The Atmosphere

The Atmosphere - SQA Key definitions : 

• Albedo - The proportion of solar radiation that is reflected by a body or surface. Value ranges from 0 (no reflection) to 1 (100% reflection).

• Biome - A large geographical area characterised by its specific climate, soil conditions, and the distinct communities of plants and animals that have adapted to live there.

• Coriolis effect - The apparent deflection (to an observer) of a path due to the changing speed of rotation between the poles and the equator, caused by the Earth rotating independently of the atmosphere above.

• Equatorial rainforest - Rainforests formed along the equator due to ascending, low atmospheric pressure coupled with wet air, rising between the two Hadley cells. 

• Ferrel cell - One of the three major atmospheric circulation cells that transport heat and moisture around the Earth. It operates in the mid-latitudes, roughly between 30° and 60° north and south of the equator.

• Global energy budget - The balance between incoming and outgoing solar radiation.

• Hadley cell - One of the three major atmospheric circulation cells that transport heat and moisture around the Earth. It operates in the lowest latitudes, roughly between 0 and 30° north and south of the equator.

• Hot desert - Deserts formed approximately 30° north or south of the equator due to descending, high atmospheric pressure coupled with dry air, falling between the Hadley and Ferrel cells. 

• Ice sheet - A huge mass of glacial ice that covers the surrounding terrain. 

• Insolation - The solar energy (sunlight) that reaches a square metre of the Earth's surface over a period of 1 second (units - W/m²). 

• Latitude - A measurement of how far a location is north or south of the Equator. measured in degrees (°). 

• Milankovitch cycles - Long-term variations in the Earth's orbit and tilt that change how much solar radiation reaches the planet. 

• Natural climate change - The long-term variations in Earth’s average temperature and weather patterns that occur due to natural processes, independent of human activity. 

• Natural greenhouse effect - Naturally occurring gases in Earth's atmosphere trap heat, keeping the planet warm enough to support life.

• Polar cell - One of the three major atmospheric circulation cells that transport heat and moisture around the Earth. It operates in the highest latitudes, roughly between 60° and 90° north and south of the equator.

• Solar flare - A sudden, massive explosion on the surface of the Sun that releases a huge amount of material and radiation into space. 

• Solar radiation - The energy emitted by the Sun in the form of electromagnetic waves.

• Sunspot - A relatively cooler, dark area on the Sun's surface where the magnetic field is preventing hot gas from rising to the surface.

• Surface wind patterns - The consistent, large-scale movement of air across the Earth's crust; The trade winds, the westerlies and the polar easterlies.

• Temperate rainforest -  Rainforests formed between 40° and 60° north or south of the equator due to ascending, low atmospheric pressure coupled with wet air, rising between the Ferrel and Polar cells. 

• Tri-cellular model -  The three-cell model, which explains how the Earth's atmosphere circulates and redistributes heat from the equator to the poles.

• Tundra - Cold desert formed approximately 60° and 90° north or south of the equator due to descending, high atmospheric pressure coupled with dry air, falling between the two Polar cells. 

The Atmosphere

The Atmosphere consists of all the gases that surround the Earth. There is no upper 'edge' to the Atmosphere, the gases just get lower and lower pressure until it becomes the vacuum of space. However, the 'end' of the atmosphere and the 'start' of space is usually defined as about 100km (this is known as the 'Kármán Line'). 

The image below shows a view from the International Space Station. The fading blue colour shows the thinning of the Atmosphere as the height increases :

There are several layers that form the Earth's atmosphere, each with distinct conditions within : 

The composition of the Earth's atmosphere has changed a lot over the lifetime of the Earth, but it has had roughly the same balance of gases for the last half a billion years:-

The current composition of the Earth's atmosphere is a mixture of mainly nitrogen, with a large proportion of oxygen and then a series of trace elements : 

The Atmosphere : Natural Greenhouse Effect

The atmosphere has an important role in the temperature at the Earth's surface. Without the atmosphere, the temperature would vary greatly across the day-night cycle. As an indication of this, the Moon (without a substantial atmosphere) experiences temperatures of around 127 °C facing the Sun and -183 °C on the night side. The Earth's atmosphere moderates this incoming heat, reducing daytime temperatures and increasing nighttime temperatures to levels tolerable for life. 

