水草情迷

小小明,
What Shui and Divergence said is correct and I believe in it. They are talking about the latent heat. It applies well in the substance in the air but not in the water.

We do not feel so bad in 35C with a RH of 60% as it is in Shanghai, but it is awful to breathe in the same temp with a RH >90%.

You are talking about the feeling in the air, why not take a look at the thermometer in your tank?

Please note that we are discussing on the fact "Is it true to lower the temp of the water below room temp by fan?" What I want to confirm is simply true or false.

Perhaps the fish / plant can give me the answer.
Forgive me if I am annoying you.

...
如果 |1+2|<|3| ===>水溫下降
否則 |1+2|>|3| ===>水溫上升
...

Can you calculate the rate at which water will flash to steam given the temperature and psia? For example, assuming approximately .8 PSIA and 100 degrees F, at what rate would the water evaporate? Would lowering the pressure or increasing the temperature change the evaporation rate significantly?

Thanks.

Rick Gresham (age 51)

Portland, OR, USA


A:
The answer is yes, the rate that water evaporates can indeed be calculated, but it depends on a few more things than you mention. The evaporation rate is influenced by

1) The temperature of the water at the air-water surface

2) The humidity of the air

3) The area of the air-water surface

4) The temperature of the air (more on this below)

In a real-world situation of evaporating water, none of these three quantities above remains constant because the process of evaporation itself changes them. Water evaporating takes quite a lot of heat away -- 540 calories per gram -- when it evaporates. That's enough to cool down 540 grams of water by a degree, or 50 grams of water a little more than ten degrees. If you are not very careful to replace the lost heat energy during the evaporation, the temperature will go down. And even then the temperature right at the surface will be lower than elsewhere in the water and it will depend on

5) water currents convecting heat and the ability to keep the temperature constant at 100 degrees F.

For a similar reason, the air near the surface of the water will become more saturated with water as the water evaporates. The evaporation rate will depend on:

6) airflow past the water/air surface.

These factors explain why:

People sweat to stay cool.

People say "It's not the heat, it's the humidity."

People use fans to keep themselves cool on hot days.