The term "temperature humidity index" (THI) is a number which aims to establish the degree of discomfort of the average individual due to the ambient temperature and the relative humidity.
It is indicated by a number. The THI is calculated by a mathematical formula as follows: THI = .4 of the sum of the wet and dry bulb temperature plus 15.
Example No. 1. On a certain day, the wet bulb temperature was 70 degrees and the dry bulb temperature was 80 degrees.
The THI = .4 x (70 +80) + 15
= .4 x (150) + 15
= 60 + 15
= 75
From this it may be seen that the relative humidity as indicated by the wet bulb temperature is quite important in establishing the temperature humidity index.
Example No. 2. On a certain day, the dry bulb temperature was 80 degrees and the wet bulb temperature was 75 degrees.
The TI-11 = .4 x (80 + 75) + 15
= .4 x (155) + 15
= 62 + 15
= 77
In example No. 1, the relative humidity was 61% and in example No. 2 it was 79%. The dry bulb or ambient temperature was the same on both days, however, the THI is two points higher on the second day due to the higher humidity.
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Basics of Refrigeration