ATMOSPH

ERE AND

WEATHER

A S LE V E L G

E O G R A P H Y PH Y S I C

A L 2

1. INTR

ODUCTION

WEATHER

/ CLIM

ATE

WEATHER• State of atmosphere at a local level and within

a shorter timescale• Can be minutes – month• Emphasis: Sunshine, Cloud, Win, rainfall,

temperature

CLIMATE• The long-term behavior/variations of the state

of the atmosphere/ patterns of weather in a larger region over a longer period of time

• Usually over 30 years• Average temperature, average precipitation,

pressure, vegetation, regular wind

1.1 ATMOSPH

ERIC

COMPOSITIO

N/

STRUCTU

RE

EXOSPHERE690 – 10000 kmParticles moving

to and from space

Lower boundary: Exobase

THERMOSPHERE• 85 – 690 km• Temperature increasing

with height• Low Air Pressure – thin air• Electronically charged

particles will interfere with radio broadcast[Aurora/ North/ Southern Light created by charged particles and earth’s magnetic field]

• Air too thin for temperature to be rightly detected

MESOSPHERE• 50 - 80 km• Boundary: Mesosphere• Temperature drops with height• Meteors burn up here – shooting stars

STRATOSPHERE• 16 – 50 km• Ozone layer is here• Volcanic gases can

affect the climate• Temperature

increases with height• Ozone absorbs heat –

hence the increase with height

TROPOSPHERE• Up to 16 km• Tropopause – the boundary• Temperature decreases with height(A result of changing insolation andThe subsequent heating of the air nearGround surface)Pressure • Wind speed increases with height• Weather occurs here – water vapor, dust

particles, clouds, rains, pollutants

1.2 EARTH SUN

GEOMETRY

1. EARTH SUN GEOMETRY• The earth’s revolution is elliptical• The part of the elliptical plane closest to the

sun: Perihelion (147300000 km)• The part of the plane with the longest distance

from the dun: Aphelion (152100000)• The earth is tilted by about 23o – remains in

such way throughout the entire revolution – no change of tilts

PerihelionAphelion

Summer Solstice

Winter Solstice

Vernal Equinox

Autumnal Equinox

SEASONS – SOLSTICE/ EQUINOX• The 4 seasons variation only exist in certain

areas in the world• When one refers to Spring, Summer, Autumn,

Winter – the time periods are different between the 2 hemisphere

• Also seasons do not exist very much near the tropic or at the poles

SEASONAL VARIATIONS• Notice in the diagram below:1. The earth is permanently tilted in one way – Axis parallelism 2. The earth doesn’t rotate in the way that the tilted side

always face the sun3. The variation in points of solar reception occurs because the

earth is tilted in such wayStudy the diagram below until you understand this fullyVideo to help:

https://www.youtube.com/watch?v=hHyQQ8UlXPk https://www.youtube.com/watch?v=X8oGfa8VKtc

PerihelionAphelio

n

DRS(Direct ray of the sun)

Summer Solstice

Winter Solstice

DRS(Direct ray of the sun)

Vernal/ Autumnal Equinoxes

DRS(Direct ray of the sun)

THE DIFFERENT SEASONS1. Summer Solstice: Summer in Northern hemisphere – winter

in the Southern Hemisphere - still hot in the tropic region – Arctic Circle sunlight for 6 months – Antarctic circle no sunlight for 6 months

2. Autumnal Equinox: Direct Ray on equatorial region – Autumn in the Northern hemisphere – Spring starting out in the Southern hemisphere

3. Winter Solstice: Winter in the Northern Hemisphere – strong summer in the Southern hemisphere (Perihelion) – still hot in the tropic region – Arctic Circle no sunlight for 6 months – Antarctic circle sunlight for 6 months

4. Vernal Equinox: Direct Ray on equatorial region – Spring in the Northern hemisphere – Autumn in the Southern hemisphere

1.3 THE ENERGY B

UDGET

- INTR

ODUCTION

THE EARTH IS SOLAR POWERS• All of the earth’s energy derives from the sun• The absorbed solar power:… Photosynthesis… Evaporation… Melting process… Warms the earth

