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Explaining heat index with cups of water and fun experiments

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BLUEFIELD, WV (WVVA) - It is that time of the year again, as the heat and humidity of summer have both made a return. When we turn up the heat and humidity, we often talk a lot about dew points, humidity, and heat index. But what do these actually mean?

The definitions of dew point and relative humidity have a difference.

The dew point temperature is the temperature that air needs to be cooled to in order to achieve saturation.

The atmosphere reaches saturation once we reach 100% relative humidity. Once saturation is reached, the atmosphere has to produce some sort of condensation. This is why we wake up in the morning and see dew or frost on the grass, sometimes you even get a mist in the mornings if the atmosphere is completely full of moisture.

The definition of relative humidity is the amount of water vapor in the air, compared to how much the atmosphere can hold at a given temperature.

Notice how the cups get larger and can hold more water at higher temperatures. This is similar to our atmosphere

The temperature of the atmosphere is very important when it comes to dew point and relative humidity. Using the example above, notice how each cup is halfway full of water, which resembles 50% relative humidity for each cup. Also, notice how the size of the cups are gradually different. The greater the temperature, the bigger the cup. This is to resemble our atmosphere, as the air can hold more moisture at higher temperatures.

Heat index in the summer is the opposite of wind chill in the winter. It is basically the feels like temperature that are body reacts to.

This does get a little tricky when we add in the heat index. The heat index is the temperature at which your body thinks it is. This is because in some instances, your body is working harder to cool you off.

Sweating leads to a process known as evaporative cooling. Dew point has a large role in how correctly are bodies perform the sweating process.

Imagine if you are running through Las Vegas or Phoenix and it is 115 degrees outside, but you notice that your body is not soaking wet in sweat.

This is because sweat is evaporating from your skin rapidly, as the dew point is much lower, indicating a drier atmosphere. This means that the air is trying to take in more moisture, thus your body is sweating properly and cooling you down. So your body acts and behaves like the temperature is much lower, which would indicate a lower heat index temperature.

However, if you run through Texas or Florida, where it is a lot more humid and the dew point is much higher, your body will be soaking wet in sweat.

This is because the atmosphere already has a lot of moisture and it is having trouble accepting more. This means that sweat does not properly evaporate from your skin correctly, which does not allow your body to cool down. So your body acts and behaves like the temperature is much higher, which would indicate a higher heat index temperature.

Can you determine if the heat index will be higher or lower for each cup using the air temperature, dew point, and relative humidity?

Now that we are getting somewhere in this fun explanation, lets add in some examples. Looking at the specifications for each cup above, lets try to determine the heat index temperature of each cup. Again, notice how each cup has 50% relative humidity, but the temperatures and dew points are gradually different.

When you look at cup one. An air temperature of 55 degrees and a dew point temperature of 37 degrees will lead to a heat index temperature that is cooler than the actually air temperature. This is because the dew point is very low, meaning there is more dry air in the atmosphere, which leads to more comfortable conditions.

With cup two, the air temperature is 75 degrees and the dew point temperature is 55 degrees. Although the air temperature has increased in this scenario, the dew point is still rather dry and comfortable, so the heat index will actually feel just like the 75 degree temperature we have.

Cup three is when we really turn up the heat and humidity. Notice how the air temperature has increased to 95 degrees, but the dew point has increased significantly to 74 degrees. This means we have a very muggy airmass, leading to a much higher heat index than the actually air temperature. The heat index in this scenario would be 106 degrees.

Here are your results. How did you do?

Here are the results. Notice that the heat index temperature was not impacted by any change in relative humidity, as the relative humidity stayed the same in each example. It was the change in the dew point temperature that made the real impact on the heat index.

Now, you are probably asking, why doesn't our viewing area see a lot of heat advisories when it is abnormally hot and humid like we are experiencing this week?

Well..... There are certain criteria's for heat advisories, excessive heat watches and excessive heat warnings. Our viewing area is split between the Charleston, WV and Blacksburg, VA National Weather Service Offices. So heat criteria's are different across the viewing area.

To view the heat criteria's for the western slopes, click here and then click on the "Non-Precipitation Events" tab.

To view the heat criteria's for the eastern slopes, click here and then scroll down to the "Non-Precipitation Events" tab.

For fun experiments you can do at home with the kids, click here and here.

Brandon Lawson

Weekend Meteorologist

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