atmosphere

Major gaseous composition of dry air

78% nitrogen, 21% oxygen, and approximately 1% argon and other trace gases.

Atmospheric layer where almost all weather events occur

Troposphere.

Average environmental lapse rate (ELR) in the troposphere

Approximately $6.5$ .

Temperature trend in the stratosphere with height and its physical cause

Temperature increases with altitude because the ozone layer absorbs incoming ultraviolet (UV) solar radiation.

Insolation

Incoming shortwave solar radiation received from the Sun by Earth.

Terrestrial radiation

Outgoing longwave infrared radiation emitted by Earth's surface and atmosphere.

Albedo

The proportion or percentage of incoming solar radiation reflected by a surface without absorption.

Surfaces with contrasting high versus low albedo values

Snow and ice exhibit high albedo, whereas dark ocean water and dense forests have low albedo.

Three shortwave radiation pathways in Earth's atmosphere

Reflection back into space by clouds, aerosols, and the surface; absorption by gases and particles in the atmosphere; and absorption at Earth's land and ocean surfaces.

Longwave radiation pathways between Earth and space

Earth emits infrared radiation outward; greenhouse gases and clouds absorb this energy and re-radiate it in all directions, including downward toward the surface.

Natural greenhouse effect

The process by which atmospheric trace gases absorb and re-emit outgoing longwave infrared radiation, maintaining surface temperatures higher than they would otherwise be.

Anthropogenic radiative forcing

The net energy imbalance in the climate system caused by additional human emissions of greenhouse gases.

Factors creating latitudinal insolation variations

Solar incidence angle (higher and more concentrated in the tropics) and atmospheric path length (shorter path in the tropics versus longer path through air at high latitudes).

Primary astronomical cause of Earth's seasons

Earth's axial tilt, which alters sun angle and daylight length throughout the year, rather than variations in Earth-Sun orbital distance.

Local controls on surface energy budgets

Cloud cover, slope aspect, altitude, vegetation density, surface moisture, and urban building materials.

Latitudinal energy imbalance driving global circulation

A net radiation surplus exists in low latitudes (tropics) and a net radiation deficit exists in high latitudes (polar regions).

Latitudinal boundaries of the idealised three-cell circulation model

Hadley cells between $0^{\circ}\text{ and }30^{\circ}$, Ferrel cells between $30^{\circ}\text{ and }60^{\circ}$, and Polar cells between $60^{\circ}\text{ and }90^{\circ}$ in each hemisphere.

Atmospheric conditions at the Intertropical Convergence Zone (ITCZ)

Intense thermal heating and surface convergence induce buoyant rising air, adiabatic cooling, condensation, deep cumuliform clouds, and heavy convectional rainfall.

Weather conditions produced by subtropical high-pressure belts

Descending air from the Hadley cell warms adiabatically and suppresses cloud development, creating stable, arid conditions responsible for major desert belts.

Dominant atmospheric features of mid-latitudes

Prevailing westerly winds, polar-front disturbances, and migrating frontal depression systems.

Circulation dynamics of polar regions

Cold, dense air sinks to generate polar high pressure, flowing outward as polar easterlies toward the subpolar low-pressure belts.

Coriolis effect

The apparent deflection of moving air and water caused by planetary rotation, directing pathways to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

Absolute humidity

The total mass of water vapour contained per unit volume of air, measured as $\text{g/m}^3$.

Relative humidity

The amount of water vapour present in the air expressed as a percentage of the maximum amount the air can hold at that temperature.

Relationship between air temperature and saturation vapour pressure

Warmer air has a higher saturation vapour pressure, enabling it to hold a greater capacity of water vapour before reaching saturation.

Dew point

The temperature to which an air parcel must be cooled at constant pressure and moisture content to attain $100\%$ relative humidity.

Steps in cloud droplet formation

Moist air rises, expands, and cools adiabatically until reaching its lifting condensation level, where excess vapour condenses onto microscopic hygroscopic nuclei.

Dry adiabatic lapse rate (DALR)

The cooling rate of an unsaturated rising air parcel, valued at approximately $10\,^{\circ}\text{C/km}$.

Saturated adiabatic lapse rate (SALR)

The cooling rate of a saturated rising air parcel, roughly $4\text{ to }7\,^{\circ}\text{C/km}$, which is lower than the DALR due to the release of latent heat during condensation.

Primary cloud classifications and appearances

Cumulus (puffy convective clouds), cumulonimbus (towering storm clouds with anvils), stratus (uniform horizontal sheets), and cirrus (high, detached, wispy ice clouds).

Two microscopic processes driving raindrop growth

Collision-coalescence in warm clouds and the Bergeron-Findeisen ice-crystal process in cold clouds.

Convectional precipitation mechanism

Intense solar heating warms surface air, causing buoyant ascent, adiabatic cooling, condensation, cumulonimbus formation, and localized high-intensity downpours.

