Understanding The Difference Between Dewpoint And Relative Humidity

The dewpoint is not the relative humidity. The relative humidity is not the dewpoint. Unless you are a meteorologist, you could go your entire life without knowing the relative humidity again. Meteorologists use relative humidity for forecasting clouds and for specific fire weather purposes…and that’s about it. If you only follow one of these two variables, choose dewpoint.

The dewpoint is an absolute measure of how much moisture there is in the air. The higher the dewpoint is, the more humid it is. When the air temperature cools to the dewpoint, the air comes saturated. Any dewpoint above 60° suggests it is humid outside; a dewpoint of 70° or higher means humidity is oppressive.

The relative humidity is different than the dewpoint. The relative humidity describes the relationship between the temperature and the dewpoint. More specificially, the relative humidity is the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated. If the temperature is close to the dewpoint, the relative humidity will be high; if the temperature is far away from the dewpoint, the relative humidity will be low.

The relative humidity does not describe how humid the air is; the dewpoint does. The relative humidity can be 100% in the winter when the air is far from humid. Likewise, the relative humidity can be below 50% when it is very humid outside.

Let’s look at the relationship between the temperature and dewpoint and how it affects relative humidity. If the temperature and dewpoint are equal to each other, the relative humidity is 100%. For example, if the temperature is 65° and the dewpoint is 65°, the relative humidity is 100%:


If the temperature is 30° and the dewpoint is 30°, the relative humidity is still 100%:


Regardless of what the temperature and the dewpoint are, if the two are equal, the relative humidity is 100%. The relative humidity can be 100% on a humid summer morning or on a very cold winter day.

Let’s go back to the first scenario. Suppose the temperature rose from 65° to 80° on a summer day, but the dewpoint remained at 65°. Because the difference between the temperature and dewpoint increased, we expect the relative humidity to decrease. In fact, it dropped to 60%:


Did the air become less humid? The answer is no. The dewpoint did not change, but the relative humidity dropped significantly; because the dewpoint is over 65°, however, it is humid. In fact, you didn’t even need to know the relative humidity to know whether the air became less humid or not; you just needed to know if the dewpoint rose or fell.

Suppose later that day the temperature kept rising, the dewpoint didn’t change, and the relative humidity fell:


Did the air become more humid or less humid? The answer is neither. The dewpoint did not change, so the air is not more or less humid.

As an aside, the air may feel more humid when the temperatures rose through the day (even when the dewpoint did not). This apparent temperature – called the heat index – rose because the temperature increased. The heat index is only calculated between the temperature is greater than 80° and the dewpoint is greater than 54°.

Let’s make another change. Suppose the temperature did not rise again, but the dewpoint and the relative humidity both increased:


Did the air become more or less humid? The dewpoint rose, so the air became more humid. The relatively humidity rose only in response to the dewpoint rising; because the difference between the temperature and dewpoint went down, the relative humidity increased.

It’s time for one more change. Suppose the temperature of the air remained the same, but the dewpoint and relative humidity dropped:


The air became far less humid because the dewpoint dropped 20°! The relative humidity dropped only because the difference between the temperature and the dewpoint increased.

In summary, the dewpoint is an absolute measure of how much moisture there is in the air. The relative humidity does not tell you how humid the air is; the dewpoint does. The relative humidity describes the relationship between the temperature and dewpoint. The relative humidity can drop and fall rapidly through the day even if the dewpoint remains the same. Despite “humidity” in its name, relative humidity is not a good judge of humidity as you feel it; it is a measure how far apart the temperature and dewpoint are.

Don’t fall into the relative humidity trap! Relative humidity almost always rises and falls more quickly than the dewpoint. Stick with the dewpoint; it is your absolute guide to absolute humidity!

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More Storms Than Usual This Month, Summer, And Year?

Many in the Tri-State feel that this has been an abnormally stormy month, summer, or year. Perhaps it is the intensity of the rain, the frequency of the storms, or the lack of days with abundant sunshine that have people thinking there have been more storms than usual this year.

