Sec 4 Geography Revision Notes - · PDF fileHwa Chong Institution 2010 Term 1 IHE Geography Revision Notes Prepared by Cao Yu 2010 ©Copyright Reserved 2010/2/19 Last Edited Distance

Hwa Chong Institution 2010 Term 1 IHE Geography Revision Notes

Prepared by Cao Yu 2010 ©Copyright Reserved 2010/2/19 Last Edited

Hwa Chong Institution (High School)

Term 1 - 2010

Sec 4 Geography

Revision Notes

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INFORMATION FOR READERS

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This note consists of 12 printed pages (including this page)



Content page

Lighthouse of this note

Section I: Terms and conditions

Section A: Atmosphere, Weather and Climate

Section B: Elements of weather and climate

Section C: Humidity and precipitation

Section D: Planetary Wind and pressure

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Section A: Atmosphere, Weather and Climate

The more original a discovery, the more obvious it seems afterwards.

STRUCTURE OF EARTH’S ATMOSPHERE

� Atmosphere

� An envelope of transparent, odorless gases

� It is held in place by the pull of Earth’s gravitational forces

� Earth’s gravity is strongest nearer to the surface, therefore, most of the atmosphere is

found near the surface.

� Consists mostly of:

� Nitrogen (78%)

� Oxygen (21%)

� Water vapor

� Dust particles

STRUCTURE AND COMPOSITION OF ATMOSPHERE

� Using temperature, atmosphere can be divided into 4 distinct layers:

1. Troposphere

2. Stratosphere

3. Mesosphere

4. Thermosphere

TROPOSPHERE

� Approximately 0 to 14 km above sea level

� Temperature decreases by 6.4 degrees for every 1000m increase in altitude

� Temp is due to solar radiation which keeps earth warm

� Falls in pressure due to the decreasing effect of gravity

� The most unstable layer

� Topped by an isothermal layer

STRATOSPHERE

� Steady rise in temp caused by ozone

� Pressure continues to fall

� Dry air; little or no weather

� Acts as a protective shield against meteorite which usually burns out as it enters earth’s

gravitational field

MESOSPHERE

� Temperature falls rapidly due to no water vapor, cloud or dust or ozone to absorb incoming

radiation

� Lowest temperature – 90 degrees

� Strongest winds – 3000km/hr

THERMOSPHERE

� Temperature rise rapidly with height to reach 1500 degrees

� Due to atomic oxygen which has the same function as ozone.



ENERGY IN THE ATMOSPHERE

� 3 ways in which solar radiation is treated by atmosphere:

� Absorption: mainly my ozone, dust, water vapour and carbon dioxide, greenhouse

gases

� Reflection: reflection radiation back into space

� Scattering: Incoming radiation is diverted by molecules of gas and becomes diffused



IMPORTANCE OF THE ATMOSPHERE

� Helps sustain life

� Protects the Earth

WEATHER VS CLIMATE

� Weather

� When this happens, we experience a change

in weather

� Meteorologists define ‘weather’ as the condition

of the atmosphere at a particular place over a short period of time

� Climate

� Weather pattern of a place over a long period of time, usually 30 years

� The basic elements of weather and climate:

� Sunshine

� Temperature

� Humidity

� Clouds

� Rainfall

� Air pressure

� Wind

HOW WEATHER AND CLIMATE AFFECT PEOPLE

� Weather affects our daily life

� Climate affects how people live generally

� Where people choose to live

� Types of houses people live in

� Climate affects food and water supply



Section B: Elements of Weather climate

We have, I fear, confused power with greatness.

DEFINITION OF TEMPERATURE

� Temperature is the degree of heat or cold in the atmosphere

� It is measured in ºC or ºF

WARMING OF AIR

Convection Hot air, being lighter than cold air, rises as a

convection current

Conduction Air receives heat from the ground it is in

contact with

Radiation Heat energy from the ground is sent back to

space. This form of heat energy can travel in a

vacuum

Factor

influencing

Temperature

Description

Latitude � Places near the Equator are hotter than places near the poles due to

the angle at which sun’s ray strike the Earth

� Amount of heat received by Earth’s surfaces decreases with latitude

due to the decreasing angle of incidence Angle at which sun’s ray hits

earth of the sun’s ray

� Earth’s axis is tilted at an angle of 23.5º from the vertical

� From June to Aug, there is a higher intensity of the sun’s ray in the

northern hemisphere

� The higher latitudes in the northern hemisphere experience higher

temperature in summer while the higher latitudes in the southern

hemisphere experience low temp in winter.