Orographic precipitation and rain-shadow mechanism

Air forced over elevated terrain cools adiabatically to produce rain on windward slopes; as air descends the leeward side, it warms and dries, producing a rain shadow.

Frontal precipitation mechanism

Warmer, lighter air is forced up and over denser, colder air along a boundary, leading to widespread cooling, cloud condensation, and steady or burst rainfall.

Atmospheric instability criterion

Occurs when an uplifted air parcel remains warmer and less dense than the ambient air around it, accelerating vertical ascent and vertical cloud formation.

Atmospheric stability criterion

Occurs when an uplifted air parcel becomes cooler and denser than the surrounding air, causing it to sink back and suppressing vertical cloud growth.

Temperature inversion

A condition where atmospheric temperature increases with altitude, acting as a stable lid that prevents convection and traps surface pollutants.

Characteristics and Northern Hemisphere circulation of a depression

A low-pressure system with convergent, rising air that brings clouds and precipitation; airflow circulates in an anticlockwise direction in the Northern Hemisphere.

Characteristics and Northern Hemisphere circulation of an anticyclone

A high-pressure system with subsiding air that brings clear skies and settled weather; airflow circulates in a clockwise direction in the Northern Hemisphere.

Monsoon circulation mechanism

A seasonal reversal of prevailing wind direction driven by differential heating between continents and oceans alongside the seasonal migration of the ITCZ.

El Niño conditions in the equatorial Pacific

Trade winds weaken or reverse, causing warm surface waters to spread east across the central and eastern Pacific while suppressing cold coastal upwelling.

La Niña conditions in the equatorial Pacific

Trade winds strengthen, enhancing upwelling and generating colder-than-normal sea-surface temperatures across the central and eastern equatorial Pacific.

Weather versus Climate distinction

Weather is the day-to-day atmospheric state at a specific time and location; climate is the aggregate statistical average of weather over a standard $30\text{-year}$ period.

Global warming versus Climate change distinction

Global warming denotes the long-term increase in global mean surface temperature, while climate change encompasses broader systemic shifts including rainfall, sea level, and storm tracks.

Major anthropogenic sources of greenhouse gases

Carbon dioxide from fossil fuel combustion and cement manufacture, methane from livestock, rice paddies, and gas extraction, and nitrous oxide from agricultural fertilizers.

Role of deforestation in atmospheric carbon accumulation

Removes active terrestrial carbon sinks and releases stored biomass carbon as carbon dioxide through burning and decomposition.

Natural forcing mechanisms influencing climate variability

Volcanic sulphate aerosol emissions, solar irradiance fluctuations, orbital Milankovitch cycles, and internal coupled ocean-atmosphere oscillations.

Observational evidence for contemporary climate change

Instrumental thermometer records, expanding ocean heat content, accelerated glacier and ice sheet melt, thermal sea-level rise, phenological shifts, and paleoclimate proxies.

Ice-albedo feedback mechanism (positive feedback)

Surface warming melts reflective ice, revealing darker surfaces that absorb more insolation, which accelerates regional and global warming.

Water vapour feedback mechanism (positive feedback)

Warming increases atmospheric evaporation, and because water vapour is a potent greenhouse gas, it enhances longwave absorption and accelerates warming.

Radiative longwave feedback mechanism (negative feedback)

As Earth warms, it emits greater quantities of outgoing longwave infrared radiation into space according to the Stefan-Boltzmann relationship, exerting a cooling effect.

Key global impacts of anthropogenic climate change

Shifts in global precipitation, increased heatwave severity, heightened coastal flooding from sea-level rise, freshwater stress, biome shifts, and agricultural yield instability.

Climate change mitigation definition and examples

Strategies that tackle the causes of climate change by reducing emissions or enhancing sinks, such as deploying renewable energy, improving industrial efficiency, and afforestation.

Climate change adaptation definition and examples

Measures aimed at reducing vulnerability to the effects of climate change, including flood barrages, drought-tolerant crops, heat action plans, and managed coastal realignment.

Key trade-offs of climate change mitigation

Delivers global collective benefits by curbing future warming, but requires complex international political cooperation, major capital investment, and extensive technological transformation.

Key trade-offs of climate change adaptation

Provides immediate localized vulnerability reduction, but faces biophysical limits under extreme warming, demands unequal financing, and risks maladaptation.

Specific impacts of El Niño on coastal Peru

Suppression of cold nutrient-rich upwelling devastates marine fisheries, while anomalous atmospheric convection brings torrential rain and destructive flooding.

Causes of the Urban Heat Island (UHI) effect

High thermal mass and heat retention of building materials, reduced evapotranspirative cooling from impervious surfaces, and anthropogenic waste heat release.

Strategies to reduce the Urban Heat Island effect

Urban greening and tree canopy expansion, installing high-albedo cool roofs and green roofs, providing structural shade, and designing urban ventilation corridors.