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Earth has a well developed atmosphere- consists of a mixture of gases called air

Density + pressure variations cause air motion- the wind

The atmosphere governs physical conditions of weather: temperature, pressure, moisture content, wind velocity, wind direction.

Climate is long term weather conditions

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Evolution through time

4.6bn years ago- earth formed

early atmosphere- hydrogen + helium, volcanic eruptions added gases

3.5 Ga - Photosynthetic cyanobacteria added free 02 + Atmospheric o2 accumulated 

By 2 Ga- o2 reached 1% of present value

Eukaryotic green algae 1.6 Ga accelerated o2 production. By 600ma o2 arrived at 20% current conditions

Present o2 by 65 ma (21% atmospheric concentration)

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Atmosphere supports life

O2 made the formation of an ozone layer possible- absorbs deadly UV radiaiton and prevents it from sterlising the surface of the Earth.

Allowed diversification of life-  especially in terrestrial environments

Present atmosphere = 78% Nitrogen, 21% Oxygen; Other=1%

Other includes: Argon, Carbon dioxide, Sulfur dioxide, Ozone and Radon

Aerosols are tiny suspended particles <1 micrometer. Can be liquid droplets or solid dusts and can influence the insolation received by the Earth by reflecting infra-red radiation back out into splace. 

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Our effect: Pollution

Humans add gases and aerosols from fossil fuel combustion + industrial processes

Environ,ental harm- acid rain (from sulfates and nitrates acidfying precipitation)

- Enhanced Greenhouse effect- CO2 and CH4 trap atmospheric heat. 

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Pressure and density

Air pressure- force due to the weight of overlying air

  • greatest near the Earth's surface because all the atmosphere is piled aboe
  • 1 atm @ sea level = 1013mbar

Air density= mass of air/volume

  • Maximum at sea level
  • Decreases upward

Temperature & air 

As air rises it expands+ cools, holding less water

As it descends it contracts+warms, absorbing more water

This is called adiabatic cooling + heating

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Relative humidity

Water content is described by the relative humidity

Ratio (%) of measured water content compared to the maximum possible

Air with 100% relative humidity= saturated

Air with less than 100% than relative humidity=undersaturated 

Dry desert air- 0.3% water - Low relative humidity

Humid tropical rainforest air- 4% water- High relative humidity

Maximum moisture content changes with temperature. Cold air holds less moisture, warm air holds more. Warm undersaturated air becomes saturated as it cools- dewpoint. Below the dewpoint- water forms dew on surfaces + water freezes to form frost. 

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Rising air adiabatically cools to form tiny water droplets

Millions of tiny droplets join together to form cloud

They can also dissipate adiabatically. 

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Atmospheric layers

Thermal layers

Troposphere (first 12km): Mixing layer- all weather occurs here. Thinner at poles (9km) and thicker at equator (12km). Temperature slowly decreases upwards to -55 degrees celsius.

Tropopause- temperature stabalizes

Stratosphere (12km-39km): Does not convect. T = consistent for 10km. Then warms to 0 degrees C.

In the mesosphere the temperature cools again.

Ionosphere: 60-400km- solar energy creates positive ions by stripping electrons from N and O. The charged particles from solar flares interact in the ionosphere and are funneled to the poles by the Earth's magnetic field. Northern lights (Aurora borealis), Southern lights (Aurora australis). 

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Origins of the wind

Troposphere experiences almost constant motion

Movement of air from one place to another = wind

Lccal- 10's to 1000s km

Global - scale that encircles the planet e.g jet stream

Solar energy is not equally distributed- vertical rays on tropics=hotter-air rises. At poles- sunlight=oblique - less insolation.  This differential solar heating drives convection.

Thermal convection. Warm air expands, becomes less dense and rises- this creates a void and air flows in, replaced by sinking, cooler, more dense air. 

Lateral pressure differences drive horizontal winds. Air flows from high P to low P perpendicular to isobars which show areas of equal P. The steeper the gradient- the faster the winds. 

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Wind complications - Coriolis effect

Coriolis effect- Earth's rotations cause the prevailing winds to be deflected

"A phenomenon that causes fluids like water and air to curve as they travel across or above the Earth's surface."

Earth is constantly spinning around its axis from west to east. But because the Earth is a sphere- the equator is spinning faster than the poles. 

In the Northern hemisphere this is to the right. This is why storms spin anti-clockwise around the eye of the storm. 

In the southern hemisphere this is to the left . Storms spin clockwise. 

