OMAHA, Neb. (KMTV) — "It's not the heat, it's the humidity!"
Over the last couple of days, eastern Nebraska and western Iowa have been sweating in the intense heat and humidity. On Wednesday, heat index (how it feels) values reached as high as 115° in Omaha, while we could see the same today. A lot of the reason why this heat is so intense is due to the humidity. Humidity is something those in the eastern U.S. have to deal with, either from the Gulf of Mexico or evapotranspiration from corn (to be discussed later).
In regards to humidity, you might hear the terms "dew point," "relative humidity," and "heat index." You might be confused about the terms we use, what is the best measure of humidity? How is the heat index calculated? What does all of it mean? Find out below!
What is Humidity? Humidity, or moisture, refers to the amount of water vapor in the atmosphere. If you remember middle school Earth science, once the water reaches a certain temperature it evaporates and rises into the atmosphere. The air accumulates water as it rises, it cools and eventually forms clouds. That period between evaporation and condensation is water vapor and mixes in with the air. The more water vapor there is, the more humid it feels.
What is dew point? The dew point is the exact measure of moisture in the atmosphere. Technically, it is the temperature at which air can be cooled down to achieve saturation or 100% relative humidity. Technical definition aside, the dew point is often better to determine how humid the air outside is. Normally, when we reach a dew point of 60°, that is when it begins to feel "humid" outside. Anything approaching 70° is very humid, and 80° is downright tropical. Omaha hit a dew point of 80° on Wednesday, which is why you began sweating the second you walked outside.
What is relative humidity? The dew point and air temperature are used to get relative humidity, which is the percentage of humidity in the atmosphere. You get this number by dividing the dew point by the air temperature. So for example, if it is 90° outside with a dew point of 60°, your relative humidity will be 66%. The major caveat to using relative humidity to gauge humidity is that it will always seem lower. Take Wednesday, with a high of 96 and a dew point of 80. Despite it being more humid than the Amazon rainforest, the relative humidity is 83%, which does not seem that high.
What happens if the relative humidity reaches 100%? We achieve saturation, which is a fancy way of saying condensation. Commonly, we achieve relative humidity of 100% during the overnight hours, which can lead to fog or dew forming on grassy surfaces. Higher in the atmosphere, as relative humidity approaches 100%, clouds form as water vapor condenses. Once enough occurs, rain happens.
So, what is it about humidity that makes it feel hotter? As our body temperature rises, the way we regulate our temperature is through sweat. Once our sweat begins to evaporate, it cools the body off. However, when there is already a large amount of water vapor in the atmosphere, sweat can not evaporate as quickly. Thus, when it is humid we feel hotter and are more sweaty. The temperature that it feels on your skin is the heat index, where the air temperature and humidity are combined.
How do we get the heat index? Through this equation:
Heat Index = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 6.83783 x 10-3T2 - 5.481717 x 10-2R2 + 1.22874 x 10-3T2R + 8.5282 x 10-4TR2 - 1.99 x 10-6T2R2
T - air temperature (F)
R - relative humidity (percentage)
...Yeah, let's just stick to the TV graphics...
Where do we get our humidity from? In the corn belt, we get our moisture from two sources. The first source is the Gulf of Mexico, where the moisture-laden air is pulled northward into our region. Outside of the summer, this is our main moisture source. However, the corn belt is special in that we have a second source of humidity, corn sweat! No, I'm not making this up, corn sweat is a factor in our humidity. Like humans, corn emits water to cool itself off, and it emits a lot of water. One acre of corn can emit 4,000 gallons of water per day! In Iowa alone, the average day will see corn emit 46.9 billion gallons a day! That much water being pumped into the atmosphere certainly adds to the water vapor and raises our humidity. This is mostly what happened on Wednesday.
So, next time you exclaim in the classic phrase "It's not the heat, it's the humidity!". You are indeed correct!
