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Why does it feel so nice? Thank the dew point

The dew point is a great measure of how humid it feels outside.
Credit: Getty Images/iStockphoto
Enjoying the Cooler Weather

Aside from the talk of a hurricane, humidity is probably the most-mentioned weather condition in Florida. Yes, there’s heat, thunderstorms and torrential rainfall too, but the word "humid" is probably uttered more than them all.

So what is humidity? 

Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye.  

But when referring to the mugginess of the air, meteorologists will usually talk about the dew point rather than relative humidity. Here’s why. Relative humidity is a measure of how much moisture is in the air relative to the current temperature. 

The problem is that 100 percent relative humidity at an air temperature of 50 degrees will not feel humid to us, because it’s cool.

But, 100 percent relative humidity at 75 degrees would feel very muggy, because it’s warm. So, 100 percent humidity doesn’t always feel the same. 

Understanding the dew point

The dew point is the temperature that air needs to be cooled to in order to condense or turn into water (like dew on the grass). This means that once the temperature equals the dew point, there is 100 percent humidity.

Credit: 10 Tampa Bay

We start to notice that it feels humid or a bit uncomfortable at about a 65-degree dew point, regardless of the air temperature. When dew points are in the 70s, it becomes very uncomfortable. 

You’ll notice this, even in cooler air like a 75-degree morning. If the dew point is 74 and the air temperature is 75 during your brisk morning walk, you’ll feel the humidity in the air and the sweat will just cling to your skin. The hotter the air, the more uncomfortable it becomes. That 74-degree dew point with a 90-degree air temperature will feel even worse. Either way, a 75-degree dew point will make most people feel miserable. 

That’s where the heat index or “feels like” temperature is often used. It’s a derived temperature of what it feels like to the human body with the combination of temperature and humidity. It’s pretty good at describing if it might feel humid out as well; because if it’s significantly higher than the actual air temperature, there’s a lot of water vapor, or humidity, in the air. However, the heat index expresses the effects of humidity better in the afternoon when temperatures are their hottest. 

What happens to the body when it’s humid?

When the air is humid and has a high moisture content, the sweat on your body cannot evaporate, leaving our bodies feeling hot and sticky. Our bodies cool off by sweating. So in humid weather, to cool off, our bodies must work even harder. This results in excessive sweating, making us feel hotter and more uncomfortable. 

Bottom line

So, there are lots of ways to determine how it’s going to feel outside today or tomorrow, but the dew point might be the best for knowing if it is going to be pleasant, a little sticky, uncomfortable, oppressive or miserable.

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