Simple Science
Why don’t clouds fall from the sky?
© Sancho McCann 1
At a glance
Clouds are made from
tiny droplets of water
and ice
It’s common knowledge that clouds
are made from water droplets. This
knowledge seems counter intuitive, if
the clouds are made of water why
don’t they fall to the ground? Surely
water is heavier than air, so it
shouldn’t hang around overhead?
© Ken Bosma 1
These droplets are so
light that they fall very
slowly towards the
ground
Air moving from the
warm ground to the
cooler atmosphere
moves up at a faster
speed than the falling
of the droplets causing
them to rise
As the cloud grows, it
contains more droplets
which collide to form
larger droplets until
the speed with which
the droplets fall is
larger than the rising
air – it’s raining
The speed with which an object falls is governed by a combination of two
factors, its mass and its shape. The shape of an object is important since
objects with a larger surface will collide with more air particles as they fall
than one with equal weight but smaller surface. This will cause the object
with a smaller surface to fall faster. A nice way to think of this is to think
of dropping a large feather and a pin. Both have roughly the same mass
but the feather has a much larger surface and so will fall to the ground
more slowly. The ice and water droplets which make up a cloud are
extremely small and so have very small masses and relatively large
surface areas for their mass. This means that the speed of their fall is far
slower than the upwards motion of the warm air from the ground. This
keeps the droplets in place. As the cloud gets bigger, more and more
droplets collide and form larger (and heavier) droplets. Eventually the
mass of these droplets has increased enough for the downwards speed of
the droplet to be larger than the upwards motion of air and so the
droplets fall as rain.
Centre for Lifelong Learning
Alex Brown, 2012
2