I keep a database of Severe Thunderstorm, Tornado, and Flash Flood Warnings for the area. The records go back to when the National Weather Service Forecast Office in Wilmington, Ohio first began issuing warnings in 1995. While we are not halfway through the month of August, the Tri-State Tornado and Flash Flood Warnings counts (from January 1st to August 31st) are below the averages and records for the same time period. The Severe Thunderstorm Warning count is above average:


These numbers suggest this has been a stormy year in some ways, but not in all. Many forget how stormy 2011 was. 2011 was also the wettest year on record in Cincinnati (with yearly records dating back to 1871).

Many have said that this August has been abnormally stormy. One way to measure this is by comparing the August Severe Thunderstorm Warning count from one August to others:


On average, there 12 Severe Thunderstorm Warnings are issued in the Tri-State each August; so far in 2015, 17 have been issued. By this measure, it has been a stormy August, but more warnings were issued in 2007 and 2010. It is worth noting that August 2015 has not yet ended, so that count is not final.

Has this been an abnormally stormy summer? If you compare the Tri-State Severe Thunderstorm Warning count from June 1st to August 31st, there have only been 3 years since 2007 with more warnings:


On average, 59 Severe Thunderstorm Warnings are issued in the Tri-State each summer (June, July, and August combined). While we are ahead of the average, we are unlikely to break the summer record of 103 warnings set in 2008.

For all intents and purposes, 2013 and 2014 were somewhat quiet severe weather years. No tornadoes were confirmed in the Tri-State during 2013, and 5 were confirmed in Tri-State during 2014. For perspective, the yearly average (1950-2014) is roughly 3 tornadoes. Flash Flood and Tornado Warning counts were below the 1995-2014 average in both 2013 and 2014. The Tornado Warning count was below average in 2013. If you remember the last couple of years, 2015 is a stormy year. Calling 2015 “stormy” compared to 2011 or 2012 is a much harder case. There were 16 confirmed tornadoes in the Tri-State during 2012.

I’ve found that when people compare seasons, they often compare it to last year or the previous season. 2011 and 2012 were stormy years and – in many ways – stormier than 2015.

There are also dozens of ways to measure how stormy a period of time is. 2015 has been a stormy year for Brown County, where three flash flood fatalities occurred earlier this year. Those are the first storm-related deaths in Brown County since March 2, 2012. Where you live, what you see, or what you doing see influences your memory of storms.

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A Sycamore Story: Putting A Weather Station On The Roof

Since I returned back home to Cincinnati in late 2011, I’ve been connected with Sycamore Community Schools in a lot of ways: through teachers, through friends, and by getting involved in district activities. Sycamore is a district with award-winning education, and Sycamore is working on some new initiatives to make sure students are better prepared for the workforce.

Before a weather station and camera network was created at Local 12, I spoke at a STEM (science, technology, engineering, and mathematics) meeting with Sycamore staff in 2012. I discussed the value of having a weather station at a school. First, the data collected by the weather station can be used in the classroom to teach math, science, technology, and computer programming. The data can also be shared with the National Weather Service and media to show current weather conditions; these data are especially important during active and severe weather.

Sycamore recent initiatives opened up an opportunity to put a weather station at Sycamore High School. With the backing of the district, the weather station arrived at the high school last week, and the installation began on Wednesday:


Mr. Chad Husting, a science teacher at the high school, Ashwin Corattiyil, the Dean of Students at the high school, and I set up the weather station on Wednesday. Mr. Husting was nothing short of MacGyver connecting the pieces together. He even came in Friday to secure the station’s pole and tripod!

The weather station is away from wind blocks and obstructions and also away from where animals and people can influence the measurements with it:


The wind speed and direction are measured at the top of the weather station, and rainfall and temperature measurement are taken in the black and white-colored units, respectively.

It is important that the station is positioned away from walls, buildings, and trees that can block the wind. That makes the roof a great spot!


Note the cinder blocks holding the weather station down. Derrick Richardson (assistant principal), Ms. Haverkos (high school science teacher), two custodians, Ashwin, and I put those there to make sure the station was secure (they did most of the work)!


The weather station has a wireless connection to a console in the building. From the console, the data are uploaded to the Internet and to various sources:


While at the school, students will be able to see the data on the console, but the data will also flow to several places online, including:

http://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?stn=E7726 (coming by mid-August)

This is an exciting time for Sycamore not just because I see a weather station on the roof but because students, staff, and the community benefit from having that data. Science teachers at Sycamore High School, including the ones listed above, seem to be very excited about this new teaching tool, and I’m hopeful that the success of this weather station is so big that it spreads to other schools throughout the district.

