The Atmosphere
The atmosphere is made up of many gases; 78% Nitrogen; 21% Oxygen; water vapour; Carbon Dioxide; Ozone; Methane; Argon; Helium; Sulphur Dioxide, and many others. It is formed of four layers, the Troposphere, Stratosphere, Mesosphere and Thermosphere. The weather and climate conditions that we experience and measure take into account what happens in the lower Troposphere, a narrow band of the atmosphere closest to the surface of the Earth.
The Structure of the Atmosphere: Characteristics of Each Layer
The Troposphere
- extends approximately 12km from the Earth's surface
- temperature decreases with increasing height (6.4 degrees per 1000m)
- wind speed increases as height increases
- pressure falls as altitude increases
- turbulence with warmer air descending and cooler air rising
- contains most of the water vapour in the atmosphere
- particularly unstable layer due to presence of cloud, water vapour, dust, pollution and turbulence
The boundary between the Troposphere and the Stratosphere is called the Tropopause, marking the limit of the Earth's weather and climate.
The Stratosphere
- extends approximately from 12km to 50km
- temperature increases with altitude, as ozone is concentrated in the lower stratosphere
- the ozone absorbs UV radiation from the Sun and gives out heat, warming up the upper stratosphere
- light winds in the lower stratosphere, increasing with height
The boundary between the Stratosphere and the Mesosphere is called the Stratopause.
The Mesosphere
- extends from approximately 50km to 80km
- rapid fall in temperature with height, caused by a lack of water vapour, cloud and dust
- extremely low temperatures and high winds
The boundary between the Mesosphere and the Thermosphere is called the Mesopause.
The Thermosphere
- extends from approximately 80km to 1000km
- the outer layer of the atmosphere
- rapid increase in temperature with increasing altitude, exceeding 1000 degrees
Throughout all four layers, air pressure decreases with altitude.
The diagram below shows the changes in temperature and pressure in the different layers of the atmosphere.
The Structure of the Atmosphere: Characteristics of Each Layer
The Troposphere
- extends approximately 12km from the Earth's surface
- temperature decreases with increasing height (6.4 degrees per 1000m)
- wind speed increases as height increases
- pressure falls as altitude increases
- turbulence with warmer air descending and cooler air rising
- contains most of the water vapour in the atmosphere
- particularly unstable layer due to presence of cloud, water vapour, dust, pollution and turbulence
The boundary between the Troposphere and the Stratosphere is called the Tropopause, marking the limit of the Earth's weather and climate.
The Stratosphere
- extends approximately from 12km to 50km
- temperature increases with altitude, as ozone is concentrated in the lower stratosphere
- the ozone absorbs UV radiation from the Sun and gives out heat, warming up the upper stratosphere
- light winds in the lower stratosphere, increasing with height
The boundary between the Stratosphere and the Mesosphere is called the Stratopause.
The Mesosphere
- extends from approximately 50km to 80km
- rapid fall in temperature with height, caused by a lack of water vapour, cloud and dust
- extremely low temperatures and high winds
The boundary between the Mesosphere and the Thermosphere is called the Mesopause.
The Thermosphere
- extends from approximately 80km to 1000km
- the outer layer of the atmosphere
- rapid increase in temperature with increasing altitude, exceeding 1000 degrees
Throughout all four layers, air pressure decreases with altitude.
The diagram below shows the changes in temperature and pressure in the different layers of the atmosphere.
The Atmospheric Heat Budget (or Global Heat Budget)
The atmospheric heat budget (or global heat budget) is the balance between incoming solar radiation (insolation) and outgoing terrestrial radiation. This budget has remained constant over the last few thousand years.
Insolation
This is the incoming solar radiation from the Sun that reaches the Earth's atmosphere or surface. It is released from the Sun in short waves, and when it reaches Earth's outer atmosphere (the Thermosphere), one of three things can happen:
- It can be absorbed by water vapour, gases, dust or clouds.
- It can be reflected by the Earth's surface and scattered by particles in the air. This reflection is known as the albedo. The albedo of an object is the extent to which it diffusely reflects light from the Sun.
- It passes directly through to the Earth's surface
Reflected heat, in the form of long wave radiation, is trapped in the atmosphere and keeps Earth warm. This known as the natural greenhouse effect.
The diagram below shows this process.
(figures approximate)
But not all energy released by the Sun reaches the Earth, and the amount of insolation received varies across the globe. This is determined by:
- the solar constant: the mean (average) solar electromagnetic radiation. Varies slightly and only affects long term climate rather than short term weather
- distance from the Sun: the elliptical orbit of the Earth around the Sun can cause a variation of around 6% in the solar constant (see diagram below)
- the solar constant: the mean (average) solar electromagnetic radiation. Varies slightly and only affects long term climate rather than short term weather
- distance from the Sun: the elliptical orbit of the Earth around the Sun can cause a variation of around 6% in the solar constant (see diagram below)
- position of the Sun in the sky: due to the Earth's tilt and the curvature of the planet, the Equator receives more solar energy as solar radiation is more concentrated on a smaller surface area ('b' in diagram below), and less atmosphere to pass through than at the Poles ('a' in diagram below). At the Poles, because of the greater angle it reaches Earth at, there is more atmosphere for the radiation to pass through, so there will be more scattered and absorbed, and less received.
