Atmospheric Moisture & Precipitation

Atmospheric
Moisture and
Precipitation

What is Moisture ?
A small amount of a liquid (such as
water) that makes something wet or
moist
Wetness caused by water; "drops of
wet gleamed on the window"

Water is known to exist in three
different states; as a solid, liquid
or gas.

Clouds, snow, and rain are all made of up of
some form of water. A cloud is comprised of
tiny water droplets and/or ice crystals, a
snowflake is an aggregate of many ice crystals,
and rain is just liquid water.

The solid state of moisture is represented
by solid forms of precipitation – snow –
hail – sleet and tiny ice crystals - Solid
state (e.g. ice) occurs at temperature below
freezing point 00
C.

The liquid state of moisture consists of rain
and of tiny drops of tiny droplets of water
that aggregate to form most clouds. Liquid
state (e.g. sea water) occurs between
freezing and boiling point temperatures
(00
C – 1000
C).

Water existing as a gas is called water
vapor. When referring to the amount of
moisture in the air, we are actually
referring to the amount of water vapor.
If the air is described as "moist", that
means the air contains large amounts of
water vapor - Gaseous state (e.g. water
vapor) occurs at temperatures 1000
C or
2120
F

Water’s Changes of State
Solid to Liquid
The process of changing of state – such as
melting ice – requires that energy be
transferred in the form of heat.
Latent Heat – is the energy absorbed or
released during a change of state.

Liquid to Gas
Evaporation – is the process of changing a
liquid to a gas
Condensation – is the process where a gas –
like water vapor – changes to a liquid -
water

Solid to Gas
Sublimation – is the conversion of a solid
directly to a gas without passing through
the liquid state.
Deposition – is the conversion of vapor
directly to a solid

What is Water Vapor?
Water vapor is water in its gaseous state-
instead of liquid or solid (ice). Water vapor
is totally invisible. If you see a cloud, fog,
or mist, these are all liquid water, not water
vapor.
Water vapor is extremely important to the
weather and climate. Without it, there
would be no clouds or rain or snow, since
all of these require water vapor in order to

evaporates from the surface of the Earth
eventually returns as precipitation - rain or
snow. Water vapor is also the Earth's most
important green house gas, accounting for
about 90% of the Earth's natural
greenhouse effect, which helps keep the
Earth warm enough to support life.
When liquid water is evaporated to form
water vapor, heat is absorbed. This helps to
cool the surface of the Earth. This "latent
heat of condensation" is released again

when the water vapor condenses to form
cloud water. This source of heat helps drive
the updrafts in clouds and precipitation
systems, which then causes even more
water vapor to condense into cloud, and
more cloud water and ice to form
precipitation. It is restricted to the lower
part of the troposphere – half of all water
vapor is found within 1.6 km of the earth’s
surface and only a tiny fraction exists
above 6.4 km.

Water vapor is unevenly distributed and
may be very low over desert areas and may
be up to 4% by volume over equatorial
areas
Evaporation of liquid water is the main
source of water vapor (gas) in the
atmosphere

Water Vapor Cools and Warms the Climate
System?
When water evaporates from the surface of the
Earth, it cools the surface. This keeps the surface
from getting too hot. But because that water
vapor is also the atmosphere's primary green
house gas, water vapor acts to keep the Earth's
surface warmer than it would otherwise be. The
surface cooling effect of evaporation (which
creates water vapor) is stronger than its
greenhouse warming effect.

Water vapor varies by volume in the
atmosphere from a trace to about 4%.
Therefore, on average, only about 2 to 3%
of the molecules in the air are water vapor
molecules. The amount of water vapor in
the air is small in extremely arid areas and
in location where the temperatures are very
low (i.e. polar regions, very cold weather).
The volume of water vapor is about 4% in
very warm and humid tropical air.

Amount of Water Vapor in the Air
There is a limit to how much water vapor can
enter the atmosphere. As more water vapor
enters the atmosphere, the amount of pressure
exerted by that water vapor increases. We call
this pressure the vapor pressure. The higher the
temperature of the atmosphere, the more vapor
pressure it can withstand. When the vapor
pressure maximum is reached, no more water
can enter the atmosphere. At this point, we say
that the atmosphere is completely saturated

Because the maximum vapor pressure
increases with temperature, warmer air can
hold more water vapor before becoming
saturated.

