WEATHER AND CLIMATE

LECTURE 1

Atmosphere: Structure and Composition

Atmosphere is made up of layers:

1. Troposhere- decreases by 6.4 degrees Celsius for every1000m increase in height (Environmental Lapse Rate)- contains most of the water vapour, cloud,dust, pollution- tropopause: upper limit of the troposhere. - temperatures remain constant with heightincrease

Atmosphere: Structure and Composition

2. Stratosphere- steady increase in temp due to ozone concentration- winds increase with height- pressure decreases with height- stratopause: layer above stratosphere- no change in temp with increasing height

Atmosphere: Structure and Composition

3. Mesosphere- temperatures fall rapidly - no gases or particles present to absorb radiation- mesopause: layer above mesosphere where no change in temperature with height is seen

HEAT/ENERGY BUDGET

Before looking at the earth’s budget, let us see what happens to incoming solar radiation:

3 General Processes act upon incoming radiation:

a) Absorption

b) Radiation

c) Reflection

HEAT/ENERGY BUDGET

Absorption.

- by gases in the upper atmosphere as well as ice particles and dust

Reflection.

- by clouds and the earth’s surface back to space

- dependent on the albedo of clouds and earth’s surface

- gas molecules also scatter radiation back to space. The rest reaches surface by diffuse radiation (scattered energy)

HEAT/ENERGY BUDGET

Scattering and Diffuse Radiation

Scattered insolation is either:

Scattered Back to Space

Absorbed by earth’s surface as diffuse radiation

HEAT/ENERGY BUDGET

Incoming radiation converted to heat energy

- radiates back to the atmosphere

- absorbed by water vapour/carbon dioxide to retain heat near the surface (Greenhouse Effect)

HEAT/ENERGY BUDGET

The amount of radiation the earth receives:

A system of inputs and outputs

Balances on the global level, but not necessarily so on a local scale

HEAT/ENERGY BUDGET

Eg: At night, no incoming radiation, yet heat is still lost, especially on cloudless days

- at any one place and time, more radiant energy is being lost than gained, vice versa

Heat Escapes to Space

HEAT/ENERGY BUDGET

This can be determined by the net radiation: difference between all incoming and outgoing radiation

surplus: radiant energy flowing in faster than it is flowing out

deficit: radiant energy flowing out faster than it is flowing in

HEAT/ENERGY BUDGET

What prevents tropics from overheating?

1 Horizontal Heat Transfers

- winds carry heat energy away from the tropics

2. Vertical Heat Transfers

Radiation, conduction, convection and transfer of latent heat

- supplementary reading

Factors Affecting Temperature

Amount of insolation varies through time and space, and from point to point

a) Long-term factors

b) Short-term factors

c) Local influences

Long-Term factors

Height above sea-level

- atmosphere heated from earth’s surface by conduction and convection

- dependent on surface area of landmass

Surface

Atmosphere

ConductionWarm, rising Air

Convection

- as heights increase on mountains, less land mass present to give off heat by above process, hence lower temperatures

Long-Term factors

Height above sea-level

- at the same time, pressure/density of air decreases with altitude

- less air molecules present to absorb and retain heat, hence as air thins with altitude, temperatures decrease Decreasing

Density of Air Molecules with Increasing HeightSurface

Long-Term factors

Altitude of the sun

- temperatures decrease with decreasing angle of the sun

SunA

B

-Less loss of energy atA as ray at A travels a shorter distance than at B

Long-Term factors

SunA

B

Therefore, the higher the latitude (moving from Poles to Equator), the higher the temperatures, vice versa

Long-Term factors

Nature of Surface (Land/sea)

- Land and water differ in their abilities to absorb heat

- specific heat capacity: the amount of energy needed to raise 1kg of a substance by 1 degree Celsius

- water has a higher S.H.C. than land/soil

Long-Term factors

Nature of Surface (Land/sea)

- water requires more energy to raise its temperature by 1 degree Celsius as compared to continents

- In summer, sea heats up more slowly than land

- In winter, land loses energy more rapidly than the sea

Long-Term factors

Nature of Surface (Land/sea)

Illustration of different rates of energy gain/loss between land and water

Swimming Pool on a hot day:- air temperatures warm

- water seems to be ‘icy’ cold when you jump in

A chilly afternoon immediately after a heavy rain:

- air temperatures cool

- water in the pool seems to be ‘nice and warm’

Long-Term factors

Nature of Surface (Land/sea)

- Continental areas therefore are more responsive to temperature changes as compared to water bodies

- this is also why coastal areas have smaller annual temperature ranges than inner continents

Long-Term factors

Prevailing winds

- where winds come from

- and characteristics of surface over which they blow

Winter:

- winds blowing from sea tend to be warmer

- coastal areas experiencing such breezes will be warmer than areas not experiencing such breezes

Long-Term factors

Prevailing winds

Warmer Sea Breeze

Cold Surface (Winter)

Warms coastal areas. Warm wind eventually cools with distance into continents

Inner Continents will be colder than coastal areas even if they may be within the same climatic region

Long-Term factors

Ocean Currents

Equator

S. Pole

N. Pole Warm Currents

Cold Currents

Short-term factors

Seasonal Changes

- due to earth’s tilt, Northern Hemisphere receives more insolation during Summer solstice (21 June) than Southern Hemisphere

- Northern Hemisphere receives less insolation during Winter Solstice (22 December)

N

S

Earth’s axis onwhich it rotatesnot on the SouthPole

N

SBut is tilted atan angle

Short-term factors

Equator

Earth during Winter Solstice

- Northern Hemisphere - less insolation (cooler ie Winter)

- Southern Hemisphere - more insolation (warmer ie Summer)

Short-term factors

Earth’s Elliptical Orbit around the sun therefore sets up the different seasons

Summer Winter

Spring

Autumn

Short-term factors

Length of Day and night

- areas experiencing longer days tend to havehigher temperaturesEquator: - Equal lengths of day and night every 24 hours

Poles: - experience 24 hours of darkness for parts ofwinter- during summer, experience up to 24 hours ofday

Local Influences on Insolation

Slope Aspect

- northern hemisphere: north-facing slopes (adret) receive less sunshine

- cooler than south-facing slopes (ubac)

In addition, steeper slopes receive more insolation due to their higher angle of incidence

Local Influences on Insolation

Cloud Cover

- reduces both incoming and outgoing insolation

- thicker cloud cover, more absorption, reflection, scattering and terrestrial radiation (radiation back to space) during daytime

- cooler temperatures during day with thick cloud cover, higher with no/little cloud cover

Local Influences on Insolation

Cloud Cover

- Night:

- Thick cloud cover during night time can act as an insulating blanket to trap heat

- lack of cloud cover during the night, loss of heat from surface by terrestrial radiation, cool/cold temperatures

Local Influences on Insolation

Urbanisation

- alters the albedo of natural landscape

- buildings, concrete and black roads tend to reflect less insolation and therefore absorb more heat

- hence higher temperatures in urban areas than grass or natural landscape

Finito