Clouds & Particles
Basics
Unit 1: Clouds
Clouds play a very important role in the climate system. In this Unit we'll look at
the different forms of water on the Earth and in the atmosphere. We'll also look
at what clouds are made of, how they form and identify the many different
types of clouds which exist.
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water in the atmosphere
formation of clouds
cloud types
Part 1: Water in the atmosphere
Water is the only substance that can exist naturally in three 3 forms: as
a liquid (in oceans, rivers, lakes..), as a solid (as ice, snow, hail..) and
as a gas (as water vapour).
The atmosphere isn't just made up of air but also contains water
vapour. This water vapour is invisible, doesn't smell and makes up less
than 0.001% of all the water on the Earth. However, this tiny amount
of water in the air is really important to our climate.
Let's now look at the role of water in the air and see what are clouds
made of.
The water we drink is water in its liquid state. When we crunch an ice cube, our
teeth know that the water is solid. These two forms of water are easy to see.
But water can also exist as a gas in the air, here the water occurs as free water
molecules. We call these free molecules water vapour or moisture.
When liquid water is converted into water vapour, the process is
called evaporation. This is what happens when you use a hair drier to dry your
hair. The water doesn't simply disappear, it's still in the room but now in the
air. Due to the high temperature of the hair drier, the liquid water has changed
to water vapour, it has evaporated!
Condensation is the opposite process where water vapour changes to liquid
water. After having a bath, the bathroom is filled with steam, or water vapour.
The warm steam condenses onto the cold bathroom mirror, returning the water
to it's liquid state and forming water droplets on the mirror.
1. Condensation and evaporation. Author: J. Gourdeau.
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Although many clouds look very solid, you
could never walk on one. Clouds are only
water in the air! Clouds form when water
vapour turns into liquid water droplets or
into solid ice crystals which are light
enough to float in the air. The
temperature at which water vapour
condenses into visible water droplets is
the saturation point. So the saturation
point is the moment when condensation
occurs or when dew forms. Cloud
formation doesn't just involve cooling,
2. Source: C. Gourbeyre.
however. In order to form clouds, water
vapour needs to condense onto tiny
particles in the air. These tiny particles
are known as cloud condensation nuclei
and we will look at how these particles
help cloud formation in Unit 2.
In some clouds the tiny water droplets
Clouds which occur in air which is
collide and form larger water droplets.
below 0°C are made of ice crystals.
As the droplets become bigger and
These ice crystals form near droplets of
bigger (their volume increases about a
super-cooled water (water that remains
million times during this collision
as liquid even when the temperature
process) they become too heavy for the is below 0°C) and increase in size when
air to support them and they fall as
water vapour from cloud droplets is
precipitation. Precipitation is
deposited on the ice crystals. As the ice
the proper term for water
crystals fall, they can collide and this
which falls out of clouds, it can be rain,
makes the ice crystals heavier. When
or snow, or hail.
the ice crystals are too heavy to float in
the air, they fall to the ground. The
crystals become snow or melt
and become rain if the air below
the cloud is more than 0°C.
3. Ice crystals (© Rasmussen and Libbrecht , Y. Furukawa, www.snowcrystals.com)
Water Cycle
Water on the Earth moves in a continuous
cycle. It's weird but true, we drink the same
water that the dinosaur drank!
You now know that liquid water evaporates
from the oceans forming water vapour. This
water vapour then rises, cools and condenses
into clouds. These clouds move over the land
and rain falls out of some of them. The water
which falls fills the lakes, streams and rivers
and eventually flows back into the oceans.
4. The water cycle. Author: J.
Gourdeau.
Once back in the ocean, evaporation starts the
process all over again. Water also falls directly
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onto the land (about 11% of the water which
falls) and moves through the soil ending up in
the rivers and the oceans. Some of the water
which falls on the land is taken up by plants.
The water moves from the roots, through the
stems to the leaves where it can evaporate.
