1. Skin temperature and latent heat release Elena Saltikoff FMI (material courtesy of prof. Timo Vesala, UH) 5.10.2015Ilmatieteen laitos / PowerPoint ohjeistus1

2. Good News Jochen: …if you have no lift, the hot and moist air will only make you sweat and wish for refreshment… Good news: we have the lift …taking you to 5th floor …for refreshments and optional hot and moist air (towels provided) 5.10.2015Ilmatieteen laitos / PowerPoint ohjeistus2

3. Hot and moist air: Sauna Three million saunas in the world 4/5 of them are in Finland Smoke saunas and saunas heated by wood or electricity Originally the word ”sauna” has meant a pit in the ground or snow (especially dug by a bird to spend the night in it) Sauna of Sibelius in Ainola, photo from Wikipedia 5.10.2015Ilmatieteen laitos / PowerPoint ohjeistus3

4. Some quantitative facts*: Temperature of air is 80-110 °C (175-230 °F) Dew point is 37-43 °C, that is 40 – 60 g of water per dry air kg, relative humidity is 10-20 %. Skin temperature is 39-43 °C * Teeri N., The Climatic conditions of the sauna. In: Sauna studies (Teir, Collan and Valtakari, Eds.), 1976.

5. Skin temperatures (Ladies): (Measured by IR-radiometer, 25.5.2005) Before sauna: 34.4 (34.3-34.5) °C After sauna: 38.3 (35.7-40.7) °C After swimming in a lake 22°C (12.1°C, immediately after swimming) After cooling down 29.1 (27.1-30.6) °C Inside the sauna, lower level 41.6 °C

6. Some qualitative facts: By throwing water on kiuas (heart made of stones) the temperature and humidity are controlled Bather feels löyly (increase in humidity, not mist) as heat on the skin (why?) Bather perspires 1 – 4 litres per hour (normal rate is 1 litre per 24 hours) Feeling of “lacking oxygen” may occur (why?)

7. The essence of the matter: Even a slight increase in humidity would suppress vaporization from the skin (sweating) Vaporization might reverse to condensation (same process but opposite character) Energy is transformed in the form of enthalpy from the kiuas to the skin

8. We have experienced latent heat Latent heat cooling, evaporationCondensation 5.10.2015Ilmatieteen laitos / PowerPoint ohjeistus8

9. Conclusions: Latent heat (including ceasing vaporization) Sensible heat

10. 5.10.2015Ilmatieteen laitos / PowerPoint ohjeistus10

11. Postscript: Oxygen content of sauna air corresponds to that at the altitude of nearly 3 km. Heat exchange due to (black body) radiation is about 1000 J/s.

12. Latent heat seriously Complex process of simultaneous forced convection and non-steady diffusion around a bather is treated by means of a stationary semiempirical Sherwood number for a sphere corrected by a factor for non-steady pure diffusion: h(x,t) = f(x) g(t) According to Aristotle the re-evaluation by Kulmala, Vesala, Schwarz and Smolik (Mass transfer from a drop – II. Theoretical analysis of temperature dependent mass flux correlation, Int. J. Heat Mass Transfer, 1995) can be neglected and we write for the latent heat flux: