Atmosphere and Climate Systems

The One Thing

The atmosphere is an integrated, layered system in which solar radiation is absorbed, reflected, redistributed, and re-emitted through processes involving heat, moisture, pressure, and circulation. Human-driven increases in greenhouse gases disturb this energy balance, producing global warming and wider climate change that require both mitigation and adaptation.

Who and What

• Atmosphere: The mixture of gases surrounding Earth, held by gravity and essential for life, weather, and climate. It consists mainly of nitrogen, about 78 percent, and oxygen, about 21 percent; argon, carbon dioxide, water vapour, and other gases make up most of the remainder.
• Troposphere: The lowest atmospheric layer, extending roughly 8 km at the poles to 18 km near the equator. Most weather and water vapour occur here. Temperature generally decreases with height at the normal environmental lapse rate of approximately 6.5 degrees Celsius per kilometre.
• Stratosphere: The layer above the troposphere, extending to about 50 km. It contains the ozone layer and relatively stable air. Temperature increases with height because ozone absorbs ultraviolet radiation.
• Ozone Layer: A region in the stratosphere rich in ozone that absorbs much of the Sun’s harmful ultraviolet radiation.
• Mesosphere: The layer from about 50 to 80 km, where temperatures generally decrease with height and many meteors burn up.
• Thermosphere: A very thin upper layer from about 80 km upward. Temperatures increase sharply because gases absorb high-energy solar radiation.
• Exosphere: The outermost atmospheric region, gradually merging with outer space.
• Insolation: Incoming solar radiation received by Earth. On a horizontal surface, Insolation is proportional to the cosine of the solar zenith angle.
• Albedo: The proportion of incoming solar radiation reflected by a surface; bright surfaces have high albedo.
• Terrestrial Radiation: Long-wave infrared energy emitted by Earth’s surface after it has absorbed solar energy.
• Greenhouse Effect: The natural warming process in which gases such as water vapour, carbon dioxide, methane, and nitrous oxide absorb and re-radiate outgoing terrestrial radiation. It is necessary for life, but increased greenhouse-gas concentrations strengthen it and contribute to global warming.
• Heat Budget: The balance between incoming solar radiation and outgoing terrestrial radiation in the Earth-atmosphere system. It can be expressed as: Incoming solar radiation = reflected solar radiation + absorbed solar radiation. Over the long term, absorbed energy is balanced by outgoing terrestrial radiation.
• Radiation Balance: At Earth’s surface, Net radiation = incoming short-wave radiation - reflected short-wave radiation + incoming long-wave radiation - outgoing long-wave radiation.
• Conduction: Transfer of heat through direct contact, mainly from the warm ground to the air touching it.
• Convection: Vertical transfer of heat through the movement of warm, rising air and cool, sinking air.
• Advection: Horizontal transfer of heat by moving air.
• Radiation: Transfer of energy through electromagnetic waves without requiring a material medium.
• Air Pressure: The force exerted by the weight of air on a unit area; it generally decreases with altitude.
• Pressure Gradient Force: The force that moves air from areas of higher pressure toward areas of lower pressure. It initiates wind, while friction and the Coriolis force modify wind speed and direction.
• Coriolis Force: The apparent deflection of moving air caused by Earth’s rotation. Winds deflect right in the Northern Hemisphere and left in the Southern Hemisphere. The force is zero at the equator and becomes stronger toward the poles; it changes wind direction but does not create wind.
• Global Circulation: The worldwide movement of air that transfers heat from the equator toward the poles.
• Hadley Cell: A tropical circulation cell in which warm air rises near the equator, moves poleward aloft, sinks near about 30 degrees latitude, and returns toward the equator at the surface.
• Ferrel Cell: A middle-latitude circulation cell between approximately 30 and 60 degrees in each hemisphere.
• Polar Cell: A high-latitude circulation cell in which cold air sinks near the poles and rises near about 60 degrees latitude.
• Trade Winds: Persistent tropical winds blowing generally from the subtropical high-pressure belts toward the equatorial low-pressure belt.
• Westerlies: Prevailing winds of the middle latitudes that generally blow from west to east.
• Polar Easterlies: Cold, dry winds that generally blow from polar high-pressure areas toward subpolar low-pressure areas.
• Jet Stream: A narrow band of very fast winds in the upper troposphere that influences weather systems and storm tracks.
• Inter-Tropical Convergence Zone: A belt of low pressure near the equator where the trade winds converge and air rises, often producing heavy rainfall.
• Humidity: The amount of water vapour present in the air.
• Relative Humidity: The ratio of the actual water vapour in air to the maximum water vapour the air can hold at the same temperature, expressed as a percentage: Relative humidity = (actual water vapour content / saturation water vapour capacity) x 100.
• Dew Point: The temperature to which air must be cooled for saturation and condensation to begin.
• Condensation: The change of water vapour into liquid water when air cools to its dew point or becomes saturated.
• Cloud: A visible collection of tiny water droplets, ice crystals, or both suspended in the atmosphere.
• Water Cycle: The circulation of water through evaporation, transpiration, condensation, precipitation, infiltration, surface runoff, and groundwater movement.
• Normal Lapse Rate: The average temperature change with height in the atmosphere.
• Adiabatic Lapse Rate: The temperature change in a moving air parcel without heat exchange with its surroundings.
• Air Mass: A large body of air with relatively uniform temperature and moisture characteristics.
• Front: The boundary between two air masses with different temperature and moisture conditions.
• Tropical Cyclone: A powerful low-pressure storm that develops over warm tropical oceans, with strong winds spiralling around a central eye. It requires warm ocean water, usually around 26.5 degrees Celsius or higher, sufficient moisture, a pre-existing disturbance, and weak vertical wind shear. Tropical cyclones generally do not form directly at the equator because Coriolis force is too weak.
• Temperate Cyclone: A low-pressure weather system that forms mainly in middle latitudes along fronts and produces changing weather.
• Anticyclone: A high-pressure system with sinking air, generally associated with clear and stable weather.
• Monsoon: A seasonal reversal of wind direction caused mainly by unequal heating of land and water.
• Weather: The short-term condition of the atmosphere at a particular place and time.
• Climate: The long-term average pattern and variability of weather in a region, usually assessed over at least 30 years.
• Climate Change: A long-term change in temperature, rainfall, winds, or other climate characteristics.
• Global Warming: The long-term increase in Earth’s average surface temperature, mainly linked today to rising human-caused greenhouse-gas concentrations.
• Mitigation: Actions that reduce greenhouse-gas emissions or increase the removal of carbon dioxide from the atmosphere.
• Adaptation: Adjustments made by people or ecosystems to reduce the harmful effects of actual or expected climate change.

