Weather and Climate

WEATHER AND CLIMATE

1.1: WEATHER
AND CLIMATE

In this topic, you will learn:
◼What is weather and climate?
◼What are climatic hazards?

LESSON OBJECTIVES

◼ Weather is the

state of the
atmosphere at a
specific place and
time.

WEATHER

What are the different
weather variables
shown?

Weather is described using variables such as:
1. Temperature - the degree of how hot or cold a place is,

as measured using a thermometer.

2. Cloud cover - the amount of sky covered by clouds.
3. Precipitation - all forms of water that fall to the Earth

from the atmosphere e.g. hail, snow, rain.

4. Wind speed - how fast the wind travels
5. Wind direction - the direction from where the wind

comes from.

WEATHER

• Refers to the average state of the atmosphere at

a particular place over a long period of time
(usually over 25 years).

CLIMATE

Seoul’s climograph

1. Tropical Equatorial Climate
2. Tropical Monsoon Climate
3. Cool Temperate Climate

TYPES OF CLIMATE

◼Between latitudes 10° north and south of equator
◼E.g.: Singapore, Malaysia, Brazil

EQUATORIAL CLIMATE

◼High temperatures all year round, about 27°C
◼Small annual temperature range of 2 °C to 3 °C
◼High annual precipitation of more than 2000 mm/year
◼Precipitation falls evenly throughout the year

EQUATORIAL CLIMATE

CLIMOGRAPHS

◼A climograph is a graph that shows a particular location's

average temperature and precipitation during the year.

Title
The title at the top shows the location

Months
The letters at the bottom show the months of the year

Precipitation
• The average

precipitation
for each month
is indicated by
the bars

• Which may be

expressed in
millimetres
(mm) or inches
(in)

Temperature
∙The average
temperature for
each month is
indicated by the
line graph

∙Which may be
expressed in
Celsius (oC) or
Fahrenheit (oF)

Steps to reading a climograph:

Look for patterns in the temperature data.
∙Is the temperature constant or fluctuating all year round?

∙Which month is the warmest? What is the temperature?

∙Which month is the coolest? What is the temperature?

∙What

isthe

annual

temperature

range?

(Subtract

the

minimum

temperature from the maximum temperature).

Look for patterns in the rainfall data.
∙Does the rainfall occur all year round?

∙Which are the driest and wettest months? State the values.

∙What is the total annual rainfall? Add each month's total together to get
the annual total.

If possible, try to identify the type of climatic region the country / city falls

under, e.g. tropical equatorial, tropical monsoon, cool temperate.

Practice: Identify the type of climate experienced in
Singapore and describe its temperature and rainfall
distribution.

◼Between latitudes 5°-30°North and South of the

Equator

◼E.g. Vietnam, Bangladesh, Mumbai
◼High mean annual temperature around 25°C
◼Small annual temperature range 3-4°C
◼High annual precipitation of about 1500mm
◼Unevenly distributed as there are distinct wet and dry

seasons

MONSOON CLIMATE

◼Between latitudes 45°- 60°North and South of the

Equator

◼E.g. London, United Kingdom
◼E.g. Paris, France
◼4 distinct seasons
◼Large annual temperature range of about 21 °C
◼Low precipitation between 300mm to 900mm
◼Precipitation falls evenly throughout the year

COOL TEMPERATE CLIMATE

◼Identify the

type of climate
experienced at
Station X and
describe its
temperature
and rainfall
distribution.

PRACTICE: READING A CLIMOGRAPH

Station X

◼Station Y : cool temperate climate
◼Characteristics: moderate mean annual temperature

of about 20˚C with

◼large annual temperature range of 16˚C OR
◼experiences seasons where temperatures are higher

than 25˚C during summer from November to March
and lower than 15˚C during winter from June to
August

◼Rainfall distribution is quite even with low annual

total of about 400mm

IDENTIFY & DESCRIBE CLIMATIC TYPE

◼Climates are changing due to natural and human

induced causes.

◼Predictable weather patterns are now beginning to

change and becoming unpredictable.

◼Extreme weather events, such as heat waves,

droughts, floods, cyclones and wildfires and expected
to occur more often or with greater severity.

◼These can impact natural and human systems

significantly.

WHAT ARE CLIMATIC HAZARDS?

DIFFERENCE BETWEEN NATURAL AND

HUMAN SYSTEMS

Natural systems

Human systems

•Natural systems consist of
natural processes that occur on
Earth, and all the living and non-
living things that are found
naturally. These natural
processes and living and non-
living things are interconnected.

•An example of a natural system
is an ecosystem.

•On the other hand, human
systems consist of human-led
processes and the interconnected
human activities.

•An example of a human system
is agriculture.

1.2 VARIATION IN
AIR
TEMPERATURE
ACROSS EARTH’S
SURFACE

In this topic, you will learn:
◼Why air temperature varies across the earth’s surface

LESSON OBJECTIVES

• Hottest temperature recorded on Earth?

– 56.7°C in Death Valley, California, on 10 July

1913

• Coldest temperature recorded on

Earth?
– Official record- Vostok station on July 21,

1983
-89.2 °C

– Vostok is a Russian station in Antarctic.

◼Air temperature at a place varies:
• Throughout the day because of Earth’s rotation

on its axis.

HOW AND WHY DOES AIR TEMPERATURE

VARY ACROSS TIME?

◼The side of the Earth that faces

the sun receives solar radiation
and experiences day time and
higher temperatures.

◼As the Earth continues to rotate,

this side will eventually face away
from

the

sun

and

experience

night

time

and

therefore

experiences lower temperatures.

◼The Earth takes about 24 hours to

complete one rotation on its axis,
hence that is the length of a day
we experience.

HOW AND WHY DOES AIR TEMPERATURE

VARY ACROSS TIME?

◼Air temperature at a place varies:
• Over time in a year due to the earth’s

revolution around the sun.

HOW AND WHY DOES AIR TEMPERATURE

VARY ACROSS TIME?

HOW AND WHY DOES AIR TEMPERATURE

VARY ACROSS TIME?

Time of
the year

Explanation for mean temperatures

June

Due to the Earth’s tilted axis, in June, the northern hemisphere leans towards
the sun. It receives more direct solar radiation. Solar radiation is spread over
a smaller area and is more concentrated, resulting in higher temperatures.

Sept

In September, both the hemispheres do not lean towards the sun. At this
time, both hemisphere experiences moderate temperature.

Dec

Due to the Earth’s tilted axis, in December, the northern hemisphere leans
away from the sun. It receives the least direct solar radiation and solar
radiation spreads over a larger area compared to the other months. Hence, it
experiences the lowest temperature.

March

In March, both the hemispheres do not lean towards the sun. At this time,
both hemisphere experiences moderate temperature.

HOW AND WHY DOES AIR TEMPERATURE

VARY ACROSS PLACES?

Latitude

Altitude

Type of
surface

Distance

from the sea

◼Latitude refers to

the distance to
the north and
south of the
Equator.
Latitudes are
imaginary
horizontal lines
running from east
to west around
the earth
measured in
degrees

FACTORS INFLUENCING TEMPERATURE AT

LOCATIONS : 1)LATITUDE

◼Temperatures decreases from lower latitude to higher

latitude. WHY?

◼The earth tilts at 23.5° on its axis. As a result, the sun’s

ray strike at various angle at different parts of the
earth.

◼The angle of incidence is the angle at which the sun’s

ray reaches the earth. When angle of incidence is
higher, sun’s rays will be more concentrated on the
area.

HOW DOES LATITUDES AFFECT

TEMPERATURES?

Higher latitudes:
Lower angle of incidence🡪
sun’s rays strike at a lower
angle 🡪 solar energy spread
out over a wider area 🡪 lower
temperatures

Lower latitudes (between 23.5oN
and S of Equator):
Higher angle of incidence🡪 solar
energy is concentrated on a small
area 🡪 higher temperatures

GLOBAL SURFACE TEMPERATURE DISTRIBUTION

2. ALTITUDE

◼Example

▪ Beijing, China(40oN of

equator) has temperature
averaging 12oC

▪ Singapore (1oN of equator)

has temperature averaging
29oC

FACTORS INFLUENCING TEMPERATURES AT

DIFFERENT LOCATIONS:2) ALTITUDE

◼Altitude: Height of a place in relation to sea

level.
- General decrease of 6.5°C for every 1,000 m increase in

altitude.

