Heat Index (HI) - marharyta/apparent-temperature GitHub Wiki

Heat Index (HI)

As defined by the glossary of Biometeorology heat index is “an apparent temperature calculation designed to determine the temperature that the human body “feels” when its evaporative cooling mechanism (perspiration) is limited due to increased relative humidity. The variables required to calculate the heat index (HI) were established originally by Steadman (1979b), but the current operational equation was created through multiple regression statistical analyses by Rothfusz (1990)”.

The Rothfusz formula as described in a picture above was used in the initial calculations as provided by the glossary of Biometeorology. After conducting the first tests, it became clear that the formula was either provided in an incorrect format or had wrong coefficients: the HI calculated with 25 C and relative humidity of 70% was 205 C, which is way outside of the normal range, whereas the expected outcome was 26 C. In relative humidity of 50% and temperature of 20 C the Heat Index is 108.80607156 C, which is very far from the table calculations.

According the the table from Methods to Calculate the Heat Index as an Exposure Metric in Environmental Health Research (Anderson, G. B., Bell, M. L., Peng R. D. 2013.)

Figure 25. (Anderson, G. B., Bell, M. L., Peng R. D. 2013.)

After analyzing the original research provided in the references from the encyclopidia of Biometeorology, In the original research (Rothfusz, L. P. ) the temperature was linked as Fahrenheit, which would make the results unapplicable in case if people provided Celsius values.

Figure 26.

If we take a Fahrenheit equivalent of the values and see the output, the values calculated by our formula match the ones provided in the Figure 25:

Figure 27. URL: https://www.weather.gov/safety/heat-index

The accuracy of Heat index is a metrics should be under a question, since there is a big number of algorithms to calculate heat index in literature. According to Methods to Calculate the Heat Index as an Exposure Metric in Environmental Health Research (Anderson, G. B., Bell, M. L., Peng R. D. 2013.) There are more than 20 methods to calculate Heat Index. After Steadmans formula a lot of researches were conducted to derive a more specific formula, yet all of them return more or less similar data. (Anderson, G. B., Bell, M. L., Peng R. D. 2013.)

According to the research, the algorithm that performed best was Heat index algorithm software, the NWS algorithm, provided heat index values that were, on average, within 0.4°C/0.7°F of the original Steadman values for both relative humidity and dew point temperature tables. “Agreement for weather conditions within Steadman’s original tables. Between Steadman’s two original tables (1979a), heat index values calculated using Steadman’s original physiological models are available for air temperature between 20°C/68°F and 50°C/122°F, relative humidity between 0% and 100%, and dew point temperature between 0°C/32°F and 30°C/86°F” (Anderson, G. B., Bell, M. L., Peng R. D. 2013.)

Figure 28. (Anderson, G. B., Bell, M. L., Peng R. D. 2013.)

After further testing the library used to calculate HI was rewritten to support NWS algorithm for more precise numbers.

This table demonstrates values of heat index for different combinations of air temperature, and relative humidity:

Figure 29. (Steadman’s two original tables. 1979)

After testing Celsius the formulas do not provide exact values.

Figure 30. URL: https://www.weather.gov/safety/heat-index

To test model for calculations, I will be checking with the table from Figure 28. It is possible that an error (slight deviation from the table values towards -1 C is due to error in conversions from Fahrenheit to Celsius units).