There is a group of gases that provides an additional warming effect; these are known as 'Greenhouse Gases'. Without this additional warming, known as the 'Natural Greenhouse effect', the heat radiated from the ground would escape quickly into space, and the average surface temperature would be about -20°C. 

The Greenhouse Gases are 

• Carbon dioxide

• Methane

• Nitrous oxide 

• Water Vapour

These gases act 'like a blanket' trapping heat within the Earth's atmosphere:-

Atmospheric Circulation : Global Energy Budget

The global energy budget is the balance between incoming and reflected solar radiation. The Earth has to balance this, as otherwise it would be too hot or too cold. When solar radiation enters the atmosphere, two-thirds of incoming solar radiation is absorbed by the atmosphere (clouds, water vapour, gases, dust) and Earth’s surface (land, water, plants). The remainder is reflected by Earth’s surface, clouds, atmospheric gases and dust. 

The proportion of solar radiation that is reflected by a body or surface is known as the albedo. This can range from a value of 0 (no reflection) to 1 (100% reflection). 

The amount of reflected energy changes with the type of surface :

Snow and ice reflect a lot of solar energy back into space and therefore have a high albedo (fresh snow ~0.85). The albedo for forests, oceans, and deserts is much lower, as more energy is absorbed by the ground and water (ocean water ~0.06). The average albedo for Earth is 0·31. 

Atmospheric Circulation : Tri Cellular Model

Due to the fact that the Earth is not flat, the Sun heats different parts of the Earth by different amounts. 

The solar energy (sunlight) that reaches a square metre of the Earth's surface over a period of 1 second (units - W/m²) is known as 'Insolation'. 

The areas that receive the highest insolation are at the Equator (as the light strikes the Earth more directly), whereas the least insolation is received at the Poles (due to the much lower angle to the Sun) : 

There is a net gain of solar energy in tropical latitudes and a net loss towards the poles due to the angle at which insolation strikes the Earth’s surface. Atmospheric and oceanic circulation redistributes this energy, so energy is moved from areas of surplus (between 38° north and south) to areas of deficit (above 38° north and below 38° south).

Hot air rises at the equator until it reaches the edge of the Troposphere and is forced to move north or south. As it does so, it cools and sinks back downwards, producing a 'convection cell'.

There are three such cells between the Equator and the Pole in each hemisphere :

It is the mass movement of air caused by the convection cells which produces the wind within Earth's Atmosphere. Combined with the Coriolis Effect, this circulation distributes heat throughout the entire atmosphere.

 The videos below provide a summary of Atmospheric Circulation : 

Atmospheric Circulation : Biomes

These convection cells affect the biomes found at different locations due to their impact on temperature and precipitation. 

At locations where the edge of the cell is descending, high atmospheric pressure coupled with dry air leads to the formation of hot Deserts (between the Hadley and Ferrel Cells) or cold Tundra (at the poles). 

At locations where the edge of the cell is ascending, low atmospheric pressure coupled with wet air leads to the formation of Tropical rainforests (along the equator) or Temperate forests (between the Ferrel and Polar Cells). 

Natural Climate Change

The Earth's climate has varied for billions of years, through a range of natural influences. Geological records spanning millions of years indicate a number of significant variations in Earth’s climate, evidenced by ice ages and warmer interglacial periods.

The diagram below shows how the average global temperature has changed over the last 500 million years : 

These variations are caused by a combination of 'Long-term' and 'Short-term' natural influences : 

• Long-term influences : Milankovitch cycles (orbital changes) & Plate Tectonics

• Short-term influences : Volcanic activity & Sunspot activity

Natural Climate Change : Milankovitch Cycles

As Earth orbits around the Sun, cyclical variations in Earth–Sun geometry combine to produce variations in the amount of solar energy reaching Earth. 

