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What's the difference between

enthalpy and entropy?

Entropy on the other hand is defined in

Thermodynamics (on a Macroscopic Scale) as

a measure of the energy that is not available

for work during a thermodynamic process. A

closed system evolves toward a state of

maximum entropy.

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What relative humidity means

The warmer air is, the more water vapor it can

"hold."

Dew point is a measure of how much water

vapor is actually in the air. Relative humidity is

a measure of the amount of water in the air

compared with the amount of water the air

can hold at the temperature it happens to bewhen you measure it.

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What relative humidity means

To see how this works, let's use the chart below.

Air temperature in

degrees C

Water vapor air can hold at

this temperature.30 degrees 30 grams per cubic meter of air

20 degrees 17 grams per cubic meter of air

10 degrees 9 grams per cubic meter of air

These numbers, which apply to air at sea level pressure, are

based on measurements over the years. They are basic

physical facts.

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Now, let's see how Dew Point and relative humidity work.

Imagine, that at 3 p.m. you measure the air's temperature at 300C

and you measure its humidity at 9 grams per cubic meter of air.

What would happen if this air cooled to 100C with no water vapor

being added or taken away?

As it cools to 100

C the air becomes saturated; that is, it can't holdany more water vapor than 9 grams per cubic meter. Cool the air

even a tiny bit more and its water vapor will begin condensing to

form clouds, fog or dew - depending on whether the air is high

above the ground, just above the ground, or right at the ground.

Back at 3 p.m., when we made the measurements, we could saythat the air's dew point is 100C That is, if this particular air were

cooled to 100C at ground level, its humidity would begin condensing

to form dew.

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How about relative humidity?

At 3 p.m. the air has 9 grams of water vapor per cubic

meter of air. We divide 9 by 30 and multiply by 100 to get a

relative humidity of 30%. In other words, the air actually

has 30% of the water vapor it could hold at its current

temperature.

Cool the air to 200C. Now we divide 9, the vapor actually in

the air, by 17, the vapor it could hold at its new

temperature, and multiply by 100 to get a relative humidity

of 53% (rounded off).

Finally, when the air cools to 100C, we divide 9 by 9 and

multiply by 100 to get a relative humidity of 100% - the air

now has all the vapor it can hold at its new temperature.

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ENTHALPY

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ENTHALPY

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ENTHALPY

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