Water vapor is indeed very important in its roll in global warming, but don't forget that water vapor in the air is a function of temperature. Fortunately, the amount of warming required to double the water vapor in the atmosphere is less than the warming that a doubling of water vapor will induce. If this were not true, Earth would have a runaway greenhouse effect, just like that of Venus. Water vapor is a feedback and not a forcing.Its atmospheric concentration is pegged to such forcings as the Sun, volcanic aerosols and man made carbon dioxide.
There MAY be a negative feedback from water vapor due to clouds, especially if relative humidity is close to constant. But relative humidity may be a diminishing function of temperature, as evidenced by the fact that hot episodes, like the dustbowl of the 1930's was very dry as well as very warm.
You are correct. H2O stops heat coming in from the sun as visible light as well as heat leaving Earth as infrared light. However, it is transparent to some colors, and not to others. It tends to block infrared light more than visible light. The sun does not send much light in the infrared range, and Earth sends nearly all of her energy out in the infrared range. http://en.wikipedia.org/wiki/File:Atmosp...
This is because the sun is hotter than Earth, http://en.wikipedia.org/wiki/Wien%27s_di...
and therefore puts out energy at a higher frequency (color). http://en.wikipedia.org/wiki/Color#Spect...
Thus, some gases (like ozone) tend to block visible light more than infrared light (anti-greenhouse gases), while others (like H2O) tend to block infrared light more than visible light (greenhouse gas). http://www.randombio.com/co2.html
However, note that some consider Ozone to be a greenhouse gas, (3) because its visible light range is mostly saturated, (2) and there is very little visible light still getting through in the range that ozone blocks. http://en.wikipedia.org/wiki/Ozone#Ozone...
Eventually, the radiation that cannot escape will be converted back to heat and re-released in another frequency which will escape. http://sg.answers.yahoo.com/question/ind...
This takes time, and the extra time spent in the atmosphere is what causes the warming. Note that the warming happens in the part of the atmosphere where the gas is trapping the heat. Thus, anti-greenhouse gases tend to warm the stratosphere (high up) (1), and cool the troposphere (near the ground) (4), while greenhouse gases tend to do the opposite. Thus, global warming refers only to warming of the Troposphere, (6) and implies Stratospheric cooling. (5)
Also, H2O changes Earth's albedo by forming clouds, snow, and ice which reflect light back to wherever it came from. (7)
It's the number 1 green house gas responsible for over 90% of the green house effect.
Water vapor is a greenhouse gas, but it would be foolish for the modern day snake oil salesmen, like Al Gore, to attempt to tax it. Even illiterates can see the benefits of water. There would be open revolt since their sinister scheme would be totally exposed.
if the water vapour doesn't allow the sun's heat to flow back out into space as quickly mean it's also doest allow the sun's radiation to penetrates through the atmosphere right but how comes the water vapour still playing a crucial role in global warming?