These include changes in: 

• The 'shape' of Earth’s orbit 

• The 'tilt' of Earth’s axis

• The 'orientation' of the Earth’s axis

Milankovitch Cycles : The shape of Earth’s orbit (100,000 year cycle)

This varies from elliptical to nearly circular. When the orbit is circular, the amount of insolation received on an annual basis is greater and the Earth’s temperature increases. 

The applet below allows changes to the shape of the Earth's orbit, showing the changes in insolation (marked as 'Solar Energy') across an orbit : 

Milankovitch Cycles : The Tilt of Earth’s Axis (41,000 year cycle)

The angle of tilt varies over time. A greater tilt means more severe seasonal variations in temperature (warmer summers and colder winters), and vice versa. Cool summers allow snow and ice to persist at high latitudes, building up into ice sheets. The high albedo of snow and ice causes additional cooling.

The applet below allows changes to the tilt of the Earth's axis, showing the changes in insolation (marked as 'Direct horizontal irradiance') as the tilt is changed : 

Milankovitch Cycles : The Orientation of Earth’s Axis (26,000 year cycle)

This changes over time and is driven by tidal changes influenced by the Sun and the Moon. This results in one polar hemisphere being closer to the Sun than the other, changing the amount of insolation reaching each hemisphere.  

Currently perihelion (the closest point of orbit to the Sun) occurs during winter in the Northern Hemisphere and in summer in the Southern Hemisphere. This makes Southern Hemisphere summers hotter and moderates Northern Hemisphere seasonal variations. 

But in about 13,000 years, axial precession will cause these conditions to flip, with the Northern Hemisphere seeing more extremes in solar radiation and the Southern Hemisphere experiencing more moderate seasonal variations.

Natural Climate Change : Plate Tectonics (Millions of Years of Movement)

Geological and fossil evidence support the theory that Earth’s continental plates have changed position and shape. 

Once part of a giant landmass at the South Pole, the breaking up and movement of the continental plates to their current positions has impacted on atmospheric and oceanic circulation, both of which have affected climate.

Natural Climate Change : Volcanic Activity

Volcanic activity reduces the Earth’s temperature due to the introduction of ash and other particulates into the upper atmosphere. This results in less insolation reaching Earth. 

The diagram below shows the impact of volcanic eruptions on the average global temperature between 520 and 680 CE.

The temperature anomaly represents the observed temperature change from the long-term average, showing the impact caused by each eruption. In the largest eruptions, they are followed by a substantial drop in average global temperature : 

In the longer term, volcanoes introduce higher levels of carbon dioxide and other greenhouse gases into the atmosphere, which trap heat. 

Natural Climate Change :  Sunspot activity (11 year cycle)

Sunspots are storms on the Sun’s surface that are marked by intense magnetic activity, resulting in solar flares and the ejection of hot gases.

A Sunspot is a region on the surface of the Sun which is cooler than the surrounding area. A Sunspot is caused by the magnetic field preventing hot gas from rising to the surface, resulting in a relatively cool patch (though still hotter than Thermite). These sunspots can be huge, covering an area greater than 150,000 km in diameter.

The diagram below shows a large Sunspot, with the Earth also shown for scale :-

The Sun follows a cycle of variations in activity on an 11-year cycle. When the Sun is very active, the Sunspot number increases and the number of Solar Flares also increases. When the Sun is inactive, the sunspot number can drop to zero, with very few Solar flares. 

The graph below shows Sunspot numbers for the last 400 years :-

During periods of maximum sunspot activity, the Sun emits more energy; therefore, insolation will be higher. The average global temperature is represented by the black line on the above graph, showing a correlation between Sunspot activity and Climate. 

The video below shows further evidence of a correlation between sunspot activity and Climate, based upon the water flow through the Iguazu Falls in Brazil :