Relative Humidity

The explanation for spatial variations in precipitation centers ~ above the concept of relative humidity.

You are watching: As an air mass cools water will condense when

The loved one humidity is the water vapor push (numerator) separated by the equilibrium vapor press (denomator) times 100%. The equilibrium vapor press occurs as soon as there is an equal (thus the word equilibrium) flow of water molecule arriving and also leaving the condensed step (the fluid or ice). For this reason there is no network condensation or evaporation (Alistair Fraser, ptcouncil.net).

Now, if the water vapor press is higher than the equilibrium value (numerator is greater), there is a net condensation (and a cloud might form, say). And that is not because the air cannot organize the water, however merely due to the fact that there is a greater flow into the condensed phase than out the it.

Relative humidity explains the lot of water vapor in reality in the wait (numerator), relative to the maximum quantity of water the air have the right to possibly organize for a offered temperature (denominator). it is expressed together a percentage:

RH= H 2 O actual H 2 O max This equation is no rendering properly due to an incompatible browser. View Technical requirements in the Orientation because that a perform of compatible browsers.

If the family member humidity (RH) is 100%, this method that condensation would certainly occur. Top top a usual hot muggy summer day, RH might be roughly 60-80%. In a desert, RH is commonly around 15-25%. 


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Figure 10. Once an wait mass includes the maximum quantity of water it deserve to hold, the is saturated through water vapor. This is displayed graphically in the plot above as the black solid bent line in number 10. With raising temperature (x-axis), the air can hold an ext water vapor (y-axis), as indicated by greater saturation values (solid black color curved line). In general, the is not feasible to have actually water components that exceed saturation (i.e. Relative humidity is 100%). In various other words, the maximum family member humidity is typically not better than 100% (i.e. Not over the solid black color curved line). Another means to think around relative humidity is that it describes how nearby the waiting is to saturation. In the example shown, the really water vapor content is about 40% of the at saturation (i.e. The blue suggest is about 40% the the way to saturation) – meaning the RH = 40%.

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Source: Michael Arthur and also Demian Saffer

One important consequence is that when air masses adjust in temperature, the family member humidity deserve to change, even if the actual amount of water vapor in the air does not (the molecule in our equation, i beg your pardon is identified by the saturation curve, continues to be the same, yet the denominator alters with temperature). Figures 11-13 below show an instance of this process. Together the wait cools, the relative humidity increases. If the wait mass were cooled sufficient to end up being saturated (hit the solid black curved line), condensation would occur. This temperature is called the dew point.


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Figure 11. RH and also cooling that an waiting mass
Click here for text different of number 11

Air mass starts at 30 levels Celsius, with 15 g H2O every cubic meter that air. It have the right to hold a maximum of 30 g H2O. RH = 50%