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The representation is made on a temperature-relative humidity, instead of a standard psychrometric chart. The comfort zone in blue represents the 90% of acceptability, which means the conditions between -0.5 and +0.5 PMV, or PPD < 10%.
In hot summer weather, a rise in relative humidity increases the apparent temperature to humans (and other animals) by hindering the evaporation of perspiration from the skin. For example, according to the heat index, a relative humidity of 75% at air temperature of 80.0 °F (26.7 °C) would feel like 83.6 ± 1.3 °F (28.7 ± 0.7 °C). [13] [14]
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Determining relative humidity: The percent relative humidity can be located at the intersection of the vertical dry bulb and diagonally down sloping wet bulb temperature lines. Metric (SI): Using a dry bulb of 25 °C and a wet bulb of 20 °C, read the relative humidity at approximately 63.5%.
When the two tendencies are in balance— and the air and food are stable—the air's relative humidity (expressed as a fraction instead of as a percentage) is taken to be the water activity, a w. Thus, water activity is the thermodynamic activity of water as solvent and the relative humidity of the surrounding air at equilibrium.
The LCL can be either computed or determined graphically using standard thermodynamic diagrams such as the skew-T log-P diagram or the tephigram.Nearly all of these formulations make use of the relationship between the LCL and the dew point, which is the temperature to which an air parcel needs to be cooled isobarically until its RH just reaches 100%.
RH is relative humidity (%) exp represents the exponential function; The Australian formula includes the important factor of humidity and is somewhat more involved than the simpler North American wind chill model. The North American formula was designed to be applied at low temperatures (as low as −46 °C or −50 °F) when humidity levels ...
This approach is accurate to within about ±1 °C as long as the relative humidity is above 50%: ; (); This can be expressed as a simple rule of thumb: For every 1 °C difference in the dew point and dry bulb temperatures, the relative humidity decreases by 5%, starting with RH = 100% when the dew point equals the dry bulb temperature.