Why does it Rain

[mezarashi]

An elementary school text book would quote something similar to:

 

As clouds rise higher and higher, the air gets colder and colder. When the water vapor in the cloud becomes too heavy, it falls back to the ground as rain or snow.

 

As you grow older, you may learn about the mechanisms and conditions in which water evaporates, and in which it condenses. Despite all that, I still can't answer the question, why does it rain. Let me explain more about my confusion.

 

I am not discussing about the precipitation that occurs at the boundary between a warm and cold front. That mechanism is fairly clear as you have an abrupt change in atmospheric temperature, and suddenly the once warm air is no longer able to accomodate the excess moisture.

 

But as we know, even without fronts, it rains. The evaporation and condensation process seems to be one that is fairly slow, but the "rain process" is rather abrupt. If we for example, leave a tub of water in an air-sealed room, it will not rain in the room. Even if we pump moisture into this room, the "raining phenomena" will not occur. I find it interesting that on Earth, the clouds can reach a "critical point" in which it just pours down everything all at once, and then nothing in between. Naturally, I would think that a kind of equilibrium would exist instead.

 

Any insight would be much appreciated, and thanks

[insane_alien]

i've read (forget the source) that the raindrops start to condense around bits of dust (some from space apparently) and they get bounced about by air currents merging and growing bigger then once the air curents cannot support their weight any more they start to fall still merging with others in the cloud.

 

The water vapour can rise due to thermals in much the same way as a glider pilot will use thermals.

[Sophist]

Yeah:Drops of water condense, releasing the latent heat used to evaporate the liquid which provides the warm air of suporting the cloud. Eventually less heat is released as the droplets are larger and less condensation occurs. As a consequence, the drops of water plummet to earth.

Hope this helps!

[mezarashi]

Yeah:Drops of water condense' date=' releasing the latent heat used to evaporate the liquid which provides the warm air of suporting the cloud. Eventually less heat is released as the droplets are larger and less condensation occurs. As a consequence, the drops of water plummet to earth.

Hope this helps![/quote']

 

So could you clarify the conditions for it to start raining, and the conditions for which it will stop raining? Or is rain triggered by some random event that builds up perpetually into rain.

[Newtonian]

iT RAINS TO PISS US OFF

[jdurg]

The analogy with the sealed room is a good one, but it is easily explained. If you have a room and seal it off, there are many, many, many, many surfaces for the water vapor to condense on. As a result, you don't get water droplets forming in the air and falling down.

 

In the clouds and atmosphere, however, there isn't anything for the water vapor to condense on except for the bits of dust and ice crystals up in the clouds. Therefore, since it's the only place the water can condense on it forms the little droplets in the clouds. When those droplets are too heavy to be supported by the air currents, they fall down to the ground as rain.

 

For it to start raining, you need a moist air mass which is moving into a cooler mass of air. This happens as warm humid air rises, or as humid air moves into cooler air masses. When this happens, the water vapor in the air condenses. Cooler air cannot hold as much moisture as warmer air, so that excess moisture condenses out onto little bits of dust or ice crystals in the air. This is how the clouds form. If there is enough of the humid air, so much moisture will condense out that the air currents in the atmosphere simply cannot hold the weight of the water. The water then falls down to the ground as rain.

 

For it to stop raining, the moisture that's condensing out of the clouds needs to either finish condensing and forming droplets, or the air has to become warmer and allow more moisture to evaporate into the air.

 

This is why in the spring and summer you tend to get more thunderstorms than in the winter. During the summer months, the humidity levels tend to rise significantly as well as the temperature. The sun heats the ground from above while the atmosphere remains relatively cool. As this hot humid air rises in the afternoon thanks to the constant heating throughout the day, it soon begins to condense out and form droplets. The large gradient between hot and col air causes pretty significant updrafts which pushes this condensing air and ice crystals far up into the atmosphere. The crystals rub against each other and form static charges which eventually discharge in the form of thunder and lightning.

 

To form rain, you need the following: A warm humid air mass cooling off in contact with a cooler air mass. If the air in the upper atmosphere is at a similar temperature as the air in the lower atmosphere, no rain will form as no water will condense.

 

For the rain to stop, the air has to become desaturated due to warming temperatures, or the moisture level in the air simply has to be exhausted.

[mezarashi]

Ah thanks, I think I see the picture better now.