This process is known as transpiration and is
another important part of the water cycle.
Part 2: Formation of clouds
A cloud is composed of millions of little droplets of water or ice crystals,
when temperature is very low, suspended in the air. Clouds can form
when water vapour becomes liquid, i.e. when humid air is cooled the
water vapour condenses onto tiny particles. We now look at the major
ways in which clouds form.
Convection
1. Formation of clouds by
convection process.
Author: J. Gourdeau.
On Earth, the density of air depends on its
temperature. This means that warm air rises and
cold air sinks because warm air is less dense than
cold air. This movement of warm air upwards is
known as convection. Convection is one of the
processes that allows clouds to form. The Sun
warms the surface of the Earth. This warmth heats
the humid air at the ground and, as a result, the air
becomes less dense and begins to rise. As the air
rises, it cools. Clouds are formed when the humid
air cools below a critical temperature: the water
then condenses onto tiny suspended particles and
forms water droplets in the air.
Topography (mountains)
Clouds also form over mountains or hills. These clouds are called orographic
clouds. The air is forced to rise over the mountain and, as it rises, it cools. If
the air cools to its saturation point, the water vapour condenses and the water
contained within the air becomes visible as a cloud.
2. Source: NOAA
The Foehn effect
3. Source: NASA
When air rises over the mountains, it cools and becomes saturated with water
vapour. Condensation occurs and the water vapour becomes liquid. This liquid
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water stays as clouds or falls as rain as the air continues to rise. When this air
descends on the other side of the mountain it contains less water so it
is warmer and dryer. The difference in temperature from one side of a
mountain to the other is known as the Foehn effect.
When different air masses meet
It's not only mountains which force air to rise. When warm air meets a mass of
heavier cold air, the warm air is forced to rise. The boundary between warm
and colder air is called a front. As the warm air rises it cools and as it cools
clouds may form.
4. Cold front. A: cold air; B: warm air. Here cold
air moves towards a warm air mass and forces the
warm air to rise. Author: J. Gourdeau.
Horizontal motion
5. Warm front. A: cold air; B: warm air. Here warm
air moves towards a mass of cold air and rises.
Author: J. Gourdeau.
Sometimes winds bring warm and moist air into a
region. If the warm moist air moves over a much
colder surface, it is cooled and the moisture it
contains will condense and form fog. This process
occurs frequently at the coast.
Part 3: Cloud types
6. Fog over a lake. Source: Ph.
Osset
The different types of clouds in the atmosphere
Clouds are classified into a system that uses Latin words to describe
their appearance and the height of cloud base. This classification was
developed by the English chemist Luke Howard in 1803. The Latin
words used are: cirrus which means "curl of hair"; stratus which means
"layer"; cumulus which means "heap"; and nimbus which means "rain".
Cloud types are divided into four groups. The identification of the first
three groups is based on the height of the cloud base above the ground:
- high level clouds with a cloud base between 5 and 13 km above the
ground
- mid level clouds with a cloud base between 2 and 6 km above the
ground
- low level clouds with a cloud base from 0 to 2 km above the ground.
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The fourth group consists of vertically developed clouds. These clouds
are so thick that they cannot be classified according to the height of
their cloud base above the ground.
High level clouds
High level clouds are named cirrus, cirrostratus and cirrocumulus. Air
temperatures at the altitudes these clouds form at can be less than -40 oC so
these clouds are made of millions of tiny ice crystals, rather than water
droplets.
Cirrus (Ci)
Cirrus clouds are curly, featherlike
clouds and are often the first
clouds to appear in a clear, blue
sky. The shape and movement
of cirrus clouds can often indicate
the strength and direction of high
altitude winds.
These clouds never produce rain or
snow at the Earth's surface.
1. Cirrus cloud. Source: JF Gayet, LAMP.
Cirrocumulus (Cc)
Cirrocumulus clouds look like
small white puff balls high in the
sky. The puff balls can occur
individually or as long rows.
