ENV1800 Climate and Weather

Define weather and climate

Weather

Day by day variations in temperature, rainfall and wind

• thermometers, rain gauges and barometers

Climate

Long term average of temperature, wind and precipitation over time

• proxies - variables known to change in response to climate e.g tree rings

6 key factors that shape patterns in earths climate are mainly determined by: DREETI

The sun

1. Different amounts of heating of the earths surface by the sun

• Different temperatures at different latitudes

2. Rising air due to convection

• Differences in rainfall at different latitudes

3. Earths rotation

• coriolis effect —> prevailing winds and ocean currents

4. Elevation

• Temperature decreases with elevation, altitude increases air becomes thinner and hold less heat

5. Tilted Earth axis

• seasonal differences in temperature and day lengths for temperate and polar regions

• seasonal differences in rainfall and wind direction in monsoon climates

6. Inter-annual variation in winds and ocean currents

• El nino southern oscillation

Hadley cell define and explain

A global scale tropical atmospheric circulation

Explains the frequent rainfall in tropical regions and dry climate in subtropical regions at around 30 degrees N and S of the equator

• Maximum sunlight and heat energy at the equator

• Heats the land and air w moisture rises due to convection

• As air rises it cools down eventually condensing causing rainfall in the tropics

• Cooled dry air descends into subtropical regions causing dry climates

What happens due to the Hadley cell

1. Low air pressure (air is rising) —> Cloudy skies and high rainfall close to the equator

2. High air pressure (air is descending) —> Clear skies and low rainfall at subtropical regions

Define the Intertropical Convergence Zone (ITCZ)

Zone of high rainfall where the trade winds from the Northern and Southern Hemispheres converge 

• Push warm moist air upwards 

• Lead to cloud formation and rain 

Spot from images → thick band of clouds stretching across the equator

Trade winds VS Prevailing winds

Prevailing winds: Winds that blow consistently in a given direction across a specific part of the Earth's surface

Trade winds: A type of prevailing wind that moves from east to west across the equatorial and subtropical regions (EW)

Influence of Hadley cells on prevailing winds

Surface air flows along a pressure gradient

• High to low —> Subtropical to tropical regions

• Along with coriolis effect, deflect the air sideways

• Causing trade winds

What causes wind to move in a direction as seen in prevailing winds

Hadley circulation + Coriolis effect

Moving air or water tends to be deflected from its path

• Deflected to the right in the Northern Hemisphere

• Deflected left in the Southern Hemisphere

caused by earths rotation on its N-S axis

Coriolis effect

the curving of the path of objects or fluids due to the earths rotation

Ferrel cells

• 30 - 60 degrees N and S of the equator

• Temperate regions

• Not driven by temperature, driven by the motion of the adjacent hadley and polar cells 

• Results in semi-permanent low- and high pressure areas 

• Transports heat from the equator to the poles

Polar cells

• Polar regions

• 60 degrees to 90 degrees

• Smallest cell 

• Cold air would sink, flows 60 degrees to 70 degrees north or south 

• Leave polar region and begin to warm and rise again

Rain shadow / orographic effect

The windward side of the mountain is wet and green, while the leeward side is dry and arid

E.g The Himalayas

Northern side is (leeward) —> dry, cold and sparse vegetation

Southern side (windward) —> rainfall and a lot of vegetation

Define tropical cyclones

Rotating storms that form in places where surface winds blow across warm tropical oceans close to the equator

• Atlantic Ocean - Hurricanes

• Pacific Ocean - Typhoon

• Indian Ocean - Cyclone

Surface winds driven by air circulation influence the Earth’s climate through

1. Surface winds → surface ocean currents transporting warm water from the tropics or cool water from cooler regions 

2. Rotating storms → formed by surface winds over warm tropical oceans → tropical cyclones

Formation of tropical cyclones

Winds + ocean are warm they move up and spiral inwards due to the coriolis effect

1. Ocean water is warm >26 degrees Celsius and warms surface air

2. Warm air evaporates from the ocean surface and comes into contact with a mass of cold air, forming clouds and rain (swirling storm) —> form a cycle

3. A column of low pressure develops at the centre.

4. Wind forms around the column and spiral inwards due to the coriolis effect

5. As the pressure in the central column weakens, the speed of the wind around it increases

Due to the coriolis effect:

• Southern hemisphere —> Clockwise

• Northern hemisphere —> Anti-clockwise

Where is the coriolis effect the strongest?

At the poles of the earth (N and S)

All tropical cyclones form _______ but not at ________. Why?

Close to the equator but not at the equator

• The coriolis effect is too weak at the equator to generate spin

• Insufficient deflection and rotation to build a cyclone

• At the equator movement of air is usually upwards not across the earth surface, forms clouds but not rotating storms

Consequences of tropical cyclones for people and ecosystems

People

• Flash flood

• High wind speed + heavy rainfall and storm surges blow waves onto the land

• E.g typhoon Haiyan, Philippines

Ecosystem

• Change structure of the forest

Surface elevation

Surface elevation influences

• Rainfall (prevailing winds blowing air containing water vapour)

• Temperature —> different vegetation
  - Low elevations: Dipterocarp forests
  - High elevations: Transition to montane and mossy forests

microclimate: immediate change in weather

Types of vegetation in rainforest at different elevations

Lowland dipterocarp rainforest

• 300m above sea level

• warm and humid

• Tall trees and dense canopy

Mossy forest

• 2000m above sea level

• Cooler

• Covered in moss

• Misty

Seasonal differences in temperature and day length (temperate and polar regions) 

→ due to earth’s tilt on its North-South axis, seasonal changes in earth's surface temperature at different times of the year 

• Northern hemisphere tilted towards the sun during Summer → long days + hot weather 

• Northern hemisphere tilted away from the sun during Winter → short days + cool temperature

E.g winter solstice, shortest day of the year, one of the poles tilted furthest away from the sun 

Monsoon

Seasonal reversal of wind direction that results from the differential heating of the land and the sea

• Land has a lower specific heat capacity than water (land heats up and loses heat faster than the ocean) 

• Continents heat up more quickly than oceans when days are long during the summer and continents cool down faster than oceans during the winter

Two types of monsoons

Southern / Southwest Monsoon (may to september)

• more heat from the sun, long days short nights

• land heats up faster than the ocean

• warm air rises due to convection and forms a low-pressure zone

• Cool wind from the ocean is higher pressure now

• Wind moves from high pressure to low pressure zone

• Moist air from the sea to land

• Heavy rain across South and East Asia

Winter / Northeast Monsoon (October to January)

• less heat energy from the sun, short day long nights

• Land cools down faster than the ocean

• Moist ocean air rises creating a low pressure zone

• Land is cooler and creates a high pressure zone

• Wind moves along the pressure gradient from land to ocean

• Dry conditions in South and East asia

El nino Southern Oscillation (ESCO)

Multi year variation in Pacific Sea surface temperature occurs when trade winds weaken or reverse direction

• appears every 3-7 years

Normal conditions without ENSO

• Trade winds move from E to W, Warm water accumulates in the west (SEA and Australia)

• Leading to rainfall in SEA and Australia

• Upwelling of cold deep water in Southern America
  - nutrient-rich —> plankton / phytoplankton
  - food for many marine animals
  - local economy

With ENSO

Trade winds weaken or reverse in direction

• Blow from west to east

• Warm water moves towards East of pacific ocean

• Heavy rainfall and reduces upwelling in South America

• Dry conditions in Southeast Asia and Malaysia

La nina

Strengthened trade winds —> Intense rainfall in SEA and Australia