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Updated 26 Sept 2016 (What’s New?)

Dew Point, Relative Humidity, and Heat Index on TI-83/84

Copyright © 2016–2017 by Stan Brown

Summary: With the unseasonal heat wave in Finger Lakes region during the last week of May 2016, I started thinking about that old saying, “It’s not the heat, it’s the humidity.” Well, it’s not the humidity either, or not exactly. Two other measures of discomfort are the dew point and the heat index. You may not be able to beat the heat, but you can compute it with my DEWPOINT program.

The program runs in Celsius or Fahrenheit. It asks for the dew point and computes the relative humidity, or asks for the relative humidity and computes the dew point. Finally, it computes the heat index.

The equations I used for dew point are valid in the range 0–60 °C (32–140 °F); the program won’t accept temperatures outside that range. Heat index as a concept is valid when the temperature is above 80 °F (27 °C); the program displays “n/a” for heat index if the temperature is lower than that.

Contents:

See also: It’s Not the Heat … contains all the equations, and an Excel workbook. The article and the workbook cover all the calculations in this article, plus computing dew point or relative humidity from heat index, which the TI-83/84 program can’t do.

Getting the Program

There are three methods to get the program into your calculator:

Running the Program

The same program runs on black-and-white and color TI-83+s and TI-84+s, including the C and CE models. It won’t run on the “classic” TI-83 (without the “Plus”).On the color models, screens will take advantage of the higher resolution, but the program produces the same information on color and b&w models.

The menu screen asks you to choose between Fahrenheit and Celsius temperatures. It also asks you whether you have the dew point and want the relative humidity, or the other way around. Heat index is always computed when the temperature is 80 °F (27 °C) or above, so the menu doesn’t ask you about heat index.

splash screen for DEWPOINT program       selection of what you have and what you want see text

Examples

Fahrenheit temps / Temperature 90 / Dew point 39.3 / Heat index 86.1 / Relative humidity 17% Example 1. The temperature is 90 °F, and the relative humidity is 17%. What’s the dew point?

Select 1 in the menu, and enter the temperature and humidity. You get the results shown here, with heat index 86.1 °F and dew point 39.3 °F.

The relative humidity is how much moisture is actually in the air, relative to the amount that the air could hold just condensation starts. At higher temperatures, the air can hold more moisture, so 17% relative humidity at 90 °F represents more actual moisture than 17% at 70 °F, but less than 17% at 110 °F.

(This is why you see condensation on the outside of your glass of iced tea. The moisture content of the air is not great enough to cause condensation at the air temperature, but the air is cooler right next to the glass, and that cooler air can’t hold that same amount of moisture.}

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The dew point is the temperature at which the amount of moisture currently in the air would start condensing out. The lower the dew point, the less moisture there is in the air, regardless of actual temperature. Thus the dew point is a better measure of comfort than the relative humidity. Most people feel uncomfortable with dew point above 65 °F, and above 70 °F is uncomfortable for almost everyone.

In this example, there’s very little moisture in the air, so little that the temperature could drop a bit below 40 °F and still the air would hold all of it. The air around you can easily dry you off via perspiration, so you don’t feel too uncomfortable, even in high heat.

Another way to look at this is the heat index. With 17% humidity at 90 °F, the heat index is 86.1 °F, indicating that it feels several degrees cooler than the actual temperature. (I’m not sure on what basis this “feel” is quantified, but everyone finds 90 °F oppressive in a humid East Coast or Midwest summer, and much less so in the desert.)

Fahrenheit temps / Temperature 85 / Dew point 68 / Heat index 88.3 / Relative humidity 57% Example 2. The temperature is 85 °F, and the dew point is 68 °F. What’s the relative humidity?

Select 2 in the menu, enter the temperature and dew point, and you get the results shown here. The heat index is several degrees warmer than 85 °F, indicating that it feels warmer than the actual temperature. The relative humidity is 57%, which would be quite comfortable in cooler weather but is uncomfortable for many people in summer, particularly for sleeping.

Celsius temps / Temperature 90 / Dew point 3.9 / Heat index 29.8 / Relative humidity 17% Example 3. The temperature is 32 °C, and the relative humidity is 17%. What’s the dew point?

Select 3 in the menu, and enter the temperature and humidity. The results are shown here. The dew point is 3.9 °C, indicating that the air holds very little moisture, and the heat index is 29.8 °C, indicating that the dryness makes you more comfortable (by carrying perspiration away more efficiently).

Fahrenheit temps / Temperature 29 / Dew point 20 / Heat index 30.8 / Relative humidity 58% Example 4. The temperature is 29 °C, and the dew point is 20 °C. What’s the relative humidity?

Select 4 in the menu, enter the temperature and dew point, and you get the results shown here. The high moisture content in the air, shown by the dew point of 20 °C, contributes to the heat index a couple of degrees above the actual temperature.

Program Variables

The DEWPOINT program uses several variables. They’re listed here because you might have occasion to use some of them after the program finishes:

References

NWS, US National Weather Service. n.d.
The Heat Index Equation. Retrieved 29 May 2016 from http://www.wpc.ncep.noaa.gov/html/heatindex_equation.shtml
Paroscientific Precision Pressure Instrumentation. n.d.
MET4 and MET4A Calculation of Dew Point. Retrieved 29 May 2016 from http://www.paroscientific.com/dewpoint.htm

What’s New

Because this program helps you,
please click to donate!
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please donate at
BrownMath.com/donate.

Updates and new info: https://BrownMath.com/ti83/

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