- length of day and night: strongly linked to the occurrence of seasons, and the tilt of the Earth, as the Sun shifts between the tropics of Cancer and Capricorn, the amount of insolation received by different areas changes. On the June summer solstice, the summer is at its height over the Tropic of Cancer, and the North Pole has constant daylight, whereas the South Pole is in darkness. This switches when the sun is at its maximum height over the Tropic of Capricorn, and the North Pole is in darkness.
There are other more local and short term factors which affect the amount of insolation received, including:
- Altitude: impacts the temperature of the air. For example, mountains have less surface area with which ti heat the air around it. Also, because atmospheric pressure decreases with height, this means there are bigger gaps between the molecules, decreasing their ability to hold and transfer heat
- Aspect: the positioning in a certain direction, for example, hillsides, which alter the angle at which the Sun's rays hit the ground. In the Northern Hemisphere, it is advantageous to have a garden that is south-facing, as it will receive sunlight for the majority of the day.
- Cloud cover: hugely important, experienced by us on a daily basis. On a cloudless night the temperature plummets, cloudy days are cooler, and cloudy nights mean it will be milder. Clouds reduce the amount of outgoing radiation, but can reduce incoming radiation in the day.
- Vegetation: affects the albedo effect. In dense tropical rainforests, dense vegetation absorbs a lot of the radiation, whereas in the polar regions, the white snow and ice give a high albedo effect, reflecting much more radiation.
Latitude and Energy Balance
As the diagram below shows, the amount of insolation received at different latitudes varies considerably, with the tropical latitudes having a net gain in solar energy (or a positive heat balance) and a net loss at the poles (a negative energy balance). So horizontal and vertical heat energy transfers occur within the atmosphere in order to balance out the energy via atmospheric circulation. If there was no atmospheric circulation, the poles would get increasingly colder and the tropics hotter and hotter.
There are other more local and short term factors which affect the amount of insolation received, including:
- Altitude: impacts the temperature of the air. For example, mountains have less surface area with which ti heat the air around it. Also, because atmospheric pressure decreases with height, this means there are bigger gaps between the molecules, decreasing their ability to hold and transfer heat
- Aspect: the positioning in a certain direction, for example, hillsides, which alter the angle at which the Sun's rays hit the ground. In the Northern Hemisphere, it is advantageous to have a garden that is south-facing, as it will receive sunlight for the majority of the day.
- Cloud cover: hugely important, experienced by us on a daily basis. On a cloudless night the temperature plummets, cloudy days are cooler, and cloudy nights mean it will be milder. Clouds reduce the amount of outgoing radiation, but can reduce incoming radiation in the day.
- Vegetation: affects the albedo effect. In dense tropical rainforests, dense vegetation absorbs a lot of the radiation, whereas in the polar regions, the white snow and ice give a high albedo effect, reflecting much more radiation.
Latitude and Energy Balance
As the diagram below shows, the amount of insolation received at different latitudes varies considerably, with the tropical latitudes having a net gain in solar energy (or a positive heat balance) and a net loss at the poles (a negative energy balance). So horizontal and vertical heat energy transfers occur within the atmosphere in order to balance out the energy via atmospheric circulation. If there was no atmospheric circulation, the poles would get increasingly colder and the tropics hotter and hotter.
Horizontal heat transfers: transfer of heat from the Equator to the Poles via air movement (80%). Winds, including the jet stream, hurricanes and depressions. Transfer of heat via water movement (20%) - ocean currents
Vertical heat transfers: transferring heat energy vertically to cooler areas via radiation, conduction and convection. Latent heat is energy absorbed or released when a substance changes state, and helps to transfer energy. For example, water takes in energy when it evaporates, and this energy is released when condensation occurs in the upper atmosphere.
Vertical heat transfers: transferring heat energy vertically to cooler areas via radiation, conduction and convection. Latent heat is energy absorbed or released when a substance changes state, and helps to transfer energy. For example, water takes in energy when it evaporates, and this energy is released when condensation occurs in the upper atmosphere.
The Coriolis Effect The Coriolis effect is a deflection of moving objects when the motion is described relative to a rotating reference frame. The anticlockwise rotation of the Earth deflects global wind patterns. In the northern hemisphere winds are deflected to the right, in the southern hemisphere winds are deflected to the left. The diagrams right and below show the Coriolis effect. |