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Unit 1: What happens in clouds?
We saw in the 'basics' section that clouds are made up of millions of droplets
which form as water condenses onto tiny suspended particles. In this 'read
more' section on clouds we'll have a look in more detail at how these water
droplets form, what happens inside clouds and how different cloud types vary.
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Formation of droplets
Cloud characteristics
Cloud chemistry
Part 1: Formation of droplets
A cloud forms when humid air is cooled enough so that the water
vapour it holds becomes liquid. In this section we look at the
relationship between temperature and the amount of water vapour the
air can hold and what affects the size of water droplets in a cloud.
Saturation of air
The quantity of water vapour that air can hold depends on its temperature. For
a given temperature, the relationship between the amount of water a given
mass of air actually holds and the total amount it can hold is known as
the relative humidity.
Air is said to be saturated when it holds as much water vapour as it can.
Saturated air, therefore, has a relative humidity of 100%. Supersaturated air
has a relative humidity of more than 100%.
The table below shows the maximum amount of water vapour the air can hold
at different temperatures, before condensation begins.
T°C
-20
-10
0
+10
+20
+30
Amount of water
vapour
1.1
2.3
4.8
9.4
17.3
30,5
(g water per m3 of
air)
See the water vapour saturation curve for more values.
For example, imagine an air parcel at 20°C that contains 9.4 g m-3 of water
vapour. The relative humidity of the air is therefore (9.4/17.3) x 100 = 54.3%.
Now assume the air parcel cools down to 10oC (for example by rising in the
atmosphere), at this point the air is now saturated and the relative humidity is
100%.
Now imagine that the air rises further and the temperature falls to 0°C. At
0oC, air can only hold 4.8 g m-3 of water vapour and the air parcel is
now supersaturated by (9.4 - 4.8) = 4.6 g of water. This extra water condenses
on available aerosol particles forming cloud droplets and the relative humidity of
the air parcel returns to 100%.
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The warmer the air, the more water it can hold. This is why
warm air is used to dry objects, it absorbs moisture. On the
other hand, if saturated air is cooled down, the water vapour in
the air is forced to condense into water droplets. This is why
water droplets are seen on the outside of a cold soft drinks can
. The cold can cools the air around it causing the water vapour
in the air to condense.
Water droplets inside clouds
1. A cold drink
can 'sweating'.
Source: C.
Gourbeyre.
Cloud droplet diameters vary from a few micrometers to more than 100 µm (0.1
mm), with the average droplet size being around 10 µm in diameter.
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Continental clouds are generally made up of many small droplets.
Marine clouds are made up of fewer droplets but the droplets are large.
There are usually between 25 000 and 1 million water droplets per litre of
air.
The distance between 2 droplets is around 1.4 mm, a distance about 70
times the diameter of each droplet (it's perhaps easier to visualise this as
a football on the ground every 20 or 30 meters).
In order to fall as rain, droplets need to grow until they reach a diameter
of around 1 mm, that's about one hundred times bigger than the droplet
was originally!
For "warm clouds" (those which don't contain ice crystals),
droplets grow to rain drop size by sticking to each others.
As times passes, the droplets become larger and larger until
they are too heavy to remain suspended in the air by the
up-drafts that counterbalance the droplets falling.
In cumulonimbus clouds, for example, the up-drafts are
very strong. This means that droplets can grow to very
large sizes before they become too large to remain
suspended in the air. This is why the raindrops are so big
during thunderstorms.
2. Author: J. Gourdeau.
Colder clouds are made up of ice crystals, liquid water and water vapor. Water
vapour condenses onto the ice crystals and liquid droplets freeze when they come
into contact with the ice crystals. As the ice crystals become bigger and bigger,
they start to fall as snow, or rain if they melt before reaching the ground.
3. Source: freefoto.com
4. Source: fond-ecran-image.com
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Part 2: Cloud characteristics
Characteristics of the different cloud types
In the 'basics' section we looked at the different types of clouds and
showed that clouds are classified into four groups depending on the
height of the cloud base and the cloud thickness. In this 'read more'
section we will look at the different cloud types in more detail.
The different types of clouds: an overview
The lower layer of the atmosphere, the troposphere, can be divided into three
layers: the lower level, the mid level and the upper level. These levels don't
have firm definitions and their height depends on the latitude considered. In
Europe, low level clouds occur at altitudes of up to 2 km whereas medium level
clouds extend to about 6 km. The highest clouds are seen at altitudes of up to
12 km.
1.
The different types of cloud in the troposphere. St: stratus, Sc: stratocumulus, Nb:
nimbostratus; Ac: altocumulus, As: altostratus; Ci: cirrus, Cs: cirrostratus, Cc: cirrocumulus;
Cu: cumulus, Cb: cumulonimbus. Author: J. Gourdeau.
Some clouds generate rain,
others don't. You can only get
drizzle from stratus clouds and
can only get hail from
cumulonimbus clouds. High level
clouds and altocumulus clouds
hardly ever rain. The table on
the right shows what type
of precipitation you can get from
each cloud type.
High level clouds: Ice crystals
As Ns Sc St Cu Cb
Rain
 

Drizzle
Snow
Hail
 
 
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The highest level clouds, like cirrus, are made of tiny ice crystals rather than
liquid water droplets. A typical small ice crystal contains between 1016 and 1018
water molecules. Although no two ice crystals are exactly alike, there are several
basic types of crystals. The crystal shape depends mainly on temperature and
this is shown in figure 2.
