Think you understand evaporation? Think again, says MIT

Water molecules have quite a number of different vibrational modes, so the absorption mechanism could be quite complex. The energy of a single photon of green light is somewhere around 2.18–2.37 eV (depending on the hue of green). So there is more than enough spare energy in the light for evaporation, depending on the absorption mechanism. Doing some quick number-crunching shows that the latent heat of vaporisation for water is 0.423 eV per molecule. This is actually quite high compared to most liquids.

What the MIT team discovered is that light in the visible spectrum is enough to knock water molecules loose at the surface where it meets air and send them floating away. In other words, while it's true that evaporation has been happening all of these years due to fluctuations in temperature, water has also been turning to vapor from the force of light beams alone. The scientists have termed the process the "photomolecular effect" after the photoelectric effect that was explained by Einstein in 1905, in which particles of light could free electrons from atoms in the material they strike. Clouds absorb sunlight often soak up more sunlight than physics say is possible. The photomolecular effect on these clouds – which causes additional, unexpected evaporation – could help solve the puzzle. See also:

• Photomolecular Effect: Visible Light Interaction with Air-Water Interface During the course of this process using laser light they found that the strongest evaporative effects happened when light that was polarized in a particular way known as transverse magnetic polarization hit the surface of the water at a 45° angle. It was also strongest with green light, which surprised the team because that's the color that makes water appear the most transparent because it interacts the least with it.

  The puffs of white condensation on glass is water being evaporated from a hydrogel using green light, without heat.

• There has been a long history of unexplained anomalous absorption of solar radiation by clouds. Collocated satellite and surface measurements of solar radiation at five geographically diverse locations showed significant solar absorption by clouds, resulting in about 25 watts per square meter more global-mean absorption by the cloudy atmosphere than predicted by theoretical models.

• A surprising finding shows that light can make water evaporate without heat could enable new approaches to desalination

• The subtleties of blue tint of water The blue color of water results from molecular vibrations instead of electronic ones and it has its own photochemistry.