The air next to the ground cools during the night.
Sometimes it cools enough to reach the dew point. Water
vapor condenses onto objects on the ground and you find everything
covered with dew (or frost) the next morning. When this
happens in the air up above the ground you might think that water
vapor would simply condense and form little droplets. This
is not the case; we will find that small particles in the air
called condensation play an essential role in cloud (and fog)
|it is much easier for water vapor
to condense onto small particles
called condensation nuclei
|rather than just
and forming small droplets
of pure water
When the air is saturated with water
vapor (the relative humidity is 100%) the rates of
evaporation and condensation above a
flat surface of water will be
There's no real reason for picking
three arrows each of evaporation and condensation, the
important point is that they are equal when the RH is
It's hard for water vapor to condense and form a small
droplet of water because small droplets evaporate at a very
high rate. This is known as the curvature effect and is
The surface of the smallest droplet above at left has the most
curvature and the highest rate of evaporation (6 arrows). If
a small droplet like this were to form, it wouldn't stay around
very long. With it's high rate of evaporation it would
quickly evaporate away and disappear.
The middle droplet is larger and would stick around a little
longer because it does not evaporate as quickly. But it too
would eventually disappear.
The drop on the right is large enough that curvature no longer has
an effect. This drop has an evaporation rate (3 arrows) that
is the same as would be found over a flat surface of water.
A droplet like this could survive, but the question is how could
it get this big without going through the smaller sizes with their
high rates of evaporation. A droplet must
somehow reach a critical size before it will be in equilibrium
with its surroundings.
Particles in the air, cloud condensation nuclei (CCN), make it
much easier for cloud droplets to form. The
figure below explains why.
By condensing onto a particle, the water droplet starts out
large enough and with an evaporation rate low enough that it is in
equilibrium with the moist surroundings (equal rates of
condensation and evaporation).
There are always lots of CCN (cloud condensation nuclei in the
air) so this isn't an impediment to cloud formation. The
following information is from p. 91 in the ClassNotes.
Note that condensation onto
certain kinds of condensation nuclei and growth of cloud
droplets can begin even when the relative humidity is below
100%. These are called hygroscopic nuclei. Salt
is an example; small particles of salt mostly come from
evaporating drops of ocean water.
To understand how this can occur we first need to learn about
the solute effect
Water vapor condensing onto the
particle in the left figure dissolves the particle. The
resulting solution evaporates at a lower rate (2 arrows of
evaporation). A droplet of pure water of about the same size
would evaporate at a higher rate (4 arrows in the figure at
right). Note the rates of condensation are equal in both
figures above. This is determined by the amount of moisture
in the air surrounding each droplet. We assume the same
moist (the RH is 100%) air surrounds both droplets and the rates
of condensation are equal.
|pure water droplet
The next figure compares solution droplets that form when the
RH is 100% (left figure) and when the RH is less than 100%.
|the droplet is able to
|the droplet is in
equilibrium with its surroundings
even when the RH is less than 100%
The solution droplet will grow in the RH=100% environment at
left. You can tell the RH is less than 100% in the figure at
right because there are now only 2 arrows of evaporation.
But because the solution droplet only has 2 arrows of evaporation
it can form and be in equilibrium in this environment.
Don't worry too much about all the details. The key point is
that particles help clouds to form.