Sources of Moisture in the
Atmosphere
Evaporation from oceans, lakes, rivers &
soil
Transpiration from plants and other
vegetation
Perspiration from people and animals.
Sublimation: change from ice to water
vapor

Evaporation is the process by which water changes
from a liquid to a gas or vapor. Water boils at 212
degrees F (100 degrees C), but it actually begins to
evaporate at 32 degrees F (0 degrees C); it just occurs
extremely slowly. As the temperature increases, the
rate of evaporation also increases.
The amount of evaporation depends on the
temperature, and it also depends on the amount of
water there is to evaporate. For example, there is not
much evaporation occurring in the Sahara Desert, but
why? Although it can get extremely hot in the Sahara,
it's just sand - there's just not much water to be
evaporated.

What is Evapotranspiration?
Evapotranspiration is an important process in the
water cycle because it is responsible for 15% of the
atmosphere’s water vapor. Without that input of water
vapor, clouds couldn’t form and precipitation would
never fall. Evapotranspiration is the combined name
for the processes of evaporation and transpiration.
When water vapor is released into the atmosphere
both processes are involved, so they have been
combined into one word to cover all bases.
The evaporation in evapotranspiration refers to water
evaporated from over land. This includes evaporation
from soil, wetlands, and standing water from places

like roofs and puddles. It can also refer to direct
evaporation of liquid water from the leaf surface of the
plant.
Transpiration happens when plants release water
vapor from tiny holes, called stomata, in their leaves.
This is caused in part by the chemical and biological
changes that occur as the plant undergoes
photosynthesis and converts carbon dioxide into
oxygen. Transpiration performs the same function as
human sweating because plants do it to cool down
their leaves.

Factors that affect evapotranspiration:
Temperature – As temperature increases, the rate of
evapotranspiration increases. Evaporation increases
because there is a higher amount of energy available
to convert the liquid water to water vapor.
Transpiration increases because at warmer
temperatures plants open up their stomata and release
more water vapor.
Humidity – If the air around the plant is too humid,
the transpiration and evaporation rates drop. It’s the
same reason sweat does not evaporate from our skin
when it’s too humid.
Wind speed – If the air is moving, the rate of
evaporation will increase. The wind will also clear the

transpiration, so the plant will increase its rate of
transpiration.
Water availability – If the soil is dry and there is no
standing water there will be no evaporation. If plants
can’t get enough water they will conserve it instead of
transpiring by closing their stoma.
Soil type – Soil type determines how much water soil
can hold and how easy it is for the water to be drawn
out of it, either by a plant or by evaporation. For areas
where the ground is covered by vegetation, the rate of
transpiration is considerably higher than the rate of
evaporation from the soil.

Plant type – Some plants, like cacti and other
succulents, naturally hold onto their water and don’t
transpire as much. Trees and crops are on the other
end of the spectrum and can release copious amounts
of water vapor in a day. For example, an acre of corn
can release 4,000 gallons of water vapor a day and a
single large oak tree can transpire 40,000 gallons of
water vapor in a year.

Humidity
Humidity is the amount of water vapor
in the atmosphere
It’s may be measured and expressed as:
- Absolute Humidity (gm/m3
)
- Specific Humidity (gm/kg)
- Relative Humidity (%)

A device to measure humidity is called
a hygrometer. Hygrometers may be
designed for indoor or outdoor use (or
both).

The psychrometer which is also called the “wet- and
dry-bulb thermometer” is a type of hygrometer that
measures the water vapor present in the air. A
psychrometer is a kind of hygrometer and is used for
the elite reasons of finding out the relative humidity,
or moisture content, in the air. Relative
humidity refers to the moisture content in the air
compared to how much moisture the air can grasp at a
given temperature. Information about the
relative humidity is obliging in understanding the
weather. A psychrometer is an instrument that
computes both the wet bulb and dry bulb
temperatures. Two thermometers are required to
compute these constraints. From the values obtained,

Absolute Humidity
Absolute Humidity measures the total
water vapor content of air.
Absolute Humidity is the maximum
amount of water vapor that a given column
of air can hold.
It’s expressed as weight of water vapor in a
given volume of air (gm/m3
)
Absolute Humidity is very sensitive to:
- temperature changes
- changes in air volume