Snowflakes are simply aggregates of ice crystals. As long as the snowflakes do
not pass through a layer of air warm enough to make them melt, the snow
flakes remain intact and reach the ground. Although they are only made up of ice
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crystals, high level clouds never produce snow because their crystals are too
small, and therefore not heavy enough to fall.
2. The different shapes of ice crystals according to the air temperature and degree of supersaturation.
Source: www.snowcrystals.com.
The record for the most snow falling
in a single day is held by Silver Lake,
Colorado in the United States of
America. On the 15th April 2001, 192
cm of snow fell!
3. A dog in the snow! Source, M. Ruinhart.
Low level clouds: stratus, stratocumulus and nimbostratus and fog
Clouds at low levels form as a result of condensation of water vapour into liquid
cloud droplets. Stratus clouds form a layer near the ground that is typically only
a few hundred meters thick. Only stratus clouds produce drizzle, which we
define simply as water droplets less than 0.5 mm in diameter. They fall so
slowly that they seem to remain suspended in the air.
Cumulonimbus clouds and thunderstorms
In summer, strong daytime heating warms the lower atmosphere making it
warm and moist. This warm air rises by convection and, since the air above is
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colder, this makes the atmosphere unstable. Water vapour in the rising air
condenses into water droplets as the warm air cools. This condensation
process releases more heat into the atmosphere and convection is further
accelerated. A towering cumulonimbus cloud forms in just 30 minutes
which can reach upto 15 km in height and millions tonnes of water are lifted
from the ground into the sky.
We still don't know the exact
way lightning forms. Water
droplets, ice crystals
and hailstones inside the
cumulonimbus cloud collide
because of the strong air
currents which exist in the
cloud. Friction then creates
static electricity. Positive
charges build up at the top
of the cloud and negative ones
build up at the bottom. The
ground underneath is positively
charged. The difference
between charges gets bigger
and bigger until lightning
sparks cross the gap. The
amount of energy stored by a
cumulonimbus cloud is huge,
about the same amount as a
small atomic bomb!
4. Illustration of the electrical structure of a thundercloud.
Author: J. Gourdeau.
Lightning travels at
upto 40,000 km per
second and can generate
100 millions volts of
electricity. This heats the
air in the path of
the lightning flash to
around 30,000°C! This
temperature is so hot that
the air expands violently,
like popcorn, and creates
sound waves which we
hear as thunder.
5. Lightning strikes seen during a night-time thunderstorm.
Source: NOAA.
Part 3: Cloud chemistry
Clouds are not an inert mixture of water droplets (or ice crystals) and
particles. Particles that allow clouds to form are called cloud
condensation nuclei (CCN). These particles have different chemical
compositions depending on their origin. They can have natural sources
and originate from the deserts, from the oceans, from volcanoes or
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from living organisms or they can come from human processes. The
cloud also contains gases which can change the chemical composition
of the droplets. So a cloud is far from inert!
Four main processes occur within the cloud
droplet. These are shown in Figure 1:
1. the composition and size of the
CCN particle changes after
the droplet evaporates.
2. the soluble part of the
particle dissolves.
3. chemical reactions occur inside the
water droplet.
4. transfer between atmospheric gases
and the liquid phase take place.
1. Various cloud chemistry processes.
Author: Justine Gourdeau.
The particle inside the droplet
The water soluble fraction of an aerosol particle governs whether it can take up
water and grow into a droplet. The chemical composition of CCN particles
controls the initial chemical composition of a cloud droplet as its soluble content
dissolves in the condensed water. The less water soluble particles, for example
soil dust, pollen and particles from biomass burning, remain in the surrounding
air.
Most clouds don't lead to rain and simply evaporate. As a result of in-cloud
chemical reactions, the particles which remain after the water evaporates have a
different chemical composition to those which entered the cloud in the first
place.
2. A sulphur dioxide molecule (1) reacts with ammonia in the air to form to ammonium sulphate (2).
This then grows to form an ammonium sulphate particle (3). These particles are hygroscopic, meaning
they rapidly grow in the presence of water (4). Author: J. Gourdeau.
The atmospheric gases around the droplet
Whether a chemical species stays in the gas phase or is absorbed by the water
droplet is determined by the Henry’s law equilibrium:
A(aq) = HA PA
where:
A(aq) is the aqueous phase concentration (mol L-1)
PA is the partial pressure of A in the gas phase (atm)
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HA is the Henry’s law coefficient of the gas considered.
Some species go back to the gas phase and move away from the drop. Others,
once captured, remain associated with the aqueous phase unless total
evaporation occurs.
Reactions inside the droplet
At least a hundred chemical reactions take place in
a droplet. These reactions can change the acidity of
the rainfall resulting in acid rains which are hazardous
to plants and animals living in lakes and streams
and can contribute to the deterioration of buildings.
The main chemical species involved in acid rain are
sulphuric (H2SO4) and nitric acids (HNO3).
3. A scientist collecting water
samples for acid rain analysis.
Look at the damaged forest!
Source: NOAA.
All this complex chemistry modifies not only the cloud itself but also the
atmosphere around the cloud. Only about one cloud in every seven results in
rain and, as a result, a single particle acting as a CCN undergoes between 10
and 25 evaporation-condensation cycles before it reaches the ground.
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