When the puffs are in rows, they
give the cloud a rippling
appearance that resembles the
scales of a fish and distinguishes
it from a cirrus or a cirrostratus
cloud.
2. Cirrocumulus. Source: NOAA.
Cirrostratus (Cs)
These sheet-like, nearly transparent clouds
form at least 6 km above the ground.
Cirrostratus clouds are so thin that the Sun
and Moon can be clearly seen through
them. When sunlight or moonlight passes
through the ice crystals of a cirrostratus
cloud, the light is bent in such a way that a
halo may form. These clouds often indicate
that rain is on its way.
3. Cirrostratus cloud. Source: J. Gourdeau.
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Mid level clouds
Mid level clouds are called altostratus and altocumulus. The prefix
"alto" indicates that they have cloud bases between 2 and 6 km above the
ground.
4. Altostratus
Source: NOAA
5. Altocumulus
Source: NOAA
Altostratus (As)
Altocumulus (Ac)
Altostratus clouds are made up of
both water droplets and ice crystals.
They cover huge areas of the sky, often
over hundreds of square kilometres.
The Sun is visible through these clouds
but it looks as if it is behind frosted
glass. Although altostratus clouds bring
very little precipitation, they often
indicate the increasing likelihood of rain
so don't forget your umbrella!
Altocumulus clouds are white or grey,
or a mixture of both. They look puffy
or like fuzzy bubbles in long rows. The
generally have dark shadowed
undersides. If this shading isn't visible,
it's quite easy to mistake these clouds
for high level cirrocumulus clouds. In
case of doubt, hold your hand at arms
length: if the puff is smaller than one
finger width, you are looking at a
cirrocumulus cloud!
Low level clouds
Clouds which form between the ground and 2 km in height are generally made
up of water droplets and are called stratus, stratocumulus and nimbostratus
clouds.
Stratus (St)
Stratus clouds form in a low layer and cover the sky like a blanket. They develop
horizontally rather than vertically like cumulus clouds. They can form only a few
meters above the ground. A stratus cloud at ground level is fog!
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Stratocumulus (Sc)
Stratocumulus clouds are grey with
dark shading and spread in a puffy
layer. They do not produce rain but
often form after a rainstorm.
6. Stratocumulus clouds. Source: JM Pichon,
Laboratoire de Météorologie Physique
Nimbostratus (Ns)
Nimbostratus clouds form a dark grey, wet
looking, cloudy layer, and are associated
with falling rain or snow. They can also be
considered as mid-level clouds as they can
be up to 3 km thick! They totally mask the
Sun.
7. Nimbostratus. Source: J. Gourdeau.
Vertically developed clouds: cumulus and cumulonimbus
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Cumulus (Cu)
Cumulus clouds look like white balls of
cotton wool. They usually occur
individually with blue sky between
each cloud and they sometimes have
funny shapes. They are formed as a
result of thermal convection (see the
chapter on cloud formation processes)
and have have flat bases and lumpy
tops.
8. Cumulus clouds. Source: JM Pichon, Laboratoire
de Météorologie Physique.
Cumulonimbus (Cb)
Cumulonimbus clouds are the King of the clouds. The top of these clouds can
reach 12 km in height (much higher than the Everest!) and are commonly topped
with an anvil-shaped head. They can sometimes even reach altitudes of 18 km
and penetrate into the stratosphere. The bottom of a cumulonimbus cloud is
made up mostly of water droplets whereas higher in the cloud, ice crystals
dominate as the temperature is well below 0 °C. Vertical winds inside the clouds
can be greater than 100 km h-1. If you like rain, thunder, lightning and even
tornadoes, cumulonimbus are your friends! If not, run quickly into your house!
9. Cumulonimbus clouds. Here you can see
the anvil-shaped top and that it's raining under the
cloud. Source: NOAA.
10. Cumulonimbus clouds from space.
Source: NASA.
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