Specific Humidity
Specific Humidity measures the mass or weight
of water vapor content in a given mass of air
It’s expressed as mass (gm) of water vapor in
one kilogram (kg) of air (gm/kg)
For any given temperature, there is a maximum
mass of vapor that a kilogram of air can hold

Specific Humidity
In cold polar air, specific humidity may be as
low as 0.2gm/kg
In warm equatorial air, specific humidity
may be as high as 16gm/kg
Specific humidity progressively increases
from the poles to a single peak at the equator

Specific Humidity
Both absolute and specific humidity measure
the quantity of perceptible water in the
atmosphere

Relative Humidity
Relative Humidity (R.H.) measures the
percentage of water vapor present in the
atmosphere relative to the maximum
quantity the air could hold at the given
temperature
Relative Humidity (R.H.) changes as air’s
capacity to hold moisture (i.e. its vapor
content) changes

Relative Humidity
There is an inverse relationship between temperature
and relative humidity such that relative humidity is
lower during the hot daytime and higher during the
cooler night

Relative Humidity and Temperature ChangesRelative Humidity and Temperature Changes

Relative Humidity
Relative Humidity (R.H.) increases if:
- evaporation increases vapor content of
air
- cooling reduces the holding
capacity of air
Relative Humidity decreases if:
- moisture is removed by condensation
or dispersal
- heating increases air holding capacity

Relative Humidity
Relative Humidity (R.H.) could be
measured using this formula:
Vapor pressure
R.H. = ---------------------------------- X
100
Saturation Vapor Pressure
R.H. is 100% at saturation
R.H. is 50% if only half of the total vapor
is present

Relative Humidity
Air that has R.H. of 100% (saturation) at
cool temperature might be far from
saturation at a warmer temperature even if
the actual amount of moisture is unchanged
Dew point is the temperature at which air
becomes saturated during cooling

Relative Humidity
Instrument used for measuring relative
humidity is called a hygrometer
A homemade hygrometer uses a strand of
human hair attached to the end of a pointer to
determine R.H.
The hair changes in length in response to
changes in R.H.

Relative Humidity
The pointer falls whenever R.H. is high, or
the hair lengthens when R.H. is high
Another method for measuring R.H. is
through the use of a psychrometer which
consists of 2 thermometers:
- a dry bulb thermometer
- a wet bulb thermometer

Relative Humidity
The difference between the dry and wet bulb
thermometer readings is called wet bulb
depression
It involves the calculation of the wet bulb
depression
The wet bulb depression is entered into a
psychrometric table to obtain the R.H.

Pole-To-Pole Variation ofPole-To-Pole Variation of
Relative Humidity (%)Relative Humidity (%)

Dew Point
The dew point temperature is the temperature at
which the air can no longer "hold" all of the water
vapor which is mixed with it, and some of the
water vapor must condense into liquid water. The
dew point is always lower than (or equal to) the air
temperature. If the air temperature cools to the dew
point, or if the dew point rises to equal the air
temperature, then dew, fog or clouds begin to form.
At this point where the dew point temperature
equals the air temperature, the relative humidity is
100%.

If there is then further cooling of the air, say
because the air parcel is rising to higher (and thus
colder) levels in the atmosphere, even more water
vapor must condense out as additional dew, fog, or
cloud, so that the dew point temperature then falls
along with the air temperature. This is how
precipitation forms...when water vapor is removed
from the air so rapidly that the liquid water drops
grow to a size where they fall out of the cloud.

CONDENSATION
Condensation is a process by which a gas
such as water vapor is changed into
liquid water. When moisture cools and
reaches saturation point, the tiny particles
of water condenses into larger drops of
water.

Forms of Condensation
Dew: Tiny drops of water formed when condensation
of water vapor occur at or near the surface of the
earth.
Frost: It is a frozen condensation that occurs when air
a ground level is super cooled below the freezing
point.
Fog: A mass of tiny drops of water that form when
water vapor condenses on a nuclei near the earth's
surface.
Clouds: A cloud is a mass of tiny drops of water that
results from condensation which takes place high up
in the atmosphere.

