Heat index, WBGT and wind chill

0:00 Intro

Heat index, WBGT and wind chill calculations, for the CHST exam. Every rule and number here is cited to its primary source, on screen.

0:13 Learning objectives

In this lesson, you'll learn to: compute indoor and outdoor WBGT, choose the right WBGT formula for the setting, apply the National Weather Service heat index regression, and calculate wind chill from temperature and wind speed.

0:30 WBGT

WBGT, the wet bulb globe temperature, has two formulas. Which one you use depends on the setting. 3 times globe temperature. 1 air. Symbols are tested: t-nwb is natural wet bulb and t-g is globe temperature.

1:06 Worked example

Air temperature, t-a, is the third input, and it appears only outdoors. Example: a foundry floor, indoors. Natural wet bulb is 24 degrees, globe temperature is 40. Indoors, so use the indoor formula. 8. 3 times 40 is 12. 8 degrees C WBGT.

1:36 Worked example

Remember, the indoor formula has no air temperature term, only natural wet bulb and globe temperature. Now outdoors, a roofing job. Natural wet bulb 25, globe 45, air temperature 35. 5. 2 times 45 is 9. 5. The sum is 30 degrees C WBGT. 0, the wrong answer. The heat index combines air temperature in degrees F with relative humidity in percent.

2:12 Heat index

In practice, the simple formula is computed first, and the result averaged with the temperature. If that value is 80 degrees F or higher, the full regression equation is applied. The regression is the long polynomial in T and RH.

2:41 Worked example

Any adjustment the National Weather Service describes is then applied. Heat index and WBGT are different measures. Example: 90 degrees F, 70 percent relative humidity. Assume the first-pass value is 80 or higher, so the full regression applies.

3:09 Wind chill

Plug T equals 90 and RH equals 70 into the regression. The result is a heat index of about 106 degrees F. It feels about 16 degrees hotter than the air temperature. Remember, heat index and WBGT are different measures.

3:42 Worked example

Wind chill uses air temperature, T, in degrees F, and wind speed in miles per hour. Convert first if your data is in other units. 16. T is in degrees F and wind is in miles per hour. The result is the wind chill in degrees F. Example: a cold storage yard. Air temperature is 20 degrees F, wind 15 miles per hour. 54. 2, so the wind chill is about 6 degrees F.

4:24 On the job

Wind chill is a cold-weather calculation, not WBGT or heat index. Convert units before you plug in. That is 14 degrees colder than the air temperature alone suggests. Always report the wind chill with the units used. You're the safety manager on a roofing job.

4:57 Common exam traps

Your meter reads natural wet bulb 26, globe 46, air 34. The crew is outdoors, so use the outdoor formula, with the air temperature term. Indoors, that term is not used. 4. 8 degrees C, calculated with the outdoor formula. Now, five traps that catch people on exam day. 1 air temperature. Two: indoor WBGT has no air temperature term.

5:33 Recap quiz

Three: if the simple-formula heat index, averaged with the temperature, is 80 degrees F or higher, the full regression equation applies. Four: heat index takes degrees F and percent humidity. Five: wind chill needs degrees F and miles per hour. Convert first. Question 1. Outdoors, natural wet bulb 25, globe 45, air 35, all degrees C. What is WBGT? A: 28. B: 30. 5. D: 35. B. 5 is 30. Question 2. Indoors, natural wet bulb 24 and globe 40. 4. 2. 8. D: 32. 8. 8 plus 12. Question 3. Air at 20 degrees F with a 15 mile per hour wind. Wind chill is closest to: A: 20. B: 14. C: 11. D: 6. D, about 6 degrees F. Want to see where you stand on every CHST domain? com.

25 questions, free, no card. You get a readiness score for each CHST domain.