Ironically, Sycamore High School is across the street from a neighborhood heavily damaged by an F4 tornado back on April 9, 1999. Those who have lived in Blue Ash, Montgomery, and Symmes Township for years know that a simple weather station is more than just education; it’s safety.

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All I Really Need To Know I Learned From My High School Physics Teacher

Meet Mr. G. Some call him Mr. Gutekunst. Some (not me) call him Mike.


Regardless of what you call him, he was my physics teacher junior year of high school. I have seen him twice since I graduated. One of the best luxuries of working in your hometown – especially as a public figure – is that you can connect with people from your past fairly easily. The picture above shows we have both managed to keep our youthful look after many, many years.

I learned a lot from Mr. G. He basically set a foundation of science that I use daily as a meteorologist. Sure, all sciences – biology, chemistry, and others – are connected to meteorology, but physics is one of the most important.

While a love for science can connect two people, I remember Mr. G most for what he taught me about life. It is cliché, but what he taught on the last day of class was revolutionary and an appropriate foreshadowing for the reality of life.

I had lunch with Mr. G this past Thursday. We spent over two hours discussing our lives. We discussed everything from our families to our journey.

This blog post is not about a lunch reunion that many of us frequently have. I recognize many of us catch up with friends and colleagues, and that this meeting over lunch doesn’t seem unique.

I share this lunch meeting story because Mr. G discussed a message about life from which we could all learn.

When I was a junior in high school, Mr. G shared the “Mr. G Story” on our last day of class. With me being in his first ever class, he was in his early 20s giving his students – just a few years younger – some valuable lessons on life. Being a Purdue University physics and engineering major, a lot of his story was about keeping the pace going, fighting the urge to stop, and finding time to enjoy what makes you happy.

Part of our lunch conversation on Thursday was about the gist of the Mr. G Story, which had become grayer and fuzzier in my mind with time. I asked Mr. G to share the gist with me, and he deferred answering my question until he could look at his notes. He told me his story had changed some over time, but the overall principles were still the same. The gist of the Mr. G Story is to answer key questions about your life, and – yes – you must answer them:

1) What do you enjoy doing most?  When are you most satisfied in your life?

2) What are your best attributes, according to you?

3) How did you get to this point in your life?  Do you like where you’re going?

4) What do you want your life to stand for?  What do you want your name to mean?

5) If money was no object, what would you do with your life?

6) What will constitute you being a success in your life?  How will you know when you’ve succeeded?

7) Will your intended career path pay you a salary that lets you live the lifestyle you prefer? (put simply – will you make enough money to be happy?)

These seem like simple questions, but if you revisit them a time or two, you realize they aren’t as simple as they seem. Even if one of these questions seems simple, the next one may not be so simple. Some of these questions make you question what you are doing with the limited number of days you have on Earth.

Some could look at me, a meteorologist working at a great station in my hometown, and say I was successful. There are, however, so many things in my life that are incomplete or could use a change. I know I’ve made my mother proud, and I believe my late father would be proud of what I have done with my life. Both of them either say or would say my life is not about them though; it’s about me being in charge of my own life.

Question 6 hits me the hardest. How do you know when you are a success? What do you do once you’ve achieved success? What’s next?

I hope these questions have meaning to you, and I hope you revisit some of them before you close this webpage, end your day, or end your week. The answers to these questions can take a lifetime. If a man inspired me to contact him and have lunch years after I left his classroom, the questions he poses to all his students can be asked of all of us.

Ask yourself the tough questions. I’ve already printed the questions above and placed them over my desk at home. When a tough decision comes, I’ll be asking myself these questions until I know the answers.

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Why Josh Knight Will Be Missed

You might know him as a a meteorologist. You might know him as a feature reporter covering local events and businesses. You might know him as both. Regardless of how you know him, he will be missed.


Josh is exactly the person you see on-air. That’s not always the case in the TV business, but it definitely is with Josh. You see his bright, bubbly personality on television, and that’s the way he is when the red light on the camera goes off.

