People around me complain how hot and humid it is these days, and I explain that when temperatures peak relative humidity (RH) is lowest and it goes up at cooler times of day. With water evaporating from over two thirds of Earth’s surface, H₂O vapour is the third most abundant atmospheric gas after Nitrogen (~78%) and Oxygen (~20%), but unlike those two the concentration of H₂O changes with temperature dependence of the water-vapour capacity of air.
At 20°C air can hold up to 2.3% (by volume) H₂O gas. Physicists call this a water vapour pressure (Pw)of 0.023 atmospheres. If water-saturated air is cooled, the H₂O gas condenses to liquid that appears as mist droplets, dew or raindrops. At 40°C, watercapacity of air is 7.2% (Pw=0.072atm), more than threefold higher.
Since H₂O gas is lighter than N₂ and O₂, humid air is lighter, rising in the atmosphere and cooling with the water condensing to give clouds and rain. This is the driving force behindweather patterns, and atmospheric water vapour is a major greenhouse gas.
Typically, we measure RELATIVE humidity (RH), using a hygrometer (see cover picture). RH is the ratio of the water CONTENT to CAPACITY. Let’s assume that humid air at 20°C (Pw=0.023,RH=100%) warms to a punishing 40°C; water content stays the same while capacityincreases to Pw=0.072, so RH falls to 32% (dry).
Evaporation is driven mostly by Pw differences. Inside leaves there is spongy, wet tissue where RH stays~100%. Pw is much higher than Pw outside driving vapour out through the stomata, the same pores that admit CO₂ for photosynthesis. Water is replaced by a "transpiration” stream from the roots, that also brings soil minerals needed for growth. The evaporation cools the leaf interior and Pw falls (though RH stays ~100%), reducing the inside vs. outside Pw difference. This slows evaporation ànd cooling rates till equilibrium is reached. Control mechanisms allow stomata to open and close according to the plant’s need for water and CO₂.
What about us? CO₂ not needed, body temperature stays around 37°C and we sweat liquid that evaporatesfrom the skin SURFACE (we lose more water in our breath). On days like these we can all use some help staying cool and keeping our skins moist.