THE HEAT ENGINE• The engine is responsible for balancing the

energy received from the sun and those radiating back

• ALS0!• The energy reception of the earth is not evenly

distributed• The equator receives the highest amount of

sunlight• The polar region receives the lowest

THE EARTH CAN NOT HEAT UP INFINITELY• For all the energy received – the earth has to

give back an equal amount• The outgoing radiation of the earth must be in a

state of radiative equilibrium with the energy received

• The heat engine of the earth is responsible for this

THE HEAT ENGINE• The Heat engine also work to redistribute the energy around

the surface of the earth• Achieving this through:• Wind• Convection• Surface water• Rainfall• Ocean circulation/ currents

2. THE GLO

BAL/ LOCAL

ENER

GY BUDGET

2.1 LOCAL E

NERGY

BUDGET

COMPONENTS OF THE DAYTIME ENERGY BUDGET• Insolation• Reflected Solar Radiation• Absorption• Long Wave Radiation• Latent Heat Transfer• Sensible Heat Transfer

DAYTIME ENERGY BUDGET

1. INSOLATION

• Incoming Solar Radiation• Main Energy input in the system• Affected by: Latitude, Season, Cloud Cover

2. REFLECTED SOLAR RADIATION

• Proportion of energy reflected back = Albedo• Albedo expressed as a percentage to the

amount of energy received from insolation• Varies with color of the surface

3. SURFACE ABSORPTION

• Energy that has the potential to heat up the surface

• Depends on the nature of the earth surface• Some materials can conduct heat to a lower

level e.g. Water• Other materials are poor conductor – heat

concentrated on the surface e.g. rocks

4. LONG-WAVE RADIATION

• Radiation from the earth surface into the atmosphere – some of which may go into space, some reflected back by the atmosphere, some absorbed by the atmosphere

• There is also downward movement of radiation from atmosphere

• The difference between the downward/upward movement of radiation = net radiation balance

5. LATENT HEAT TRANSFER

• Latent is the amount of heat energy needed to change the state of the substance without changing its temperature

• Melting, Evaporation --- uses up the heat energy by 2238 Joules

• Such absorption of heating – cools the atmosphere• Condensation, Sublimation – releases heat energy by 336

joules

6. SENSIBLE HEAT TRANSFER

• Movement of air parcels in and out of a certain area• Wind, convection• Warm air rises, Cold air sinks• Pressure gradient causes movement of air

NIGHTTIME ENERGY BUDGET

1. SUB SURFACE SUPPLY

• Heat absorbed by rocks during the day form a supply of heat

• This may be released during night time• Compensates the cooling effect of nighttime

2. NET RADIATION

• Radiation of energy from the earth is dominant in the process of heat loss

• There is no insolation – earth just loses heat by long wave

• In cloudless night – there is less downward radiation

• Cloudless night = large energy lost = colder

3. LATENT HEAT

• Cooling effect at night causes cooling of the surface

• Warmer air on cool surface may result in condensation

• Condensation will release heat by 336 J• Compensates for the cooling effect of the night

time

4. SENSIBLE HEAT TRANSFER

• Movement of wind in and old of the area• May be land – sea breezes

2.2 LOCAL IN

FLUENCES

ON CLIMATE

1. ASPECT• Hillsides – change the angle at which the sun’s

ray is received• Northern hemisphere – South facing slope

(Ubac) receives more sun ray• Southern hemisphere – North facing slope

(Adret) receives more sun ray

2. CLOUD COVER• Reduces both incoming/outgoing solar radiation• Thick cloud – greater amount of

absorption/reflection/scattering• Therefore cloud reduces daytime energy

(reflecting, absorbing and scattering insolation)• But it increases nighttime energy (absorbs/

reflects back long-wave radiation from ground up)

• Therefore – the climate of deserts

3. URBANIZATION• Changes the albedo – dark concrete reflects

more light• Heat Island effect

2.3 GLOBAL E

NERGY

BUDGET

WHAT DO WE KNOW FROM PREVIOUSLY (SUMMARIZED)1. The earth receives all of its energy from one input – Incoming

