Weather Wise: Dew Point and Humidity

Hello everyone! I’m going to try to start blogging on a regular basis, at least once a week, covering topics that the general public finds either confusing, intriguing and or a combination of the both. The current title of these postings will be Weather 101 for lack of a better name, but don’t hesitate to offer alternative suggestions! I’d also like to throw it out there that I am no English Major, and that these blogs are semi-informal. In other words, don’t look at the quality of the writing, rather, look at the content. Before I get into it, if you have any topic suggestions or meteorological questions, feel free to ask us anytime on our Facebook Page or email me at Brad@SWATChasers.com!

As many of you have noticed this week, it is smokin’ hot outside! So hot in fact that you might have wished the Rapture didn’t flop! This is especially the case if you are living in the Midwest or central United States. Hot enough to cook an egg on the sidewalk? Probably, but don’t expect me to eat it! I’m just not a fan of eggs. But the question remains… Why does it feel so hot? I’ll try my best to explain the differences between Dew Point and Humidity as well as some other meteorological jargon like the Heat Index.

-Why eggs? Everyone knows concrete goes much better with bacon!

Let me throw out a scenario. You wake up, brush your teeth, and stare at your scary morning hair in the mirror before jumping into the shower. After you step out, you notice something in the bathroom has changed! The mirror has “fogged” up and you cannot see your reflection anymore! Well why did that happen?  The answer is pretty simple. When the hot water shot out of the shower head, some of it ended up saturating the air inside the bathroom. You might have noticed this with the steam that was appearing to drift through the air. When this warm, saturated air hit the cold surface of your mirror, the moisture within the air hit or reached it’s Dew Point, or in other words, the temperature at which it took for that moisture to cool, condense and form condensation, or dew.  Condensation is just a fancy word to describe the change of matter from a gaseous state to a liquid state, which is the opposite of Evaporation.

Dew Point is a good measurement for the amount of moisture or water vapor content in the air. Don’t be confused, Dew Point is not the same as Relative Humidity, but they are related. I’ll get to that in just a few. The higher amount of moisture that is contained within the air raises the Dew Point temperature, thus in theory making it easier to form a cloud. When the air temperature and dew point temperature are the same, it results in the saturation of the air. This, depending on other circumstances, can result in dew on the grass, fog or even rain.

Take a sip of coffee or any other caffeinated beverage of choice and stay with me! It will all make sense shortly!

So what is Relative Humidity? Relative Humidity or RH for short, is a calculated value that takes into account both temperature and dew point. When the Dew Point Temperature and Air Temperature are closer together, your RH value will be higher, and the opposite is the case for lower RH values. For example: Temperature is 70° and the Dew Point is 67°… You might hear yourself walk outside and say, phew, it’s humid/uncomfortable, and you would be right! The RH would be just over 90%! However, you walk outside the next day and the temperature is 80° with a Dew Point of 60°. Although the temperature has risen ten degrees, the Dew Point has fallen slightly and the spread between the two is now greater. The calculated Relative Humidity is now only 50%! (Humidity Calculator) This is much more comfortable to the touch.

-Check out this completely unrelated photo of a horse!

Why does a higher water vapor or humidity percentage make us feel more uncomfortable? It has to do with the way the human body keeps itself cool. We humans use the evaporation of sweat on our skin to stay cool on a hot summer day. Since I am no expert on the human body, I’ll try to keep this as simple as possible. When matter changes it’s state, in this case from a liquid to a gas via a means of evaporation, it requires energy in the form of heat. When sweat evaporates off of your skin, it takes some of the heat with it and releases it into the air and away from you, thus having a cooling effect. You can read all of the jargon riddled scientific explanation here.  Now, if the Relative Humidity outside is particularly high, it will be more difficult for the sweat on your skin to evaporate and cool your skin. Think about it like a crowded movie theater. The more popular the movie is (Higher RH), ex: Harry Potter (90% RH), the more difficult it is going to be for you (The Sweat) to find a seat (Evaporate). Basically, the more saturated the air is, the harder it will be for the sweat on your skin to evaporate. This will not only make you extremely sweaty, but it will also make you unable to cool yourself as easily. However, if the RH was lower, the sweat would have ample space in the air to evaporate, and in return cool yourself.

You might be typing away feverishly in a message to my email account asking, “When does RH become uncomfortable?”, and before you click send, let me answer that!

Have you ever heard a meteorologist on television refer to the Heat Index during the Dog Days of Summer? Well now that you have the basic understanding of how Relative Humidity is calculated, Heat Index (HI) is much easier to explain. The Heat Index is another calculated value that takes into account both Relative Humidity and Air Temperature. The result is the “human-perceived” equivalent temperature, or, how hot it feels. Lately, you would say “Too Damn Hot!” I can’t say I would disagree. The following is the equation for calculating the Heat Index.

 HI = c_1 + c_2 T + c_3 R + c_4 T R + c_5 T^2 + c_6 R^2 + c_7 T^2R + c_8 T R^2 + c_9 T^2 R^2\ \,

-There are no parentheses, so where do you start now?

If you are a human being like myself and not a super computer, you can use this much easier to comprehend chart that I have stolen, sorry, borrowed from Colorado State University.

Heat Index Chart

-(HI) Chart Credit: Colorado State University

The Heat Index is the number where your Air Temperature and Relative Humidity cross on the X and Y axis. For example, if your air temperature is 90° and the RH is 85%, it will feel like a balmy 117° outside. Not too tough. Lately, 2/3 of the United States have been in or on the fringe of the “DANGER” zone. If you would like to read a brief history of the Heat Index in regards to recent records in Cincinnati, OH, click this link to travel to the Wilmington, OH NWS Page.

Well, I believe I have typed enough here for everybody to digest for a few days. I know this because 6 of my 8 fingers have fallen off and I am typing with my two remaining pinkies. Hopefully you learned something today in today’s WEATHER 101 class, taught by Brad Maushart, co-founder of the SWAT Chasers and SWAT Tours. Always Remember, some are Weather Wise and some are otherwise… don’t be the latter. Again, if you have any questions, comments, complaints or suggestions, please email them to Brad@SWATChasers.com. If your questions are good enough they may be featured in the next Weather Wise segment!

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5 Responses to Weather Wise: Dew Point and Humidity

  1. Steele says:

    Question. What does it mean when the Humidity and the Dew Point are the same? For example, the temp is currently 80F, Humidity is 69% and the Dew Point is 69°.

    And take this into consideration, this was in Las Vegas at midnight…… Officially monsoon season, no?

    • swatchasers says:

      Hello Steele,

      Dew Point and Humidity are measured differently. Dew point is measured in degrees, whereas humidity is measured as a percentage. When the two have the same numerical value, they aren’t necessarily the same reading. Simply, temperature, dew point and humidity are all interrelated with one another. Using complicated equations, or this handy calculator, if you have two of the measurements, you can figure out the third missing value.

      -Brad Maushart
      http://www.SWATChasers.com

  2. shawn says:

    I live in Omaha. It seems like the dewpoint is incredibly high quite a bit this year. It feels like the air is hard to breath.

    • swatchasers says:

      Shawn,

      Temperatures in the Gulf have been a few degrees warmer this year than in previous years on average. If my guess is right, that would have allowed more moisture to evaporate into the air and flow into the interior plains making things considerably more humid. I could be wrong, but that’s the best explanation I can come up with.

      -Brad Maushart
      http://www.SWATChasers.com

  3. […] level. The point at which air becomes saturated is known as the dew point. I explained this in the Dew Point and Humidity posting a few weeks ago. Eventually, warm air rises high enough to the point in which it becomes […]

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