The Blue Machine

As we have seen, seawater forms a liquid storage system for the sun’s energy. That energy is held in the jostling of individual water molecules, which bump and swirl around each other faster and faster as the temperature increases. At the surface, some of those molecules get up enough speed to escape from the masses, freeing themselves from the liquid to zoom up into the atmosphere. That’s the process of evaporation. But only the most energetic molecules can escape, and the transition comes with an energy price tag (known to physicists as latent heat). An escaping molecule must carry this booty whenever it’s gas rather than liquid. This means that the process of evaporation cools the ocean surface, because the ocean loses the additional energy that must travel with the evaporating water. But it also means that high up in the atmosphere, when the molecule condenses to join a cloud droplet and become liquid again, this extra energy must be dumped. The cloud droplet carries on, growing and travelling and eventually falling back to Earth. But the energy stays in the sky, powering convection and winds, driving our dynamic weather.