CLOUDS
A visible collection of particles of water or
Ice suspended in the air, usually at an
elevation above the earth's surface.
They are a spectacular feature in the sky.
They are the source of precipitation.
Not all clouds precipitate – but all
precipitation comes from the clouds.
Clouds absorb – reflect scatter and re-radiate
insolation. The function of the clouds in the

CLOUDS
Clouds are made of tiny drops of water or ice crystals
that settle on dust particles in the atmosphere. The
droplets are so small - a diameter of about a hundredth
of a millimeter - that each cubic meter of air will
contain 100 million droplets.
Clouds will either be composed of ice or water
droplets depending on the height of the cloud and the
temperature of the atmosphere. Because the droplets
are so small, they can remain in liquid form in
temperatures as low as -30 °C. Extremely high clouds
at temperatures below -30 °C are composed of ice
crystals.

How do clouds form?
Clouds form when the invisible water vapor in the air
condenses into visible water droplets or ice crystals.
There is water around us all the time in the form of
tiny gas particles, also known as water vapor. There
are also tiny particles floating around in the air - such
as salt and dust - these are called aerosols.
The water vapor and the aerosols are constantly
bumping into each other. When the air is cooled, some
of the water vapor sticks to the aerosols when they
collide - this is condensation. Eventually, bigger water
droplets form around the aerosol particles, and these
water droplets start sticking together with other

droplets, forming clouds.
Clouds form when the air is saturated and cannot hold
any more water vapor, this can happen in two ways:
The amount of water in the air has increased - for
example through evaporation - to the point that the air
cannot hold any more water.
The air is cooled to its dew point - the point where
condensation occurs - and the air is unable to hold any
more water.
The warmer the air is, the more water vapor it can
hold. Clouds are usually produced through
condensation - as the air rises, it will cool and
reducing the temperature of the air decreases its ability
to hold water vapor so that condensation occurs. The

height at which dew point is reached and clouds form
is called the condensation level. Clouds mostly form
and remain at some elevation above the earth’s surface
- They can come into direct contact with the surface
and this is known as fog.

What causes clouds to form?
There are five factors which can lead to air
rising and cooling and clouds forming.
1. Surface heating - This happens when the
ground is heated by the sun which heats the
air in contact with it causing it to rise. The
rising columns are often called thermals.
Surface heating tends to produce cumulus
clouds.

2. Topography or orographic forcing - The
topography - or shape and features of the area -
can cause clouds to be formed. When air is
forced to rise over a barrier of mountains or hills
it cools as it rises. Layered clouds are often
produced this way.
3. Frontal - Clouds are formed when a mass of
warm air rises up over a mass of cold, dense air
over large areas along fronts. A 'front' is the
boundary between warm, moist air and cooler,
drier air.

4. Convergence - Streams of air flowing from
different directions are forced to rise where they
flow together, or converge. This can cause
cumulus cloud and showery conditions.
5. Turbulence - A sudden change in wind speed
with height creating turbulent eddies in the air.
The range of ways in which clouds can be
formed and the variable nature of the
atmosphere results in an enormous variety of
shapes, sizes and textures of clouds.

Clouds
Clouds are composed of water droplets and
ice crystals
The base of a cloud is clear-cut and
corresponds with the LCL (lifting
condensation level)
Clouds grow upward from the base or LCL
(lifting condensation level)

Clouds
Cloud may be classified on the basis of
height of its base – general structure and
appearance:
- High cloud or cirrus cloud
(> 6 km cloud base)
- Middle cloud or alto cloud
(2 – 6 km cloud base)
- Low cloud (0 – 2 km cloud base)

Cloud Names
Names of specific types of clouds are
created by combining the name of the
cloud's shape with the name of the cloud's
height
There are 3 main types of clouds:
Cumulus or fluffy clouds
Stratus or layered clouds
Cirrus or thin feathery clouds

Low Clouds
•Stratus
•Cumulus
•Nimbostratus
Middle Clouds
• Stratocumulus
•Altocumulus
•Altostratus

High Clouds
• Cirrus
• Cirrostratus
• Cirrocumulus

Cirriform Clouds
The word cirrus comes from a Latin word
and means a tuft or curl of hair. Cirrus
clouds are very wispy and feathery looking.
Cirrus generally occur in fair weather and
point in the direction of air movement at
their elevation.
They are found at high elevations.