40

Reason –
Air density
1. Lower air density and pressure at

higher altitude 🡪 lower ability to
absorb and radiate heat 🡪 lower
temperature

2. Higher air density and pressure at lower

altitude🡪 higher ability to absorb and radiate
heat 🡪 higher temperature

ATMOSPHERIC STRUCTURE= ALTITUDE &

PRESSURE RELATIONSHIP

Distance from Ground

Gas Molecules

1. Molecules closely
packed at lower heights
– gravity & compaction

2. Higher altitude,
molecules more
spread out

∴Higher altitudes
🡺

less gravity

🡺
lower air

pressure, since air
is less dense.

Static

Situation

2. ALTITUDE

◼Example

▪ Johor Bahru in Malaysia has a

lower altitude of around 30m
above sea level and a higher
mean annual temperature of
27oC

▪ Cameron Highlands in

Malaysia has a higher altitude
of around 1,500m above sea
level and a lower mean
annual temperature of 18oC

◼Study Fig. 1, which shows the average monthly

temperature at Mishima, situated at the foot of Mt.
Fuji, and the average monthly temperature at the
peak of Mt. Fuji.

◼With reference to Fig. 1, describe and account

for the effect of altitude on temperature.

PRACTICE 1: LINE GRAPH

FIG. 1

◼The annual average temp for Mishima (foot of Mt.

Fuji) is much higher at 15.3˚C as compared to -7.1˚C at
the peak of Mt Fuji

◼The peak of Mt. Fuji experiences extremely low

temperatures during the winter months, as temp
ranges between -15˚C to -20˚C while the winter temp
at Mishima ranges from 5˚C to 10 ˚C

◼In summer, the temp at the peak of Mt Fuji ranges

from 0-5 ˚C whereas, at Mishima which is its foot,
summer temp range from 20-25˚C.

DESCRIBE: WRITE WHAT YOU SEE

◼This is because the atmosphere is not warmed directly

by the sun, but by heat radiated from the earth’s
surface.

◼In addition, the density of the air decreases with

increasing altitude, so does the ability to trap heat.

◼This is because the molecules in the air which receive

and retain heat become fewer and more widely
spaced as height increases

ACCOUNT FOR : EXPLAIN

◼Dark surfaces such as soil, mud, forests absorb most of the solar

radiation and radiate more heat.

◼ On the other hand, light-coloured surfaces, such as snow, ice or

clouds, tend to reflect high proportions of solar radiation, instead of
absorbing. Hence, they radiate less heat.

3. TYPE OF SURFACE

◼Urban areas tend to have warmer temperatures than

rural areas due to the type of surfaces found at urban
areas.

3. TYPE OF SURFACE

◼Urban environments like the CBD

tend to comprise large areas of
dark surfaces such as tarmac
roads. These darker surfaces are
able to absorb and radiate heat
better than lighter surfaces,
causing air temperatures to rise.

◼ In addition, glass-covered

skyscrapers in the CBD reflect
sunlight to the ground surface,
thereby increasing absorption and
radiation of heat

3. TYPE OF SURFACE

• Land absorbs and loses heat faster than the sea hence,

• Seas and oceans takes a longer time to heat up, but once

heated up will retain heat longer.

• Land, on the other hand, heats up very quickly and loses heat

quickly.

FACTORS INFLUENCING TEMPERATURE AT
LOCATIONS: 4) DISTANCE FROM THE SEA

VS

Maritime Effect

Criteria

Continental Effect

• Maritime effect is the effect

large ocean bodies have on
coastal area.

• During summer, the air over the

land heats up quickly while the
sea remains cool as the sea
gains heat slower than land.

• The cooler air over the seas

helps lower the temperature of
the areas along the coast,
leading to cooler summers.

Effects of
surface on

climate of area

Summer

Temperature

• Continental effect is

the effect that
continental surface
have on the climate of
inland areas.

• During summer, the

air over the land heats
up quickly leading to
warmer summers.

MARITIME VS CONTINENTAL EFFECT

MARITIME VS CONTINENTAL EFFECT

Maritime Effect

Criteria

Continental Effect

∙During winter, the sea
remains relatively warm
compared to the air over the
land because the sea takes a
longer time to lose heat.

• The warmer air over the seas

helps to raise the
temperature of the coastal
areas, leading to warmer
winter.

Winter

Temperature

Temperature

Range

∙During winter the air
over the land loses
heats quickly leading
to colder winters.

