Is the wet bulb temperature always higher than the dry bulb temperature and what temperature does the wet bulb have to reach to restrict outdoor practice?
The wet bulb temperature is always lower than the dry bulb temperature but will be identical with 100% relative humidity (the air is at the saturation line). Combining the dry bulb and wet bulb temperature in a psychrometric diagram or Mollier chart, gives the state of the humid air. Remember, evaporation is a cooling process. Therefore, the drier the air, the faster moisture evaporates, and the lower the web-bulb temperature. Wet-bulb is always lower than dry-bulb. It is always lower than the dry air temperature because as evaporation takes place, heat escapes with the water, hence cooling down the thermometer's bulb. When WBT is low, that means the air is dry, and water vapour can easily evaporate. The term comes from how it is measured. If you slide a wet cloth over the bulb of a thermometer, the evaporating water from the cloth will cool the thermometer down. This lower temperature is the WBT, which cannot go above the dry temperature. If there is 100% humidity, dry-bulb and wet-bulb temperatures are identical, making the wet-bulb depression equal to zero in such conditions. As vapor pressure reduces at the dew point so is the partial pressure of water vapor in the air. The ratio remains = 1 or 100% Thus at saturation WBT and dew point temperature are different. Dews always generate at a lower temperature while RH remains 100%. Specific guidelines and restrictions are in place for wet-bulb globe temperatures between 82.0 °F (27.8 °C) and 92.0 °F (33.3 °C); wet-bulb globe temperatures of 92.1 °F (33.4 °C) or greater require all outdoor activities to be canceled. An evaporative cooling tower can generally provide cooling water 5°F-7°F higher above the current ambient wet bulb condition. That means that if the wet bulb temperature is 78°F, then the cooling tower will most likely provide cooling water between 83°F- 85°F, no lower. The wet bulb temperature will always be lesser than the dry-bulb temperature, except when there is 100% relative humidity. Wet bulb globe temperature is a type of apparent temperature used to estimate the effect of climatic conditions and is used to assess potential heat-stress on humans.
- Badges
- Science topic