Stratiform Clouds
Low clouds are of made of water droplets.
However, when temperatures are cold
enough, these clouds may also contain ice
particles and snow.
The word stratus comes from the Latin
word that means "to spread out." Stratus
clouds are horizontal, layered clouds that
stretch out across the sky like a blanket.

Cumuliform Clouds
Cumulo- means “heaped” or “piled”
Cumulus clouds are flat-based, billowing
clouds with vertical doming. Often the top
of cumulus clouds have a "cauliflower-like"
appearance. Cumulus clouds are most
prominent during the summer months.
Cumulus or fluffy clouds form when air is
forced up rapidly and therefore rises higher.

They can be associated with both fair
weather and bad weather when they get
really tall

High Clouds or Cirrus (Ci)
clouds
- cloud base is above 6 km
- small amount of water vapor present
- low temperature
- thin and white clouds of ice crystals
- examples: cirrus, cirro-cumulus,
cirro-stratus

Middle clouds or Alto clouds
- occurs between 2 and 6 km
- examples: altocumulus, altostratus
Low clouds or Stratus Clouds
- occurs between 0-2 km
- examples: stratus, stratocumulus,
nimbostratus and cumulonimbus
- cumulonimbus cloud has vertical
extent extending up to 15 km or more

Vertically Developed Cumulus
Clouds
Some clouds do not fit into any one of the
three height categories mentioned. Such
clouds have their bases in the low height
range but often extend upward into the
middle or high altitudes. Probably the most
familiar of the classified clouds is the
cumulus cloud. Generated most commonly
through either thermal convection or frontal

lifting, these clouds can grow to heights in
excess of 39,000 feet (12,000 meters),
releasing incredible amounts of energy
through the condensation of water vapor
within the cloud itself.
Cloud types include: fair weather
cumulus and cumulonimbus.
They are also called the clouds with Anvil
head.
They indicate very active vertical air
movements.

Precipitation
The definition of precipitation is any form
of water - liquid or solid - falling from the
sky. It includes rain, sleet, snow, hail and
drizzle plus a few less common occurrences
such as ice pellets, diamond dust and
freezing rain.
Precipitation comes only from clouds that
have the root “Nimb” in their name –
Nimbostratus - Cumulonimbus

Precipitation
For clouds to produce precipitation that
will fall to the ground:
- the tiny water droplets in clouds
must form large raindrop sizes
- the raindrop sizes must be large
enough to overcome:
 atmospheric turbulence and
 fall through evaporation

Precipitation forming Process:
The mechanism for producing larger
raindrops in clouds include:
- ice-crystal formation process or
Bergeron process in cold clouds
- collision-coalescence process in warm
clouds

Precipitation forming Process:
Ice-Crystal Formation – Bergeron Process
- Super-cooled water evaporates to
replenish the vapor supply
- Hence, ice-crystals grow in size at the
expense of the super-cooled water
droplets
- Ice-crystals grow large enough to fall
and picks up more moisture en-route

- It occurs in cold clouds, especially in
temperate regions
- In cold clouds, ice crystals and super-
cooled water droplets coexist
- Ice-crystals attract most of the water
vapor because of lower vapor pressure
around them

The enlarged ice-crystals precipitate to
form:
- snowflakes when atmospheric
temperature is below freezing
- rain when the ice-crystals melt enroute
to the ground

First proposed by the Swedish
meteorologist Tor Bergeron in the early
1920s, the Bergeron Process takes place
when ice crystals form high in the cloud
tops. These small, often microscopic ice
crystals attract more water vapor, causing
them to increase in size. As the ice crystals
increase in size, the vapor pressure drops.
This allows surrounding water droplets to
evaporate, becoming smaller and smaller as
the ice crystals grow. Eventually these ice

crystals become large and heavy enough
that they begin to fall towards the Earth’s
surface. As they do, they pass through the
lower warmer portion of clouds, attracting
even more water vapor and growing larger
still. These ice crystals can then fall to the
surface of the Earth as snow, or they can
melt, becoming rain drops. It is believed
that most precipitation happens in this
manner.