• Therefore the annual

temperature range is
usually larger than
coastal areas.

FACTORS INFLUENCING TEMPERATURES AT

DIFFERENT LOCATIONS (DISTANCE FROM SEA)

LOCATION

LATITUDE

DISTANCE
FROM SEA

TEMPERATURE

RANGE

ANCHORAGE

61.1919° N

1 KM

(Near Coast)

23°C

FAIRBANKS

64.8378° N

420KM
(Inland)

39°C

The sea heats up and cools downs more
slowly than land which influences the

temperatures of inland and coastal areas.

55

◼Anchorage has a smaller annual temperature than

Fairbanks. Anchorage is located near the sea and is
influenced by the maritime effect.

◼During summer, the land heats up more quickly than

the sea, resulting in air over the sea being cooler than
air over the land. The cooler air over the sea helps
lower the temperature in Anchorage.

◼During winter, the air over the sea remains warmer

than the air the land. The warmer air over the sea
increases the temperature in Anchorage.

EXPLAIN THE DIFFERENCE IN THE ANNUAL TEMPERATURE RANGES

OF FAIRBANKS AND ANCHORAGE.

◼Due to the maritime effect, Anchorage experiences cooler

summers, warmer winters, and a smaller annual
temperature range than Fairbanks.

◼In contrast, Fairbanks is located inland and is influenced by

the continental effect. Being located further from the sea,
the temperature in Fairbanks is not influenced by the sea.

◼Fairbanks experiences warmer summers and colder

winters.

◼There is therefore a larger difference between summer and

winter temperatures, and Fairbanks experiences a larger
annual temperature range than Anchorage.

EXPLAIN THE DIFFERENCE IN THE ANNUAL TEMPERATURE RANGES

OF FAIRBANKS AND ANCHORAGE.

1.3 VARIATION IN
PRECIPITATION
ACROSS THE
EARTH’S
SURFACE

In this topic, you will learn:
◼What is the water cycle?
◼How does water move on and below the earth’s

surface?

◼How does temperature affect relative humidity of a

place?

◼How are clouds formed?
◼How is rain formed?

LESSON OBJECTIVES

WHAT IS THE WATER CYCLE?

Pair Work:
Each pair will discuss
and annotate the
diagram to illustrate
the hydrological cycle.

◼ The rainwater that reaches the earth’s surface

moves on and below the surface in the form of
infiltration, surface runoff and groundwater
flow. These movements of water occur at
different rates, depending on various factors:
1. Type of soil and ground surface
2. Gradient of ground surface

FACTORS AFFECTING THE RATES OF

WATER MOVEMENT

◼The rates of infiltration

and groundwater flow are
faster in soil which has
larger pores (such as
sandy soil) than in soil
which has smaller pores
(such as clayey soil).

◼ The larger pores allow

more water to infiltrate,
and the water moves
much more quickly.

1. TYPE OF SOIL AND GROUND SURFACE

◼ In built-up areas, where ground surfaces are covered in concrete roads

which have tiny pores, the rate of infiltration is lower.

◼ The speed of surface runoff is higher on smoother ground surfaces such

as roads and pavements in built-up areas.

2. HOW BUILT-UP AN AREA IS

◼ On the other hand, in vegetated areas, the roots of vegetation loosen the

soil, creating larger pores, thereby increasing the rate of infiltration .

◼ The speed of surface runoff is lower on vegetated areas as the stems

and roots of grasses and plants act as barriers and increase the surface
roughness.

2. HOW BUILT-UP AN AREA IS

WHAT IS HUMIDITY?

Have you ever heard

tourists complain about the

weather in Singapore?

Very often, you’ll hear

them say this, “The sun is
great but I can’t stand the

humidity!”

RELATIVE HUMIDITY

• Relative humidity (RH) refers to the ratio of the

amount of water vapour in the air to the maximum
amount of water vapour the air can hold at a given
temperature.

Actual amt. of water vapour x 100%

Max. amt. of water vapour air can hold

RELATIVE HUMIDITY

◼Warm air can hold more water vapour than cool air
◼When temperature increases (but amount of water vapour

stays the same)
▪ The air becomes less dense and has more capacity 🡪 able to hold more

water vapour

▪ Therefore, relative humidity decreases

RELATIVE HUMIDITY DECREASES WITH INCREASING

TEMPERATURE AS WARM AIR CAN HOLD MORE WATER

VAPOUR THAN COOL AIR

S/N Temp (C)

Max. (g)

Actual (g)

Relative Humidity (%)

1.