This occurs no matter which direction the wind is moving in to begin with within each hemisphere 

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Air cells

If the Earth did not spin- we would have 2 convection cells- air rising at equator and sinking at poles

But instead- we have 3!: Hadley, ferrel and polar in each hemisphere

Low pressure at equator/ low latitudes (30o N and 30oS)  due to rising equatorial air- forms from converging air-  winds are deflected towards the equator- meet (intertropical convergence zone/doldrums) air= forced up.  air cools and condenses- wind moves air North + south- rain- tropical rain forests either side of the equator.

High pressure at horse latitudes due to descending air- air spreads out- divergence zone

Low pressire at subpolar highs due to more convergence

before polar highs

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Jet streams

At high altitudes over the polar front and horse latitudes- air masses of different temperatures come into contact.- Steep pressure gradient forms- 

Fast-flowing, high-altitude westerly winds known as jet streams 


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Local scale conditions of temperature, pressure, humidity and wind speed

Reflects the prevailing winds and variations in the topography, water and vegetation

A weather system may affect an area for a small period of time

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Air masses

Weather is controlled in part by meeting air masses-

Packages of air with unique, recognizable properties- reflecting their original locations e.g. cold air mass from poles etc

over a couple 1000km in width- Flow across a region for a couple of days

Contact/boundary= called a front

Cold- where cold air replaces warm air.Cold dense air flows underneath and pushes warm up. Moisture released makes big thunderstorms.

Warm- where warm air replaces cold air.+  Warm air flows up a gentle incline over the colder air-pushes the cold air away as a wedge. Less steep T and P gradients. Broad cloud cover.

Occluded- where fast moving cold fronts overtake warm fronts. cold front lifts warm from ground- strong winds and heavy precipitation are produced. 

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Rotational flow

High pressure systems:

  • As air flows from high pressure to low pressure- a void is created
  • This is filled by cold dry air being pulled down from above- as the air compresses it warms and "wicks"? up moisture.
  • Therefore associated weather is often clear + dry. 
  • Also known as Anticyclonic flows

Low pressure systems:

  • Air piles up in the centre- sucked up and cools- condensing as clouds- rain. cloudy, rainy weather. 
  • Also known as cyclonic flow

The flowing air develops a spiral motion due to the coriolis effect

In the nothern hemisphere: Anti-cyclonic flows= clockwise; Cyclonic= clockwise. 

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Cloud formations

Air lifting mechanisms

Convective- where warmed air is buoyed upwards

Frontal- when air is carried upwards along fronts- 

Convergence- When two converging air masses push each other up

Orographic- air is pushed up when it meets mountains

Clouds form in the troposphere- the type that forms is controlled by air stability, the elevation+ wind

Shape: Cirrus (wispy, thin, feathery); Cumulus (puffy, cottony); Stratus (stable, layered)

Prefixes: Cirro (high altitude); Alto (mid-altitude); Nimbo (rain producing)

Unstable air= vertically building cumulus clouds e.g. Cumulonimbus- immense rain clouds

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Storms are episodes of severe weather- producing lightning, wind, rain, hail, sleet and/or snow

Develop along steep pressure gradients, fuelled by warm moist air.  Named after wind speeds >60km/h.

At lower latitudes <20o N - Hurricanes form. - Huge low pressure/cyclonic storms over the warm, tropical ocean waters. They do not form near the equator due to insufficient lateral winds. 

Hurricanes develop in summer/late fall when low p tropical disturbances pull air inward- as the air rises and cools it produces heat which pulls in more air and creates even lower pressure- over time the storm builds in size and strength.

  • Size range 100-1,500km
  • Speed >119km/h 

Landfall of storms removes the storm fuel of warm, moist air. 

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Hurricane damage

High velocity winds- destroy infrastructure + buildings

Waves- can create enormous waves + storm surges

Intense rainfall can cause flooding and mudslides

Saffir- simpson scale ranks intensity according to wind speed (km/h), pressure (millibars) and damage. 

  • 1- Minimal- <119km/h wind speeds. No building damage. 1.5m storm surge. Branches broken. 
  • 2- Moderate- Some trees blown down. Mobile homes seriously damaged. 2.4m storm surge
  • 3- Extensive- Some structural damage to small buildings.
  • 4- Extreme- Some roofs completely destroyed. Evacuation up to 10lm from coast. 
  • 5- Catastrophic - over 250km/h wind speeds + coast up to 16km evacuation
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