Solar Radiation (Insolation)2. The out put from the global system is in the form of outgoing

long-wave radiation3. The Insolation may be reflected/absorbed by the atmosphere4. The Insolation may also be reflected by the earth surface5. Number 3 and Number 4 are the limits of the Insolation received6. However, the amount of insolation received varies between

different regions 7. This is due to Latitude, altitude, Solar constant, heat absorption,

heat reflection(albedos) , amount of gases in the atmosphere 8. Overall, the equator receives the most insolation9. The polar receives the least insolation and reflected back most of

it too10.Therefore, most of earth’s energy is concentrated to 0 degree

latitude11.Hence the earth’s heat engine works to redistribute the energy

evenly across the surface12.To put it absolutely simple: The Heat Engine = Weather conditions

FACTORS AFFECTING INSOLATION• The Solar Constant: The amount of energy radiated to earth

by the sun – this is usually constant – affects very long term climate

• Distance from the sun (Elliptical orbit – perihelion - aphelion) – Annual variation

• Latitudes: Heat has to go through more atmosphere in the polar and is spread out over larger areas in the polar

• Length of nights/ days – years/ points on the earth surface

HEATING IMBALANCES• Throughout the year – heating in different areas change• This is due to the earth’s elliptical orbit• However the differences between the heating imbalances of

different regions (Equator, tropics, poles) of the earth is actually caused by the tilted axis

INSOLATION/ ABSORPTION/ REFLECTED SOLAR RADIATION

UNDERSTAND THAT THE HEATING IMBALANCE PLAYS HUGE ROLE IN DRIVING THE HEAT ENGINE

– SINCE IT INSPIRED THE HEAT TRANSFERS:LATENT HEAT TRANSFER

(CLOUD/PRECIPITATION/DEW/FOG)SENSIBLE HEAT TRANSFER

(SURFACE WIND/ HURRICANES/ CYCLONES/ OCEAN CURRENTS)

HEIGHT ABOVE SEA-LEVEL• REMEMBER: Atmosphere is not warmed by the

sun – but by radiation from the earth surface + distribution through conduction/ convection

• Higher area – less land present – less heating effect

• Reduction in air pressure – less likely to hold heat

LAND-SEA DISTRIBUTION• Sea is more transparent –

reflects less heat• Sea can absorb heat to

deeper level• Can transfer heat to greater

depth with convection• The sea has a higher specific

heating capacity• Sea requires more energy to

heat up a same amount of area to land

• Hence sea heats up more slowly

• In winter: Sea also loses energy more slowly – THERMAL RESERVOIR

• Coastal environment has smaller temperature range

PREVAILING WIND• Wind of different temperatures change the

temperature of the receiving areas• Wind from the mountain – usually cold• Wind from the sea – warmer in the winter,

colder in the summer

OCEAN CURRENTS• Different areas of the ocean have different

temperatures• Near to the equator the sea is warm• To the pole, the se is cold• Warm ocean current carries warm water poleward

and cold water to the equator• The ocean current is influenced by the prevailing

wind• The ocean conveyor belt also plays a part• See all these in details in later section

3. HEAT

TRANSFER

ACROSS

THE W

ORLD

3.1 DIFFERENT T

YPES OF

HEAT TRANSFER

(INTR

ODUCTION)

V E R T I CA L / H

O R I ZO N T A L T

R A N S F E R

HORIZONTAL HEAT TRANSFER• Heat is transferred away from the tropic• Prevents the equator from getting too cold• Warms up the poles• 80% - Wind• 20% - Ocean Currents

VERTICAL HEAT TRANSFER• Most radiative cooling occurs at the earth’s atmosphere• Most insolation heating occurs at the earth’s surface• Prevents earth’s surface from getting too hot• Radiation• Conduction• Convection• Latent Heat