-2

4.0

3.5

(3.5 ÷ 4) X 100% = 87.5

2.

20

18.0

15.0

(15 ÷ 18) X 100% = 83.3

3.

2

6.0

2.5

(2.5 ÷ 6) X 100% = 41.7

4.

10

9.3

8.0

(8 ÷ 9.3) X 100% = 86.0

5.

16

13.0

11.0

(11 ÷ 13) X 100% = 84.6

6.

20

18.2

13.0

(13 ÷ 18.2) X 100% = 71.4

CALCULATE THE RELATIVE HUMIDITY USING

THE FORMULA ABOVE

71

RELATIVE HUMIDITY (RH)

◼Expressed as a percentage

▪If the air currently holds half the moisture
possible at the present temperature, then the
relative humidity is 50%.
▪When the humidity is 100%, the air holds the
maximum possible, is saturated, and will be at
the dew point temperature.

◼Why is Singapore’s humidity high? I thought as temperature

increases, relative humidity decreases?

◼Some of the most humid cities on earth are generally located

closer to the equator, near coastal regions

◼Because of

▪ Proximity to water bodies 🡪 high rates of evaporation
▪ Proximity to the Equator 🡪 high temperatures encourage high rates of

evaporation

RELATIVE HUMIDITY INCREASES WHEN …

Temperature

decreases but amount

of water vapour
remains the same

Temperature remains
the same but amount

of water vapour

collected increases

Water
vapour

Air

Water
vapour

Air

OR

WHAT HAPPENS WHEN TEMPERATURE

DECREASES FURTHER?

When the amount of water

vapour is more than what the air
can hold, the excess water vapour

will condense at dew point

temperature to become water.

Air

Water
vapour

Water
vapour

When the air holds the maximum

amount of water vapour, it is

saturated and Relative Humidity is

100%.
The temperature at which

saturation occurs is called dew

point temperature.

76

HOW ARE CLOUDS FORM?

◼As air rises up in the atmosphere, it cools,

reducing the amount of water vapour it can hold.

◼Eventually, the amount of water vapour in the air

exceeds the maximum amount of water vapour
the air can hold (relative humidity becomes
100%, and air is saturated).

◼At this dew point temperature, air condenses

into water droplets on condensation nuclei.

◼Condensation nuclei (e.g. dust and ash particles

from volcanoes)provide surfaces where water
vapor can change into water droplets or solid ice
crystals and form clouds

77

PRECIPITATION

◼Water droplets in the

clouds will collide and
coalesce and become
bigger and heavier.

◼Once the droplets are

large enough (over
0.5mm) they will fall to
the Earth as raindrops,
sometimes breaking
apart as they do so.

◼Occurs when the earth’s surface is heated intensely.
◼Common in the tropics(e.g.: Singapore) which receives

a great amount of sunlight.

CONVECTIONAL RAIN

79

CONVECTIONAL RAIN

◼Most common rainfall type in the tropics.

- Linked to lightning and thunder.
- Short but intense periods of rain.

◼Formation

Sun’s rays heat up earth’s surface and the air above it gains heat

Warm air rises and cools, and condenses on condensation nuclei at

dew point temperature to form clouds

Water droplets in the clouds collide and coalesce, and when they

become large and heavy enough, it falls as rain

80

◼As convectional rain is associated with lightning and

thunder, Singapore has one of the highest rate of
lightning activity in the world.

IN SINGAPORE

• Prevailing winds pick up moisture over the sea, and push the

moist air up the windward side of a mountain.

• The rising moist air cools and condenses on condensation

nuclei at dew point temperature. Clouds are formed.

• Water droplets in the clouds collide and coalesce, and when

they become large and heavy enough, they fall to the ground
as rain on the windward side.

• As most of the moisture has fallen on the windward side, the

leeward side experiences dry descending air and is thus dry.

