絕對濕度和蒸發

Evapouration is due to the escape of energetic water molecules from the surface of water into the air. But at the same time, because air also contains an amount of water vapour in it (unless the air is completely dry), water molecules from the air would enter into the water surface and becomes liquid water. If the rate of escape of water molecules from the water surface is higher than the rate at which water molecules in the air enter into the water surface, there is a net loss of water molecules from the water, this is evapouration.

Therefore, it is not difficult to realize that the rate of evapouration indeed depends very much on how many water molecules would enter into the water from the air. In other words, the more water molecules leaving the air into the water, the lower will be the evapouration (assuming that the number of molecules leaving the water remains the same). Conversely, a higher evapouration rate will be observed if there is less water molecules entering into the water from air.

Now, it should be clear that why the absolute humidity is low, the evapouration rate is high. Absoulte humidity is a quantity used to measure the amount of water vapour present in the air. A high humditiy indicates more water molecules are present in the air than when the humdity is low. Thus, when the absolute humidity is low, there are fewer water molecues in the air, the number of these molecules entering into the water would be less. The net escape of water molecules from the water becomes high, i.e. a high evapouration occurs.

【濕度】表示大氣乾燥程度的物理量。在一定的溫度下在一定體積的空氣裡含有的水汽越少，則空氣越乾燥，水汽越多，則空氣越潮濕。空氣的乾濕程度叫做“濕度” 。在此意義下，常用絕對濕度，相對濕度，比較濕度，混合比，飽和差以及露點等物理量來表示，若表示在濕蒸汽中液態水分的重量佔蒸汽總重量的百分比，則稱之為蒸汽的濕度。
【絕對濕度】單位體積空氣中所含水蒸汽的質量，叫做空氣的“絕對濕度”。它是大氣乾濕程度的物理量的一種表示方式。通常以1立方米空氣內所含有的水蒸汽的克數來表示。水蒸汽的壓強是隨著水蒸汽的密度的增加而增加的，所以，空氣裡的絕對濕度的大小也可以通過水汽的壓強來表示。由於水蒸汽密度的數值與以毫米高水銀柱表示的同溫度飽和水蒸汽壓強的數值很接近，故也常以水蒸汽的毫米高水銀柱的數值來計算空氣的乾濕程度。
【相對濕度】空氣中實際所含水蒸汽密度和同溫度下飽和水蒸汽密度的百分比值，叫做空氣的“相對濕度”。空氣的乾濕程度和空氣中所含有的水汽量接近飽和的程度有關，而和空氣中含有水汽的絕對量卻無直接關係。例如，空氣中所含有的水汽的壓強同樣等於 1606.24Pa（12.79毫米汞柱）時，在炎熱的夏天中午，氣溫約 35℃，人們並不感到潮濕，因此時離水汽飽和氣壓還很遠，物體中的水分還能夠繼續蒸發。而在較冷的秋天，大約 15℃左右，人們卻會感到潮濕，因這時的水汽壓已經達到過飽和，水分不但不能蒸發，而且還要凝結成水，所以我們把空氣中實際所含有的水汽的密度ρ1與同溫度時飽和水汽密度ρ2的百分比ρ1/ρ2× 100％叫做相對濕度。也可以用水汽壓強的比來表示：

例如，空氣中含有水汽的壓強為 1606.24Pa（12.79毫米汞柱），在35℃時，飽和蒸汽壓為 5938.52Pa（44.55毫米汞柱），空氣的相對濕度

而在15℃時，飽和蒸汽壓是1606.24Pa（12.79毫米汞柱），相對濕度是100％。
絕對濕度與相對濕度這兩個物理量之間並無函數關係。例如，溫度越高，水蒸發得越快，於是空氣裡的水蒸汽也就相應地增多。所以在一天之中，往往是中午的絕對濕度比夜晚大。而在一年之中，又是夏季的絕對濕度比冬季大。但由於空氣的飽和汽壓也要隨著溫度的變化而變化，所以又可能是中午的相對濕度比夜晚的小，而冬天的相對濕度又比夏天的大。由於在某一溫度時的飽和水汽壓可以從“不同溫度時的飽和水汽壓”表中查出數據，因此只要知道絕對濕度或相對濕度，即可算出相對濕度或絕對濕度來。