# B How can there be rain?

1. Mar 28, 2016

### Edison Bias

Hi!

I wonder how water vapour can reach the sky while its temperature is far from the boiling point? I have learned that water is lighther than air and this is said to be the reason for water vapour to reach the sky. The trick question is however, how does water become vapour in the first place? I have also learned from Wikipedia that temperature is an average concept. So what we have is an average temperature, meaning some molecules has a higher temperature some a lower temperature.

Considering

$$E_k=\frac{mv^2}{2}\propto kT$$

we then have different speeds of the sitting water molecules and some of them actually has a temperature (or speed) higher than the boiling point so things then "dry up" in the sense that water is vaporized. Let't take an example: if the temperature on the ground is 25C, molecules with just twice of that average speed actually get vaporized. I may be far from the truth but what is then the true explanation of water becoming vapour so much that it can rise to the clouds?

Best regards, Edison

2. Mar 28, 2016

### PeroK

3. Mar 28, 2016

### Edison Bias

• Poster has been reminded to be civil when others are trying to help them learn.
That article was just bla, bla, bla like most Wikipedia articles.

If you know something, why not simply tell it?

Best regards, Edison

4. Mar 28, 2016

### davenn

you would do well to play nice if you expect people to help you !

so what was it about that article that didn't answer you Q on how surface water becomes a vapour ?

Dave

5. Mar 28, 2016

### ProfuselyQuarky

Evaporation is all about the energy level of individual molecules--not the average energy level of the entire mass of water. That's why evaporating water turns to vapor slower than boiling water does.

If the Wikipedia article was too hard for you, maybe this is better: http://www.chem4kids.com/files/matter_evap.html

6. Mar 28, 2016

### Edison Bias

Actually, I did not get that far. I read everything in the first pargraph and it kind of confirmed my theory but I am not sure because Wiki-articles tend to have lots of information that really do not explain things in a manner that ordinary people can understand. This is also the reason why I am here, I wish for competent people like you to explain things, that's why it's called a forum, right? And I'm sorry, I really do not think it is a good forum if people just serve links. If I ask a question I expect an answer, wouldn't you? And I do not expect to have to read Wiki-articles of several pages where most of it is academic. I just want a rough explanation.

Best regards, Edison

7. Mar 28, 2016

### ProfuselyQuarky

You're right, a forum is a place to discuss. However, one should always learn to research for oneself and then ask questions after. Why have somebody here answer a question for you here if the question has already been answered elsewhere?

Now, this is a science forum, so back to the science. What else don't you understand, given the links provided?

8. Mar 28, 2016

### Edison Bias

This was interesting to hear! This is a textbook way of answering a forum question. But does this not confirm "my" theory?

Best regards, Edison
PS

And maybe I should explain why I think this a textbook way of answering a forum question. This guy has made a short attempt into explaining things while at the same time supplying a link for further studies. It's the combination of trying to answer AND supplying a link that is A+

Last edited: Mar 28, 2016
9. Mar 28, 2016

### ProfuselyQuarky

Secondly, what exactly is you're "theory"? What theory is there to be had?

10. Mar 28, 2016

### micromass

Staff Emeritus
The mentality of a science student would not be one where (s)he expects to be spoonfed the answer. It is one where (s)he would be content with a good link to an explanation. (S)he would then read the link and ask further questions if necessary.

If you're going to be lazy, entitled and spoiled, then science is not for you.

11. Mar 28, 2016

### ProfuselyQuarky

correction: this *girl*

12. Mar 28, 2016

### Edison Bias

Wikipedia, throw youself into a wall :D

This was about the most simple and interesting link I have ever read, I have even made it a bookmark in my browser because I saw there's more to read.

As I understand it, I am actually right. The only difference is that this link talks about energy instead of kinetic energy or kT and that is actually the same. I have by the way learned that kT is related to kinetic energy in a way that depends on number of freedom degrees (in a gas anyway).

Best regards, Edison

13. Mar 28, 2016

### Edison Bias

Sorry, how could I know?

Best regards, Edison

14. Mar 28, 2016

### ProfuselyQuarky

I read the Wiki article myself and it's very understandable, as well. I don't know how old you are, but if you're old enough to ask the question that you did, you should have been able to understand it, too. The link I gave you is a website meant for children (the word "child" being subjective, I suppose). I'm glad you like the link, but try to make more of an effort to understand articles for more "older" audiences, too.

Cheers,
PQ

15. Mar 28, 2016

### Edison Bias

So I'm a child it seams :D

Actually, I'm 47 but as you have understood I do not like academic Wiki-links that really do not give me a notch of more understanding other than fancy words. They only consumes time reading and I still think that if there's a simpler way of explaining things in a rough way, why not do so when you are sitting on the knowledge?

Best regards, Edison

Last edited: Mar 28, 2016
16. Mar 28, 2016

### ProfuselyQuarky

I'm sorry! I'm only 15 and I guess I'm just used to the way the adults I know write . . .

17. Mar 28, 2016

### Edison Bias

Fifteen, and knows more about physics than me at 47.

Interesting, to say the least :D

Best regards, Edison
PS
I hold a Master Degree in Electronic Engineering to make things even more pathetic :D

18. Mar 28, 2016

### Staff: Mentor

Color me impressed.

19. Mar 28, 2016

### Ophiolite

In many cases the wikipedia articles are superior to what a forum member could produce in a reasonable time frame. I do agree with you that the combination of a very simple explanation accompanied by one or more references is the preferred route. However, I think the presumption here is that most members will be academically oriented and would be accustomed to studying appropriate material once it had been identified for them. in that regard I was taken aback to learn you have a Masters degree. . . . .and yet you characterise wikipedia articles as being bla, bla, bla. You might want to rethink that approach.

20. Mar 31, 2016

### Edison Bias

Back to science (or at least an attempt into being scientific).

I think that all matter, regardless of state, gets it temperature due to particle speed, right? Both PQ's science article for children and the short Wikipedia article I found tells about vaporization as a consequence of some particles having a higher (kinetic) energy than others. This got me thinking "what is the limit of this Ek deviation?". Suddenly it struck me "Maxwellian distribution" might tell it.

The Maxwellian distribution may be written:

$$f(v)=e^{-\frac{Ek}{kT}}=e^{-\frac{mv^2/2}{kT}}=e^{-\frac{T_{k}}{T_{av}}}$$

where the last expression is just me thinking aloud.

For general purposes the expression for v is actually

$$v=v'-v_0$$

because we have a distribution of v around a certain vo or average temperature (Tav).

Now, the Maxwelllian distribution (MD) is not just taken from the air. It is a well-known fact that many things follow the natural logarithm (e). One thing is how the voltage of a capacitor decreases with a load of a resistor, another thing is that solutions to many differential equations follow e. So why may not the deviation of Ek to kT follow e?

Just as reference we may state that 1/e=37% and 1/(3e)=5%

If we consider MD and the hypothetical fact that Ek=kT we have that 37% of the particles has a higher Ek (and if Ek=3kT only 5% has a higher Ek).

Considering a "state-jump" from 300K to 400k (the boilingpoint, roughly) this gives a Ek/kT need of 4/3, speed then only need to change sqrt(4/3)=15%

I think I totally understand now :)

I didn't calculate with absolute temperatures (thus needed a factor of 4 times 25C to reach 100C) while going from 300K to 400K is totally different. In other words, the change in Ek need not be so large to give the particles enough "temperature" to vaporize.

A bit shorter than a Wikipedia link, don't you think? :)

Best regards, Edison

Last edited: Mar 31, 2016