RELIEF RAIN

RELIEF RAIN

Relief rain can be observed in the Sierra Nevada mountain ranges in California, USA.
The windward side of the mountain ranges receives higher rainfall, and thus has lush
forests. The leeward side is hot and dry, creating the Death Valley desert

84

1.4 Variation in
wind direction and
wind speed across
earth’s surface

In this topic, you will learn:
◼How are winds formed?
◼What determines the speed of wind?
◼Why does wind direction vary across earth’s surface

on a local and regional scale?

LESSON OBJECTIVES

◼Wind

is

the

horizontal

movement of air across the
Earth's surface.

◼In

places

with

higher

temperatures, air is heated,
becomes less dense, and
rises. This results in a void
and

lower

atmospheric

pressure.

◼ In

places

with

lower

temperatures,

air

cools,

becomes denser, and sinks,
which

results

in

higher

atmospheric pressure.

WINDS

WINDS

◼The

difference

in

pressure between the
two

areas

forms

a

pressure gradient.

◼Air then moves from

an

area

of

high

pressure to an area of
low pressure to fill in
the void. Hence, wind
is formed.

Wind speed depends on:
1. Strength of pressure gradient.
❑ Smaller pressure gradient: Smaller difference in pressure between

two areas, leading to slower wind speeds.

❑ Larger pressure gradient leads to faster wind speeds
2. Friction.
❑As moving air comes in contact with variations in the Earth's

topography, it experiences frictional drag which reduces wind
speed.

❑The effect of frictional drag is greatest where there is uneven relief

and rough surfaces on land, e.g. valleys and mountains.

WHAT DETERMINES THE STRENGTH OF

WIND?

LOCAL WINDS: SEA BREEZE

◼During the day, air over the land is heated up faster

than air over the sea.

◼As warm air rises, a low pressure area is formed.
◼Air over the sea is cooler as the sea heats up slower.
◼The cool air sinks and forms a high pressure area.
◼As air moves from a high pressure area to a low

pressure area, this causes air to move from the sea
towards the land, forming a sea breeze.

LOCAL WINDS: SEA BREEZE

LOCAL WINDS: LAND BREEZE

◼During the night, the land cools down faster than the

sea.

◼As cool air sinks, a high pressure area is formed.
◼Air over the sea is warmer as the sea loses heat

slower.

◼The warm air rises and forms a low pressure area.
◼As air moves from a high pressure area to a low

pressure area, this causes air to move from the land
towards the sea, forming a sea breeze.

LOCAL WINDS: LAND BREEZE

MONSOON WINDS

◼Regional wind pattern that reverses direction

seasonally. This will result in seasonal changes in
precipitation.

◼Monsoon winds affected by the Coriolis effect.

- Coriolis effect is a force produced by the earth’s rotation
- Earth’s rotation will deflect the wind’s direction.

◼The Coriolis effect deflects wind to the right at the

northern hemisphere, while it deflects wind to the left
in the southern hemisphere

94

MONSOON WIND — CORIOLIS EFFECT

95

SOUTH-WEST MONSOON WIND (JUNE-SEPT)

◼The northern hemisphere experiences summer while the

Southern hemisphere experiences winter between June and
September.

◼The land in Central Asia is heated up and air warms and rises,

causing a low pressure area.

◼However the air over Australia cools and sinks, causing a high

pressure area.

◼This results in winds blowing from Australia, towards Central

Asia, forming the Southeast Monsoon winds.

◼However, due to the Coriolis effect, wind is deflected to the

right as it crosses the equator.

◼The Southeast Monsoon winds will become the Southwest

Monsoon winds in SEA due to deflection.

SOUTH-WEST MONSOON WINDS

SW monsoon
blows across the
Indian Ocean,
carrying lots of
moisture,
bringing heavy
rain to Indian
sub-continent

NORTH-EAST MONSOON WINDS (OCT-FEB)

◼The northern hemisphere experiences winter while the

Southern hemisphere experiences summer between October
and February.

◼The land in Australia heats up and air warms and rises, causing a

low pressure area.

◼However the air over Central Asia cools and sinks, causing a high

pressure area.

◼This results in winds blowing from Central Asia, towards

Australia, forming the Northwest Monsoon winds.

◼The Northwest Monsoon winds will become the Northeast

Monsoon winds due to deflection as it crosses the Equator due
to the Coriolis effect.

NORTH-EAST MONSOON WINDS

NE monsoon
blows across
the Indian
Ocean, carrying
lots of
moisture,
bringing heavy
rain to
Australia