Ice-Crystal Formations in CloudsIce-Crystal Formations in Clouds

The collision-coalescence process
The collision-coalescence process is an
important mechanism in forming raindrops
in warmer clouds (those with tops warmer
than -15°C = 5°F). In these warm
clouds raindrops form exclusively by this
process. Most tropical rain is formed in this
way. The collision-coalescence process is
of relatively little importance in middle and
high latitudes where, even in the summer,
most precipitation begins high in the clouds

where temperatures are well below freezing
and the dominant precipitation-producing
mechanism is the so-called
ice-crystal or Bergeron process.
However falling raindrops in
these clouds do grow by the collision-
coalescence process.

In many parts of the world the air is too warm
for ice crystals to form. This being the case, rain
and snow cannot develop following the
Bergeron Process. Instead, tiny droplets form as
they collide into one another creating larger and
larger droplets. The coalescence of droplets into
larger droplets can only take place if the droplets
have an opposite electrical charge. That is to say
that if one droplet has a positive charge and the
other a negative charge, then they will coalesce
(combine) upon collision. Otherwise they will
just bounce off of one another.

Raindrop Formation By Collision andRaindrop Formation By Collision and
CoalescenceCoalescence

Forms of Condensation
Dew - is a type of precipitation where water
droplets form on the ground, or on objects near the
ground in a process called condensation of
moisture. Dew forms during calm, clear nights,
when the ground surface and other exposed objects,
such as tips of grass or leaves, lose heat by
radiation to the sky. The greater the amount of
water vapor in the air the lower the temperature of
the ground – the more abundant the dew. The dew
point is the temperature at which the water vapor
in the air becomes saturated

Mist - is tiny droplets of water hanging in
the air. These droplets form when warmer
water in the air is rapidly cooled, causing it
to change from invisible gas to tiny visible
water droplets. Mist can reduce visibility to
between 1 and 2 kilometers (0.6 - 1.2
miles).

Fog - There is absolutely no difference
between fog and clouds found high in the
air. Fog is simply a cloud that has formed
near the surface of the Earth. Fog shows up
when water vapor, or water in its gaseous
form condenses. During condensation,
molecules of water vapor combine to make
tiny liquid water droplets that hang in the
air. You can see fog because of these tiny
water droplets. Water vapor, a gas, is
invisible.

There are four main types of fog: Radiation
fog, advection fog, upslope fog and
evaporation fog.
Radiation Fog
As the ground becomes cooled by the night
sky, the air above the ground also gets
cooled. This can bring the air down below
its dew point, causing water vapor to
condense around the dust and other
particulates in the atmosphere creating fog.

Advection Fog
Similarly, advection fog takes place when
warm air moves in horizontally over a cool
surface, such as when air from the ocean
blows in over land. The land cools the air
down below the dew point, causing fog to
occur.

Upslope fog
Upslope fog takes place as warm air passes
over the slope of a cool mountain as it rises.
The mountain cools the air below the dew
point causing it to condense, forming fog.

Evaporation fog
This form of fog is caused when additional
water vapor enters air that is already near
the maximum vapor pressure. Because the
air is already nearly saturated, the
additional water vapor causes the air to
reach the dew point, forming fog.

Frost
Is water vapor, or water in gas form, that
becomes solid. Frost usually forms on
objects like cars, windows, and plants that
are outside in air that is saturated, or filled,
with moisture. Areas that have a lot
of fog often have heavy frosts. Frost forms
when an outside surface cools past the dew
point. The dew point is the point where the
air gets so cold, the water vapor in the

This liquid freezes. If it gets cold enough,
little bits of ice, or frost, form. The ice is
arranged in the form of ice crystals.

Forms of Precipitation
Snow
Snow forms when water vapor turns
directly into ice without ever passing
through a liquid state. This happens as
water condenses around an ice
crystal. Snow can take the form of ice
pellets or snow flakes. As snow falls to the
ground, it often melts on the warm surface
of the Earth. If the surface of the Earth is

chilled sufficiently, snow begins to pile up,
creating snow drifts. In some locations,
such as mountains, these snow drifts can
reach several feet or meters in depth.

Sleet
Sleet refers to a mixture of snow and rain as
well as raindrops that freeze on their way
down. Unlike snow, the raindrops pass
through a liquid form before freezing. The
result is that they are not light and fluffy.

