Cooling limit temperature

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The cooling limit temperature , measured as wet bulb temperature  , is the lowest temperature that can be achieved by direct evaporative cooling . The water released by a moist surface is in equilibrium with the water absorption capacity of the surrounding atmosphere . Due to the evaporation cold , the cooling limit temperature is below the air temperature, depending on the relative humidity :

Legend:
t FK ... wet bulb temperature
t L ... air temperature before the start of cooling
φ ... relative humidity of the air (initial condition)

Explanation:
The diagram shows the temperature curve to which humid air can be cooled by adiabatic energy extraction . It falls as the initial humidity of the air decreases. In other words, an air volume of a certain relative humidity can give off more heat than the same amount of air at the same starting temperature and the same ambient pressure but with a higher relative humidity.

The temperature drop is greater, the drier the surrounding air; conversely, the air humidity can be determined from the temperature difference.

The wet bulb temperature is measured by a psychrometric measurement with a thermometer , which is provided with a moistened fabric or cotton cover ( psychrometer ). With this measurement, the wet bulb is well ventilated so that no boundary layer forms on it in which the ambient air carries a locally higher proportion of vapor .

The cooling limit temperature is (except for a relative humidity of 100%) higher than the temperature of the dew point :

The cooling limit temperature is relevant wherever liquid is evaporated in large quantities. The cooling effect can be a desired goal (e.g. with evaporative cooling , wet cooling , water injection ) or a side effect (e.g. in drying , convection drying).

Load limit of the human body

In human biometeorology , the cooling limit temperature is used to indicate heat stress . It is either used directly or is included in the calculation of indices that quantify the perceived temperature and thermal stress, for example in the WBGT index ( wet bulb globe temperature ).

A sustained cooling limit temperature of over 35 ° C (95 ° F) is dangerous for the human body - even for healthy people in the shade - and leads to hyperthermia . Above this temperature value, the body can no longer give off heat to the environment, but absorbs the heat. Sherwood and Huber (2010) assume that a person can survive cold limit temperatures of about 35 ° C for about six hours. According to this study, the highest measured wet bulb temperatures in most places on earth are around 26 to 27 ° C, with maximum temperatures of around 30 to 31 ° C also being measured. It shows that the wet bulb temperatures on earth are very similar across some regions. While the highest air temperatures are reached in deserts , the cooling limit temperature there is no higher than in the tropics due to the low humidity.

In the long term, global warming that is not contained can lead to rising wet-bulb temperatures, so that in a few parts of the world people can no longer survive. According to a prognosis by Im et al. from 2016, such conditions could be achieved in parts of South Asia towards the end of the 21st century. The highest values ​​of the cooling limit temperature are measured in the Gulf region . In 2015, the cooling limit temperature there approached 35 degrees Celsius. A cooling limit temperature of 35 ° C is achieved, for example, at a relative humidity of 50% and an air temperature of 46 ° C at the same time.

Individual evidence

  1. ^ Grassmann et al .: Introduction to thermal process engineering , 3rd edition, Walter de Gruyter Berlin 1997.
  2. ^ RE David, GR McGregor, KB Eneld: Humidity: a review and primer on atmospheric moisture and human health . In: Environmental research . 2016, doi : 10.1016 / j.envres.2015.10.014 .
  3. ↑ a b c d Steven C. Sherwood, Matthew Huber: An adaptability limit to climate change due to heat stress . In: Proceedings of the National Academy of Sciences of the United States of America . tape 107 , no. 21 , May 25, 2010, p. 9552-9555 , doi : 10.1073 / pnas.0913352107 .
  4. ↑ Eun-Soon Im et al .: Deadly heat waves projected in the densely populated agricultural regions of South Asia . In: Science Advances . tape 3 , e1603322, 2017, doi : 10.1126 / sciadv.1603322 .
  5. ↑ Jeremy S. Pal, Elfatih AB Eltahir: Future temperature in southwest Asia projected to exceed threshold for human adaptability . In: Nature Climate Change . ( nature.com ).
  6. ↑ Otto Wöhrbach: Human air conditioning is reaching its limit. Sweating cools you down - but not always and everywhere. Climate change is threatening more and more regions of the world with heat death. ( Online version under other title ) In: Der Tagesspiegel , August 27, 2019, p. 19.
  7. ↑ Persian Gulf could be too hot for humans by 2100. In: The Telegraph . AFP , October 26, 2015, accessed on August 28, 2019 .

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