Chronology

Causes and Consequences

• Unequal heating of Earth’s curved surface causes differences in temperature and air pressure. These pressure differences generate atmospheric movement and global circulation, while the Coriolis force modifies wind direction.

• Solar radiation is Earth’s primary energy source. The average incoming solar radiation at the top of the atmosphere is about 1,360 watts per square metre on a surface perpendicular to the Sun’s rays; the globally averaged value over the whole Earth is about one-fourth of this because Earth is spherical.

• Heating, cooling, and atmospheric circulation redistribute solar energy. Conduction transfers heat from the warm ground to adjacent air, convection moves warm air upward and cool air downward, advection transfers heat horizontally, and radiation transfers energy through electromagnetic waves.

• Global circulation consists of Hadley, Ferrel, and Polar cells in each hemisphere. Subtropical high-pressure belts occur near 30 degrees north and south, subpolar low-pressure belts occur near 60 degrees north and south, and polar high-pressure belts occur near the poles. These pressure belts produce the Trade Winds, Westerlies, and Polar Easterlies.

• The Inter-Tropical Convergence Zone forms where the Trade Winds converge near the equator. Rising air produces low pressure and often heavy rainfall.

• The apparent path of the Sun and the angle of solar rays vary with latitude and season, producing unequal heating and seasonal changes. The seasons result mainly from Earth’s axial tilt of about 23.5 degrees and its revolution around the Sun, not from large changes in Earth’s distance from the Sun.

• Cooling and saturation produce condensation and clouds. Cloud formation generally requires rising air, cooling, saturation, condensation, and condensation nuclei such as dust or salt particles. The water cycle links evaporation, transpiration, condensation, precipitation, infiltration, surface runoff, and groundwater movement.

• Pressure differences, air masses, fronts, Earth’s rotation, moisture, surface conditions, and upper-air circulation influence weather systems. Tropical cyclones develop over sufficiently warm tropical oceans, whereas temperate cyclones form mainly in the middle latitudes along fronts. Anticyclones involve sinking air and are generally associated with clear and stable weather.

• The natural greenhouse effect keeps Earth warmer than it would otherwise be because water vapour, carbon dioxide, methane, and nitrous oxide absorb and re-radiate terrestrial radiation. Human activities, including burning coal, oil, and gas; deforestation; agriculture; industrial processes; and waste decomposition, increase greenhouse-gas concentrations.

• Human enhancement of the greenhouse effect changes Earth’s energy balance and contributes to global warming. Consequences include more frequent or intense heatwaves in many regions, changing rainfall patterns, glacier and ice-sheet loss, sea-level rise, ocean warming, and ecosystem stress.

• Climate change responses involve both mitigation and adaptation. Relevant measures include renewable energy, energy efficiency, afforestation, sustainable transport, water conservation, disaster preparedness, climate-resilient agriculture, and improved coastal planning.

What Gets Asked

• Compare the structure and temperature patterns of the Troposphere, Stratosphere, Mesosphere, Thermosphere, and Exosphere, including the significance of the Ozone Layer.
• Explain how unequal heating, the Pressure Gradient Force, friction, and the Coriolis Force produce global circulation, including the Hadley Cell, Ferrel Cell, Polar Cell, Trade Winds, Westerlies, Polar Easterlies, and Jet Stream.
• Distinguish between weather and climate, and explain how humidity, Relative Humidity, Dew Point, Condensation, clouds, air masses, fronts, Tropical Cyclones, Temperate Cyclones, Anticyclones, and Monsoons influence weather.
• Explain Earth’s heat budget using the equations Incoming solar radiation = reflected solar radiation + absorbed solar radiation, Insolation is proportional to the cosine of the solar zenith angle, and Net radiation = incoming short-wave radiation - reflected short-wave radiation + incoming long-wave radiation - outgoing long-wave radiation.
• Evaluate why the greenhouse effect is natural and necessary for life but becomes a cause of Global Warming when human activities increase greenhouse-gas concentrations.
• Compare mitigation and adaptation as responses to Climate Change, using the Paris Agreement, renewable energy, energy efficiency, afforestation, sustainable transport, water conservation, disaster preparedness, climate-resilient agriculture, and improved coastal planning.