3.2 ATMOSPH

ERIC

MOISTURE

ATMOSPHERIC MOISTURE• Water in the atmosphere maintains life on earth• Reflects/ scatters insolation• Important in horizontal transfer• Important in vertical transfer• Latent heat transfer• WATCH THIS:

https://www.youtube.com/watch?v=sP5ceNJTKVo

HUMIDITY• Absolute Humidity: The mass of water vapor in a given

volume of air measured in grams per cubic meter (g/m3)• Cold air can hold less water than warm air (cold air is

denser)• Relative Humidity: The amount of water vapor in the air

at a given temperature expressed as a percentage to the maximum amount of water vapor the air can hold at that temperature

• When RH is 100% - air is saturated

DEW POINT• If unsaturated air cools down where

atmospheric pressure is always constant– a DEW POINT will be reached

• Dew point: The critical temperature where a specific parcel of air becomes saturated

• Further cooling beyond the dew point = condensation

• Condensation at points below 0oC – snow

CONDENSATION RESULTS FROM COOLING• 2 Types of cooling processes• Radiative cooling• Advection Cooling

RADIATION COOLING• The land loses heat through outgoing long-wave

radiation• The air in contact with the land is cooled by

conduction• Occurs in clear calm evening – no mixing of the

air• If air is moist – dew point is reached – forms

RADIATION FOG• If temperature is below 0 – Hoar fog

ADVECTION COOLING• Warm moist air move over a cooler land/ sea• Warm air is cooled down to the dew point• Fog is formed• California/ Atacama desert (Warm air from land

drifts over cold ocean current)

SUBLIMATION• When vapor condenses directly into ice crystals

– doesn’t pass through liquid state• Condensation doesn’t quickly occur in clean air

(lack of hygroscopic nuclei)• If the air is cold super clean air – it becomes

SUPER SATURATED• It falls below its dew point without condensing• They may go on to sublime

HYGROSCOPIC NUCLEI• Particles that attract

water• They act as condensation

nuclei – encourages condensation

• Important in fog formation

• Volcanic dust (usually result in condensation into cloud then rain – hence volcanic eruption always result in heavy rain)

• Dust from soil blown by wind

• Urban industrial area – smog/ Sulphuric acid

• Where there are a lot of hygroscopic nuclei – condensation may occur even at 75% RH

DEW• Dew occurs with condensation• Usually at dawn – as the temperature is still cool• The colder earth surface cools down the air above it• This causes vapor pressure to decrease as cold air can hold

less water vapor• Eventually leads to condensation on surfaces – leaves,

ground etc.• Occurs in stable anticyclone condition – rapid radiation

cooling

CLOUD TYPES

PRECIPITATION1. RAIN2. SNOW3. SLEET4. FROST5. HAIL

RAINFALL1. Convergent/ Cyclonic/ Frontal Rainfall2. Orographic/ Relief Rainfall3. Convectional Rainfall

CONVECTIONAL RAINFALL

OROGRAPHIC RAINFALL

FRONTAL RAINFALL

2 THEORIES OF PRECIPITATIONCollision/ Coalescence theory: Occurs at Topical Zones –

raindrops of different sizes are forced into updraught at different rates

SNOW• Forms like rain (2 theories of precipitation)• Where temperature is under 0oC• If hygroscopic nuclei present – ice crystals –

which will form snow flakes• Since warm air can hold more moisture – It is

best that the air is JUST below 0oC and not very much colder

SLEET• Mixture of ice and snow• Forms when temperature is indeed below

freezing point – forming ice crystals• However, lower layer is warm enough o allow

partial melting

GLAZED FROST• Reverse of Sleet• Water droplets at higher surfaces• Freezes when passing through colder air• May occur during temperature inversion

HAIL• Frozen raindrop of more than 5 mm• Forms with cumulonimbus cloud• Uplift of air by convection current• Uplift at cold front• Hail falls through cumulonimbus cloud (he lower

layer is not warm enough to melt the hail because of latent heat absorbed in evaporation)

