I am a teacher planning a unit on climate change. I came across this statement: "Clouds are water vapor (green house gas) and water molecules will reflect heat into space… actually clouds are condensed water and the water molecule absorbs the heat, lowering the temperature of the atmosphere around the cloud." I don't understand why water vapor is a green house gas whereas the water molecule is not. I also don't understand why the water molecule absorbs heat and does not reemit it.... Please explain!
A water molecule can absorb infrared radiation. Thus, water vapor can do the same. That makes it a greenhouse gas. Note that radiation that is absorbed will eventually be re-emitted. This occurs in a random direction, which is ultimately responsible for the greenhouse effect:
Heat radiation that would otherwise leave earth into outer space is absorbed and then a part of it is emitted back to earth.
The statement that water molecules are not a greenhouse gas, in the way it is presented in your short excerpt, strikes me as wrong. Both water vapor and individual water molecules absorb and emit infrared radiation.
Liquid water turns out to have a different absorption spectrum, because there certain vibrations of the water molecule are suppressed due to the bonding between individual molecules.
The claim that water just takes the eat and then does not re-emit also strikes me as bogus; if that was the case, water would get hotter and hotter and hotter, whereas in thermal equilibrium, the rate of absorption and emission must be in balance. Of course, when cold water is introduced into an otherwise warm atmosphere, the atmosphere will heat the water and cool in that process. However, the water heated in that way will still radiate in the infrared...
When water molecules condense into clouds they release heat to the surroundings, they become bound into droplets and will need energy to become free molecules again.
A kernel is needed for the water molecules to adhere on and start forming droplets, releasing latent heat, a small ice crystal, dust, aerosols in general. That is why we observe the trails from airplanes, the supersaturated with water molecules air condenses on the trail impurities.
Maybe you should stop trusting that the article you found this statement in is written by somebody who knows what he/she is talking about. They are probably confused with the cooling properties of clouds by reflecting incoming radiation back to space, increasing the earth's albedo.
Edit: You were asking:
and I edited it to the present title because the old one has an implicit contradiction : water in droplets is not a gas, whereas we talk of green house gases.
The various phases of matter, solid, liquid, gas, have different behaviours to energy absorption and emmission. As the other answer says H2O will absorb and re emit infrared whether in solid liquid or gas, BUT, it is a big but, the phase is very important. Ice and water droplets offer only a thin surface to infrared absorption and re emission and mainly reflect incoming electromagnetic radiation of all frequencies n from their collective molecular surface properties. In addition any heat absorbed by individual H2O could be transferred to the bulk, in contrast to the gas which re emits radially .
All water molecules do absorb infrared radiation (contributing to the greenhouse effect), but the phase of the water molecule does influence how effectively it absorbs radiation.
If you look at this graph of water's absorption coefficient: (http://t.co/SP3RyofTug)
You see that clouds are represented by the red line (water as a liquid). Water vapor is the green line (water as gas).
Per the Stefan-Boltzmann Law, the earth's average temperature of about 15C gives an average infrared emission wavelength of about 10 μmeters.
As you see in the graph, at 10 μmeters, water vapor is much less effective than liquid (cloud) at absorbing the earth's (infrared) radiation.
The issue is made more complicated, however, because of effects of cloud altitude and albedo (clouds reflecting some incident sunlight back into space).
The radiation from the Sun is peaked in the short wavelengths (ultraviolet, visible, and near infrared). Therefore, most of the radiant energy from the Sun passes through the atmosphere, and the Earth (land and water) absorbs it. This warms the Earth. Because the Earth is in thermal equilibrium, the absorbed energy is reemitted, but it is reemitted in longer wavelengths, far infrared. The atmosphere absorbs the longer wavelength radiation more strongly. The atmosphere is thermal equilibrium, as well, so the molecules in the atmosphere reemit long wavelength radiation, but they reemit in all directions. So while the reemitted energy from the Earth has a net outward radial flow, the reemitted energy from the atmosphere goes in all directions.
So there is an energy flow radially inward (relative to the Earth) from the Sun to the Earth from short wavelength radiation and an energy flow radially outward from the reemitted long wavelength radiation. If that were it, the Earth would be in equilibrium. The absorption of the radially outward long wavelength radiation and reemission in all directions of long wavelength radiation by the atmosphere disrupts the equilibrium, and the result is the atmosphere slowly warms.
Water in gas state, along with carbon dioxide and methane, absorb long wavelength radiation, and therefore they are largely responsible for the greenhouse effect. Nitrogen and oxygen do not absorb long wavelength radiation, and therefore they do not contribute to the greenhouse effect.
Finally, the temperature of the atmosphere is rising, so it is not thermal equilibrium as a whole. However, on short time scales the molecules of the Earth (land and water) and the atmosphere are in thermal equilibrium, and that's why they reemit the radiation they absorb.