Glaze (Freezing Rain)
Freezing rain, which is sometimes referred
to as glaze, takes place when water droplets
become super-chilled. They do not freeze
in air, but rather freeze the instant they
strike an object, such as a road or car. The
result can make roads very slippery, and
can cause car doors to become frozen shut.

Hail
Hail forms in a complex dance between
moisture and wind. Deep within
cumulonimbus clouds ice crystals form and
begin to fall towards the Earth’s surface. As
this happens, wind gusts pick up the ice
crystals, pushing them back up high into
the clouds. As they begin to again fall
down, they continue growing in size.
Again, a wind gust might catch the growing
hail stones, pushing them back up high into

the clouds. This process may be repeated
several more times until the hail stones
become so large that they are too heavy for
the wind to carry, causing them to fall
towards the Earth.

Rain
Rain is by far the most common type of
precipitation in our atmosphere. Rain takes
place when drops of liquid water fall all the
way to the surface of the Earth. Rain often
takes one of two main forms. These two
forms are showers and drizzles. A shower
lasts just a brief period of time, and usually
is made up of large heavy drops. Drizzles
generally last much longer, and are made
up of smaller, finer droplets of water.

Rain can either form as ice crystals melt or
as a coalescence of many smaller water
droplets. In drizzle the rain drops are
extremely small like a fine spray – diameter
of droplets 0.2 to 0.5 mm. Rain drops have
a diameter of more than 0.5mm. Rain
usually from the cumulonimbus clouds.

Types of Rainfall
Physical/Orgographic/Relief Rainfall
1. Prevailing winds bring warm, moist air
to a mountain barrier
2. Air is forced to rise over high areas.
3. Air cools and condenses.
4. Clouds form and it rains.
5. Air descends on the other side of the
mountains.
6. It warms up and therefore becomes drier.

Frontal rainfall
When warm and cold air meet,
a depression forms:
1.When a cold polar air mass meets a warm
tropical air mass they do not mix – they
form fronts.
2.The colder air mass is heavier than the
warmer air mass, therefore the lighter,
warmer air rises over the top of the
heavier, colder air.

3.As the warm air is forced to rise it cools.
Also, the warm air is in contact with the
cold air along the fronts, and this also
cools.
4.Condensation occurs and clouds form.
5. Rain occurs along the front.

Convectional rainfall
When the land warms up, it heats the air
above it. This causes the air to expand and
rise. As the air rises it cools and condenses.
If this process continues then rain will fall.
This type of rainfall is very common in
tropical areas.

Coastal rainfall
This is caused by unequal heating and
cooling of seawater and adjacent lands.
When moist air comes from the sea – if the
land of the coastal area is cool – it
condenses and precipitates. Coastal rainfall
often occurs within 10km of the coastline

Global Patterns of PrecipitationGlobal Patterns of Precipitation
ISOHYET: a line on a map or chart
connecting areas of equal rainfall

Global Patterns of PrecipitationGlobal Patterns of Precipitation
 The global average annual precipitation
is about 99 cm (39 inches)
 Total annual rainfall tends to decrease
from the equator toward the poles
 Hence, equatorial region receives the
highest amount of precipitation, with an

Global Average Annual PrecipitationGlobal Average Annual Precipitation

Global Pattern of PrecipitationGlobal Pattern of Precipitation
The subtropics, between lat 15o
and 35o
N & S are substantially drier due to air
subsidence
Hence, the continental west coasts of the
subtropics washed by cold currents are
the world's driest deserts
However, the continent east coasts
washed by warm ocean currents are often
wetter

Global Pattern of PrecipitationGlobal Pattern of Precipitation
The middle latitudes, between lat. 35o
and
65o
N and S, receive total precipitation
that is close to the global average – rain is
heavier on the east coast and decreases
travelling towards the west
It is the zone of Westerly wind with
winter cyclones
California in this zone have summer
drought when STH shifts pole ward

The high latitudes, between lat. 65o
N &S
and the Poles, receive low annual
precipitation because high latitude cold
air has very low capacity for moisture
The zonal distribution of precipitation
described is often complicated by:
- land and sea distribution
- ocean currents and
- topography
Global Pattern of Precipitation

Global Pattern of Precipitation
• The coastal areas of the world receive a
great amount of rain fall than the interior
of the continents
•

Atmospheric Moisture & Precipitation

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