TROPICAL CYCLONES• Intense low pressure system – hurricanes, typhoons,

cyclones• Found near equator, near the ITCZ• Wind of extreme velocity• Torrential rainfall• Develop over warm tropical regions• In autumn – highest sea temperature• Trade wind belt• Coriolis force needed for a ‘spin’ – hence not nearer

to the equator

ADIABATIC PROCESSES• The cooling/ warming up of air in response to changes with

altitude• In the troposphere – temperature decreases with altitude• In the upper area – air is less dense – allows for cooling• Less and area – less surface absorption and hence less long

wave radiation• So – temperature of air changes internally• Diabetic process – involve mixing of air

LAPSE RATE• An idea that the atmosphere reduces in temperature as it

increases with height at the rate of 6 degree Celsius per 1 km

• Air is a poor conductor of heat• Air expands and rises – it uses up the energy in the process• Air rising can be caused by:1. CONVECTION2. OROGRAPHIC UPLIFT3. FRONTAL SYSTEM4. TURBULENCE

LAPSE RATE• Environmental Lapse rate: The general rate at which air

decreases in temperature: 6 degree Celsius/ 1 km• Dry Adiabatic Lapse (DALR): Lapse rate of a dry parcel of air• Saturated Adiabatic Lapse rate (SALR): Lapse rate of a

saturated, condensing air – is lower than DALR since saturated air will be condensing and releasing energy hence slowing down the cooling process

• The DALR and SALR are used with a specific parcel of air as compared to the ELR which is the average of normal air

• WATHC THIS: https://www.youtube.com/watch?v=AVdNAlDyFR8

INSTABILITY• When the ELR is greater than DALR• When the air is warmed and it begins to rise• Adiabatic cooling is still slower than the surrounding

atmosphere• At any given altitude, the rising air will still be warmer and

it will continue to rise• Air will soon cools to a dew point and continue to cool at

SALR – here clouds will already have been formed• Results in formation of clouds and precipitation• With moist air – vertical cloud development

STABILITY• When DALR and SALR are higher than ELR• Which means the air at any given altitude will be cooling at

a much faster rate than the surrounding atmosphere and hence it will be cooler and HENCE it will be sinking

• Uplift can not be sustained if at any point this air is cooler - even when it is saturated

• This creates an anticyclone condition – subsiding air• Stable can only rise when it’s forced to

CONDITIONAL INSTABILITY• ELR is between DALR and SALR• So if the air is dry – it cools faster and remains sinking• If the air is saturated then it may be warmer than ELR• If there is a case of orographic uplift or turbulence – there

maybe instability

3.3 PRESSURE GRADIEN

T/

SURFACE W

IND

P R E S S U R E FO R C E S / W

I ND

WIND• Wind are moving air masses both at the surface (surface

wind) and in the upper layer of the atmosphere.• Wind is a result of the differences in pressure on the surface• Air always moved from areas of higher pressure to areas of

lower pressure• Watch this before continuing:

https://www.youtube.com/watch?v=Vjd87n4qIFE

OTHER FORCES ACTING ON WIND• There are two other forces aside from pressure gradient

that are influence the direction and patterns of wind1. Surface Friction2. The Coriolis force• Balance between the forces of pressure gradient and the

Coriolis force is called the Geostrophic forces• The wind resulting from such force – Geostrophic wind

THE CORIOLIS FORCE• Since the earth is rotating from

WEST TO EAST• Any object moving north-south will

always be deflected to a direction• Imagine a spinning disc – and you

roll a ball on it – the ball with deflected to an opposite direction because the ground underneath the ball is moving

• Wind in the northern hemisphere are deflected to the right

• Wind in the southern hemisphere are deflected to the left

• I know it’s complicated… so watch these videos.

FRICTION• The drag exerted on the wind as it flows across the earth’s

surface• Mountains, terrains, urban areas – all causes friction• Friction can reduce the effect of the Coriolis force at the

surface

WATCH THE FOLLOWING VIDEO TO UNDERSTAND DIFFERENT PRESSURE

GRADIENT

https://www.youtube.com/watch?v=oCdqGkn-B1Ehttps://www.youtube.com/watch?v=8ixT7D3f8Qohttps://www.youtube.com/watch?v=O4x_DVSseIk

THE TRICELLULAR MODEL• After watching the video, you will be able to understand

that the unequal solar heating of the earth surface first causes differential pressure.