Altitude � Temperature falls by 6.4ºC for every 1000m increase in height.

� This fall in temperature is called the normal lapse rate.

� Air is warmed by the heat radiated from the ground due to conduction

and convection. It is much warmer as there is larger land

� Most of Earth's surface gases are also concentrated at the Earth’s

surface.

� Temperature sometimes rise with altitude in the troposphere due to:

� Rapid outgoing terrestrial radiation on a cloudless night.

� Cold air rolling down slope displacing the Warmer air

� Warm air rises over cold air at a warm front

� Cold air undercutting warm air at a cold front



Distance from

the sea

� Sea heats up and loses heat more slowly than land

� Coastal area temperature is influenced.

� In temperate latitudes, coastal areas have a smaller temperature range

between summer and winter

� Inland areas far from the sea are not affected by the ocean thus having

a larger annual temperature range.

Ocean

currents

� Change in temperature is due to ocean currents in the area

� Ocean current are set in motion by prevailing winds blowing over the

surface

� 2 types of ocean currents:

� Warm – Originates from equator

� Cold – Originates from higher latitudes

Aspect � Refers to the direction of the slope of the land relative to the sun

� More obvious in temperate regions rather than tropical regions

� In northern hemisphere, south facing slope have higher temp than

north facing slope

Humidity � Humidity is the state of the atmosphere with respect to its amount of

water vapour

� The higher the humidity, the more clouds there will be as clouds are

formed by condensation of water vapour.

� Thick cloud cover prevents solar radiation from entering in the day and

outgoing terrestrial radiation escaping in the night.

� Therefore, equatorial areas which have high humidity and thick cloud

cover have low diurnal temp range.

Land surface � An area with vegetation is much cooler than an area of concrete.

� Vegetation reduces the incoming radiation and prevents absorption of

heat into the ground.

� Concrete surfaces absorb heat quickly in the day and releases heat

lowly at night.

SIX’S THERMOMETER

� Maximum temperature is measured by the maximum thermometer

� It consists of a glass tube containing mercury and a metal index

� When temperature rises, the mercury expands and pushes the metal index upwards

� When temperature falls and the mercury contracts,

the metal index stays in place

� The minimum thermometer works in the same way, but contains alcohol instead of mercury.



Section C: Humidity and precipitation

The shortest distance between two points is always under construction.

HUMIDITY

� The state of atmosphere with respect to its water vapor content

� Warm air can hold more water vapour than cold air

� Water vapour comes from the evaporation of water from ponds, lakes, rivers, seas and

oceans.

� Saturation point is reached when air contains all the water vapour it is able to hold at a

particular temperature

RELATIVE HUMIDITY

� The ratio of the actual amount of water vapour in the air at a particular temperature to the

maximum quantity of water vapor the air can hold at that temperature

� Usually measured as a %

SATURATION POINT

� when the air reaches the maximum capacity to hold water vapor

DEW POINT TEMPERATURE

� The temperature at which Saturation Point is reached;

Name of the cloud Description

Cirrus

Thin and wispy

Cumulus

Round and wooly

Stratus

Flattish low clouds,

occurs in layers

Cumulonimbus Huge, towering



TYPE OF PRECIPITATION

Type of

precipitation

Description

Rainfall � The air rises or is lifted

� The air that has risen or has been lifted cools

� The cool air becomes saturated at dew point ie, it is holding all the water

vapour it can hold at a particular temp.

� When air is cooled further, it continues to rise, the water vapour in it

condenses into water droplets around atmospheric particles

� The water droplets forms clouds

� When the water droplets in clouds are too heavy to be suspended in air rain

falls

Snow � Forms under similar conditions to rain

� Except that as dew points temperatures are under 0ºC, then water vapour

condenses directly into solid (Sublimation)

Sleet � Is a mixture of ice and snow formed when the upper air temperature is

below freezing, allowing snowflakes to form and lower air temperature is

around 2 to 4ºC which allows them to melt partially

Hail � Made up of frozen raindrops which exceeds 5mm in diameter

� It usually forms in cumulonimbus clouds resulting from the uplift of air by

convection currents or at a cold front.

