If we wanted to make a cloud entirely from scratch, we’d first need a fleet of jumbo
jets or several hundred hot-air balloons to haul hundreds of tons of water up to the sky.
And then, somehow, we’d need to disperse all that liquid into a mist of droplets small
enough to float.
In short, it wouldn’t be easy.
And yet, our atmosphere manages to pump out one cloud after another all over the world
at altitudes of up to 20 kilometers above sea level, using water and fuel carried all
the way from Earth’s surface.
Cumulus clouds, for example get their start when solar energy evaporates water from oceans,
plants, and soil by breaking the bonds that hold water molecules together.
As the patch of air above collects moisture and heat cooler, heavier air sinks around
it pinching it off and pushing it aloft like an invisible hot-air balloon.
Surprisingly, this balloon’s cargo doesn’t weigh it down – in fact, the more water
vapor it collects before lift-off, the lighter it gets.
As weird as that sounds, it’s because water vapor is a gas just like the nitrogen and
oxygen that make up most of the atmosphere.
Basic physics dictates that a given volume of gas has the same number of molecules regardless
of what those molecules are.
And water is made of H plus H plus O, which is lighter than both two Ns and two Os.
So warm, humid air is even more buoyant than warm, dry air.
As the invisible balloon goes up, the falling pressure outside allows it to keep ballooning,
which spreads out its internal heat and lowers its temperature.
Eventually, the air at the top cools enough for the water vapor there to condense into
droplets, which look from afar like a thin wisp of cloud.
And as the rest of the balloon rises, water vapor continues to cool and condense at the
same altitude, creating a flat-bottomed cloud that appears to grow upward out of nothing.
What’s more, as the condensing water vapor molecules bond together into liquid droplets,
they release the energy they absorbed from Earth’s surface when they evaporated.
This heats the surrounding pocket of air, giving it lift and sucking more moist air
up behind it [updraft], which cools and condenses and releases heat, which fuels lift and strengthens
the updraft.
Even in a small cumulus cloud , the total energy released from condensation is huge
– equivalent to about 270 tons of TNT.
And if the supply of water vapor is much larger, the energy released can produce stratosphere-high
pillars of cloud with violent updrafts, fierce electrical storms, and grapefruit-sized hailstones.
Not good weather for hot air ballooning.