• The differential pressures are the major cause for air movement and wind

• The model you saw in the videos previously could be referred to as the Tricellular model

• Simply stated, it is a model divided into three cells: The Hadley, the Ferrell and the Polar.

THE THREE CELLS

• These 3 cells exist in pairs• Basically the North and the Southern hemisphere echoes

each other• They both have 3 cells which are identical to each other.• Imagine the equator being a mirror

THE HADLEY CELLS• Let’s imagine the air masses of the equator

as a single parcel (which they absolutely are not)

• Maximum solar radiation at the equator = maximum heating

• That air parcel expands – lower density – air parcel rises – creating a low pressure belt

• It reaches the tropopause – where it diverges into two – one flows beneath the tropopause to just above the 30 degree latitude north the other to the Southern equivalent

• At which point the air has been cooling enough that it has became denser

• Now it sinks at 30 degree – creating a high pressure belt

• At the surface – air moves from the high pressure belt to the low pressure belt DOWN the concentration gradient

• The air is returned to the equator in the form of trade wind

• At the equator – air from the two high pressure belts converge in the ITCZ (Inter Tropical Convergence Zone)

• Here it rises once again• WE HAVE A CONVECTION CELL!

https://www.youtube.com/watch?v=kRdiMqUBjgM

THE FERRELL CELLS• At 30 degree high pressure areas (aka the

Subtropical High Pressure belts) – sinking air diverges into 2 types of surface winds

• One of which we know are the trade winds flowing to the ITCZ

• The other type is the what we will be calling the westerly

• We will explore these winds in a second, but for now, concentrate on the new developing cells

• So a stream of surface wind flows from the STHP (Sub Tropical High Pressure belt) at 30 degrees to the 60 degree

• At 60 degrees the wind converges with another set of wind flowing down from the pole, once again down the pressure gradient because the pole is cold and hence air there will be dense

• SO… Convergence of air here causes air to rise at 60 degrees

• We therefore call this belt the Sub Polar Low Pressure Belt (SPLP)

• Our air from the STHP now rises here, hitting the tropopause – flowing back to the 30 degree STHP where it sinks at 30 degree after cooling

• WE HAVE THE FERRELL CELL!• However this cell is not actually a

convection cell as you have seen

THE POLAR CELL• Air sinks at the 90 degree poles – creating a high pressure

region• This is probably the coldest areas in the world because of

low angle from the sun and high albedo• Air naturally moves across the surface to 60 degree SPLP

(Sub Polar Low Pressure Belt)• Once again, a convergence• Air rises – diverging at the tropopause• A specific mass travels to the Pole where it sinks back down

again• This is our Polar Cell

INTER TROPICAL CONVERGENCE ZONE• The ITCZ• Locations of the Doldrums/ the equatorial trough• Her the surface wind converges inward – there may be area where

ships can not move because of the gap between the 2 converging streams of wind (the doldrums)

• Rising air here releases latent heat with condensation – stimulates convection

• A lot of tropical rainforest• Latitudinal variation – changes in overhead sun• June: this moves north – in December it moves south• It goes where the summer si• Over the sea there is less variation – because sea has higher specific

heating capacity and rarely change in temperature

JET STREAMS/ ROSSBY’S WAVE• These are currents of moving air at the upper layers of the

atmosphere• It is through the discovery of the Jet Streams/ Rossby wave

in the 1940s that allow Rossby himself to create the Tricellular model

• The Jet streams and the Rossby wave help mix the air at the upper layer and assist in the transfer of heat from the equator to the Poles

JET STREAMS/ ROSSBY WAVES• Jet Streams are caused by differential air masses• There are 2 streams – the Sub Tropical and the Polar• They also meander in a wave called Rossby Wave• The energy of the Ferrell cell is actually obtained by the

transfer of heat in the upper layer of the atmosphere by the Jetstream