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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]
When the temperature is 30 °C (86 °F) and the dew point is 15 °C (59 °F), the humidex is 34. If the temperature remains 30 °C (86 °F) and the dew point rises to 25 °C (77 °F), the humidex rises to 42. The humidex is higher than the U.S. heat index at equal temperature and relative humidity. The humidex formula is as follows: [7] [8]
Apparent temperature, also known as "feels like", [1] [2] is the temperature equivalent perceived by humans, caused by the combined effects of air temperature, relative humidity and wind speed. The measure is most commonly applied to the perceived outdoor temperature.
The temperature of the mirror is controlled by electronic feedback to maintain a dynamic equilibrium between evaporation and condensation, thus closely measuring the dew point temperature. An accuracy of 0.2 °C is attainable with these devices, which correlates at typical office environments to a relative humidity accuracy of about ±1.2%.
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%.
Water activity is the thermodynamic activity of water as solvent and the relative humidity of the surrounding air after equilibration. As temperature increases, a w typically increases, except in some products with crystalline salt or sugar. Water migrates from areas of high a w to areas of low a w.
The wet-bulb temperature is the lowest temperature that may be achieved by evaporative cooling of a water-wetted, ventilated surface.. By contrast, the dew point is the temperature to which the ambient air must be cooled to reach 100% relative humidity assuming there is no further evaporation into the air; it is the temperature where condensation (dew) and clouds would form.
Schematic of the LCL in relation to the temperature (T) and dew point and their altitude (Z); the moist adiabatic temperature curve above the LCL is also sketched for reference The lifting condensation level or lifted condensation level ( LCL ) is the height at which the relative humidity (RH) of an air parcel will reach 100% with respect to ...