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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]
A relative humidity of 100% indicates the dew point is equal to the current temperature and that the air is maximally saturated with water. When the moisture content remains constant and temperature increases, relative humidity decreases, but the dew point remains constant.
Relationship to relative humidity: The relative humidity (RH) of air in equilibrium with a sample is also called the Equilibrium Relative Humidity (ERH) and is usually given as a percentage. [2] It is equal to water activity according to E R H = a w × 100 % . {\displaystyle \mathrm {ERH} =a_{w}\times 100\%.}
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.
T a is dry-bulb temperature (°C) 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 ...
A given value of relative humidity causes larger increases in the heat index at higher temperatures. For example, at approximately 27 °C (81 °F), the heat index will agree with the actual temperature if the relative humidity is 45%, but at 43 °C (109 °F), any relative-humidity reading above 18% will make the heat index higher than 43 °C. [5]
The relationship between water content and equilibrium relative humidity of a material can be displayed graphically by a curve, the so-called moisture sorption isotherm. For each humidity value, a sorption isotherm indicates the corresponding water content value at a given temperature. If the composition or quality of the material changes, then ...
At a constant relative humidity of air, the EMC will drop by about 0.5% for every increase of 10 °C air temperature. [2] The following table shows the equilibriums for a number of grains (data from [1]). These values are only approximations since the exact values depend on the specific variety of a grain. [2]