� More common in summer areas where more heat triggers off the uplifting

of air.

THEORY OF PRECIPITATION

� Minute water droplets (less than 0.05mm in diameter) or ice crystals (if dew point

temperature is below freezing) are produced from condensation

� Very tiny and weigh very little

� Constantly kept buoyant by the same rising air currents that produce them

� Water droplets and ice crystals need to become sufficiently large to overcome this uplift

COLLISION & COALESCENCe

� ‘warm’ clouds (containing no ice crystals) are found in tropics contain numerous water

droplets of differing sizes

� different-sized droplets swept upwards at varying velocities – collide with other droplets

� the larger the droplet, the greater the chance of collision and coalescence with smaller

droplets



� when coalescing droplets reach a radius of 3mm, their motion causes them to disintegrate

to form a fresh supply of droplets

� thicker clouds provide more time for droplets to grow, with them falling faster (thundery

showers)

TYPE OF RAINFALL

Type of

rainfall

Description

Convectional � On a hot day, the air in contact with the warm ground is warmed by

conduction.

� The warm air with higher temperature than the surrounding air rises

and is said to be unstable.

� This unstable warm air which holds a lot of moisture rises in a

convection current.

� As it rises to great height, it cools.

� Further cooling after it reaches dew point causes water vapor to

condense into water droplets around particles such as dust and smoke.

� The water droplets form towering cumulonimbus clouds.

� When the water droplets are too heavy to be suspended, conventional

rain falls.

Relief � Orographic rain occurs when near-saturated, warm maritime air is

forced to rise when confronted by a coastal mountain barrier.

� As air rises, it cools.

� Air becomes saturated when dew point is reached at the level of

condensation.

� Condensation occurs upon further cooling, forming water droplets that

coalesce to form clouds.

� When the water droplets become too heavy, they fall as relief rain on

the windward slop.

� The leeward side of the mountain forms a rain shadow area.

� After crossing the mountain, descending dry air is warmer.

� Forms warm dry wind on the leeward slope and it brings very little

moisture

Frontal � Lighter warm air rises gently over heavier cold air which remains close

to the ground.

� Warm air rises, cools and water vapor condenses to form low to

mid-level clouds like nimbostratus/altocumulus clouds.

� Light rain over wide area for several hours

� Heavy cold air slips under lighter warm air.

� Warm air rises aggressively.

� Cumulonimbus clouds form with heavy rain over a small area with a

short duration.



Section D: Planetary pressure system

Life being what it is, one dreams of revenge.

WINDS

� moving air- caused by difference in air pressure

� Wind always moves from area of high pressure to area of low pressure

� Air pressure is measured using a mercury barometer

PLANETARY PRESSURE SYSTEM

1. Equatorial low pressure belt/doldrums (from 5°N to 5°S)

2. Polar high pressure belts (around 90°N to 90°S)

3. Sub-tropical high pressure belts/horse latitudes (around 30°N to 30°S)



4. Sub-polar low pressure belts (around 60°N to 60°S)

CORIOLIS

� Due to rotation of the earth and the unequal distribution of land and sea surfaces, winds do

not blow directly from areas of high pressure to low pressure.

� Moving air appears to be deflected to the right in the northern hemisphere and left in the

southern hemisphere.

MAJOR PLANETARY WINDS



� Trade winds (Both northeast and southeast)

� Blow from sub-tropical high pressure belts (horse latitudes) to the equatorial

low-pressure belt (doldrum).

� Zone of convergence: ITCZ

� Westerlies

� Blow from sub-tropical high pressure belts to sub-polar low pressure belts.

� Generally warm and dry

� Easterlies

� Blow from polar high-pressure belts to sub-polar low pressure belts.

MONSOON IN SEA & HURRICANE KATRINA

� Regional winds which influence the climate of huge land masses. They are seasonal by

nature and are caused by changes in pressure during the changing seasons between

summer and winter

LOCAL WIND



Documents

Sec 4 Geography Revision Notes - · PDF fileHwa Chong Institution 2010 Term 1 IHE Geography Revision Notes Prepared by Cao Yu 2010 ©Copyright Reserved 2010/2/19 Last Edited Distance