier.

 

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