Why 37 C 80% rH feels far worse than 37 C 40%

"The heat of vaporization of water is the highest known. The heat of vaporization is defined as the amount of heat needed to turn 1 g of a liquid into a vapor, without a rise in the temperature of the liquid. This term is not in the list of definitions given by Weast (1964), so the definition comes from Webster's New World Dictionary of the American Language (1959). The units are cal/gram and values for the heat of vaporization of water at different temperatures are given in Table 3.1. The heat of vaporization is a latent heat. Latent comes from the Latin latere, which means to lie hidden or concealed. Latent heat is the additional heat required to change the state of a substance from solid to liquid at its melting point, or from liquid to gas at its boiling point, after the temperature of the substance has reached either of these points. Note that a latent heat is associated with no change in temperature, but a change of state. Because of the high heat of vaporization, evaporation of water has a pronounced cooling effect and condensation has a warming effect (Kramer, 1983, p. 8)."

The above is standard University Physics. We live in the equatorial or tropics where it is hot and wet most of the year. This year for the El Nino, we are experiencing searingly hot days interspersed with some rainfall. On the searingly hot days, we in the tropics seem to feel that we are under some heavy 'cloud'. That feeling is the humidity in the air. Our bodies are magnificent machines which have the ability to moderate and keep ourselves relatively hot or cold when the temperature changes - but up to a certain extent only.

So my post title is this, why does 37 C and 80% which is very humid feel so much worse than in the temperate countries where the humidity is much less ?

The answer is because, when it is hot, our bodies will sweat naturally, and since sweat is 90% water, it will stay on our body. The fact that water evaporates, because there is heat needed - which is from the body - to make it change to vapour without any drastic change of the external temperature. 

So, when that happens, a loss of heat or energy from our bodies translates to a cooling effect when water evaporates from our skin. This is presumed so, WHEN the air is dry. Hence, the feeling is that there is some cooling, even at 37 C.

However, when the air is damp or humid, like in the tropics, at 80% rH. Then when the water forms as sweat on the body, it is more difficult to evaporate or there is both evaporation (body loses heat) and condensation on skin (body gains heat) with a net heat exchange which is far less than when in a dry condition.

Thus, the feeling is that your body feels so much hotter as the sweat does not seem to evaporate or that the water is always there on your body - thus leaving the person feeling so much worse than at the same temperatures but much dryer.

The body does not lose as much heat and hence it feels much hotter than during dry conditions.

This can be seen at the marathons as there are NO timings which are world class times, even though the same world class runners come here to run at the same marathon distances and have just clocked far better times in the temperate countries.

 Temperature Vapor pressure Heat of vaporization,    ∆Hvap

   [°C]             [kPa[100*bar] [J/mol][kJ/kg] [Wh/kg]

0.01          0.61165 45054 2500.9 694.69 1075.2

2          0.70599 44970 2496.2 693.39 1073.2

4          0.81355 44883 2491.4 692.06 1071.1

10           1.2282 44627 2477.2 688.11 1065.0

14           1.5990 44456 2467.7 685.47 1060.9

18           2.0647 44287 2458.3 682.86 1056.9

20           2.3393 44200 2453.5 681.53 1054.8

25           3.1699 43988 2441.7 678.25 1049.7

30           4.2470 43774 2429.8 674.94 1044.6

34           5.3251 43602 2420.3 672.31 1040.5

40           7.3849 43345 2406.0 668.33 1034.4

44           9.1124 43172 2396.4 665.67 1030.3

50           12.352 42911 2381.9 661.64 1024.0

54           15.022 42738 2372.3 658.97 1019.9

60           19.946 42475 2357.7 654.92 1013.6

70           31.201 42030 2333.0 648.06 1003.0

80           47.414 41579 2308.0 641.11 992.26

90           70.182 41120 2282.5 634.03 981.30

96           87.771 40839 2266.9 629.69 974.59

100           101.42 40650 2256.4 626.78 970.08

110           143.38 40167 2229.6 619.33 958.56

120           198.67 39671 2202.1 611.69 946.73

140           361.54 38630 2144.3 595.64 921.88

160           618.23 37508 2082.0 578.33 895.10

180           1002.8 36286 2014.2 559.50 865.95

200           1554.9 34944 1939.7 538.81 833.92

220           2319.6 33462 1857.4 515.94 798.54

240           3346.9 31804 1765.4 490.39 758.99

260           4692.3 29934 1661.6 461.56 714.36

280           6416.6 27798 1543.0 428.61 663.37

300           8587.9 25304 1404.6 390.17 603.87

320            11284 22310 1238.4 344.00 532.42

340            14601 18507 1027.3 285.36 441.66

360            18666 12967 719.8 199.9 309.5

373.946            22064 0 0.0 0.0 0.0