I often joked with him off camera that he is every ray of sunshine on a cloudy day. When something “less than optimal” came his way, he found a way to make it positive. That thing that would make most of us say “I really have to do this?” became “This is a great learning experience” to him. He always found a way to make put a bright spin on a bad situation…or at least try to bend it positive.

While his positive bias has served him well, Josh is also a very good meteorologist. When I say very good meteorologist, I mean it. An Ohio Valley forecast is always a challenge. When my shifts followed his, I rarely made changes. He would often say – either through notes or in person – that his confidence in certain weather situations wasn’t high; despite what he says, his forecast is ahead of the curve. He takes the time to look through lots of model data and make reasonable decisions that made the most sense. Where I struggle in pattern recognition and specifics, he excels. When my confidence to break away from computer guidance is weak, his is strong. When I bend away from guidance, he bends more. His confidence makes me more confident. He did this without bragging or boasting; he did it without even knowing it.

Josh is moving to WJLA in Washington D.C., closer to his hometown. As a guy who works in his hometown, there is no place like home, and it’s great to be home. I am happy for him because he will use his talents to the fullest there. WJLA is a station I respect and many in D.C. respect, and they are adding a great meteorologist to their team. His problem solving and critical thinking skills are top notch, and he will be a great asset to WJLA.

I hope you join Josh one last time on Good Morning Cincinnati from 8am to 9am and from 11am to 12pm.

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What Happened Near New Vienna, Ohio Friday Afternoon?

It’s not often I get to go out and survey storm damage. I’m usually in a studio under bright lights. When storms hit today, the newsroom dispatched me into the field. Initially, I saw tree damage along Pausch Road near Leesburg, Ohio:


This photo was taken looking northeast and all of the downed trees are pointing towards the southeast, where radar suggested the winds from the storm were pointing to. In nearly the same spot and facing the opposite direction, damage to barns suggested a northwest wind when it occurred. There was siding in the field from the leftmost barn pictures just southeast of the barn:


With all of the damage fanned out in a uniform direction, this suggested straight-line winds caused this damage.

Shortly after we left the scene to head home, the newsroom directed us to a damaged home north of New Vienna, Ohio. Here’s the approximate location of the house relative to New Vienna:


Imagine what I felt arriving the scene and seeing this:


Whoa. What could cause this? I immediately went into investigation mode. Here’s a wide shot of this house and the yard around it:


Notice anything, even that this resolution? Most of the debris is to the left of the house. With this photo looking southeast, most of the debris is on the east or southeast side of the house, including all of this debris along the road:


Closer to the home, I found this wood board driven into the ground:


Whoa. That’s some force. The home owner (pictured above) is actually an electrical engineer at General Electric. He was thinking like I was; he wondered how there could be all of this debris so far away from the house, especially east of the house. The wind was coming from the northwest at the time; if damaging straight-line wind was the cause of this damage, why was there so much damage to the east of the house (including large, heavy parts of the walls)? In addition to the debris field, that board driven into the ground suggested to me this was a tornado.

After we shot our video at the house, we drove through New Vienna (north on State Route 73); there was a lot of tree damage there:


I did not see any structural damage, and all of the tree damage seemed to lean towards the south, east or southeast. The alignment of buildings and tree along the road reminded me of the Venturi Effect, possibly explaining how winds were accelerating through the town. More on the Venturi Effect is here: http://www.tech-faq.com/venturi-effect.html. In other words, winds – moving northwest to southeast through the town, or basically down S.R. 73 – were accelerating or at least traveling through the town like this:


This damage appears to be caused by straight-line winds. As I drove home, I had a visual of what the radar data might look like. While I had looked at radar briefly in real-time as the storm moved through Highland County, I had not looked at the radar data in detail.

Here is the radar loop from 3:04 to 3:49pm for this Highland County storm:

Here is the storm relative velocity (the Doppler part of Doppler radar or how the winds are moving relative to the radar [minus the motion of the storm to see rotation] in Wilmington, Ohio) loop of this storm from 3:04pm to 3:49pm:


From the radar’s lowest scan angle, red colors are winds moving away from the radar, green colors are winds moving towards the radar, and yellow colors are severe winds moving away from the radar. So the overall wind flow relative to the radar looked like this:


There’s no strong rotation here. Radar suggests mainly outflow winds. But there’s more! Let’s look specifically at the radar snapshot around 3:15pm:


There’s no hook echo or strong inflow notch. Let’s look at the base velocity data for the same area:


Winds were moving away from the radar near New Vienna at the time damage occurred. Normally, strong winds towards and strong winds away from the radar are close together near a tornado. So there’s no tornado right? Not so fast. The magnitude of the wind speeds near New Vienna matter:


See how wind speeds over New Vienna are stronger than where the blue arrow is? Imagine a pinwheel facing the sky just north of New Vienna. Which way would it rotate? Counter-clockwise…like most tornadoes do. If you’re having a tough time visualizing this, see what normalized rotation looked like:


The green area shows significant counter-clockwise rotation based on raar; in other words, this is where radar is detecting rotation and the possibility of a tornado.

The National Weather Service in Wilmington is responsible for determining if damage was from straight-line wind or a tornado. I don’t know if they will survey this Saturday. Based on the damage I saw, the Leesburg damage looks to be from straight-line wind, but the New Vienna damage is more complicated. After seeing it with my own two eyes, the damage north of New Vienna looks tornadic, but the damage in town is a close call.

We will see what the verdict is from the NWS!

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Remembering The Harrison Area F4 Tornado 25 Years Ago

When you think of Tri-State tornadoes, you may think of March 2, 2012 or April 9, 1999. If you’ve lived in the Cincinnati area for a while, you may also remember April 3, 1974. The tornadoes of June 2-3, 1990 are often forgotten because there were no fatalities from tornadoes that night. There were, however, roughly 40 injuries in southeastern Indiana and southwestern Ohio during the event; all of these injuries came from an F2 tornado extending from Ripley to Dearborn County and from an F4 tornado extending from Dearborn to Warren County:


The highest rated tornado in the Tri-State that night was an F4 that went through Harrison, Ohio and also caused damage up to Mason. This tornado began two miles west of Bright, Indiana and continued into northwestern Hamilton County, where 32 homes and five businesses were destroyed. Two 18-inch, 75-foot long, 5/8″ steel beams designed to withstand winds up to 250mph were twisted to the ground at a restaurant in Harrison. The tornado continued into southern and southeastern Butler County where 19 homes and 4 mobile homes were destroyed. 58 homes, 22 mobile homes, and five apartment buildings were damaged. The tornado ended about 1 mile southwest of Mason, Ohio in Warren County.

A little known fact about the Harrison area tornado is that the path of the tornado is not continuous despite official records listing the damage from Bright to Mason as one tornado. The tornado briefly lifted near New Baltimore, Ohio and settled back to the ground in Colerain Township near Pippin Road. While the tornado lifted, the path’s interruption was brief enough to count as one tornado per NWS directives. Current NWS directives (specifically, NWS Directive 10-1605) state that if a tornado’s path is interrupted for more than 2 miles OR more than 4 minutes, the tornadoes will be rated separately. This NCDC website suggests “a tornado that lifts off the ground for less that [sic] 5 minutes or 2.5 miles is considered a separate segment. If the tornado lifts off the ground for greater than 5 minutes or 2.5 miles, it is considered a separate tornado.” Official NWS records (from the National Climatic Data Center) suggest the tornado lifted near New Baltimore, Ohio at 10:10pm EST and touched down again in Colerain Township, Ohio at 10:14pm EST. While close to being two separate tornadoes, official records list the damage from Dearborn County to Warren County as one tornado. It is unclear to me which definition is correct and/or was the correct definition at the time. Here is a map showing the tornado’s path on the evening of June 2, 1990:


The F2 tornado that affected Ripley and Dearborn County that night produced 3 injures and caused mainly tree damage. The Hopewell Church in western Ripley County near Holton was, however, destroyed.

Radar images of this event are of poor quality, but it appears that a cyclic supercell was responsible for the damage extending from Holton to Mason. Other, weaker tornadoes were confirmed in Boone, Clermont, and Clinton County that night.

No one died in the Harrison area tornado of June 2, 1990. Including this one, there have only been three violent (F4, F5, EF4, or EF5) tornadoes in the Tri-State since 1950 that have not produced fatalities (the others being on April 3, 1974 and April 25, 1964).

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