Atmospheric Processes and Weather Phenomena (Cambridge IGCSE/A‑Level Geography)
1. Energy Budgets and Global Atmospheric Circulation (syllabus 2.1)
Energy inputs – short‑wave solar radiation absorbed by the Earth’s surface and atmosphere.
Energy outputs – long‑wave infrared radiation emitted to space.
Key transfers
Reflection (albedo) of solar radiation by clouds, ice and the land surface.
Absorption of long‑wave radiation by greenhouse gases (natural greenhouse effect).
Sensible heat (conduction & convection) and latent heat (evaporation/condensation) transfers.
Global energy budget diagram (short‑wave vs. long‑wave) – useful for visualising the balance of incoming and outgoing energy.
Three‑cell atmospheric circulation model
Hadley cell (0°–30°) – warm air rises at the equator, moves poleward aloft, descends in the subtropics.
Ferrel cell (30°–60°) – mid‑latitude westerlies; air rises at the polar front and descends in the subtropics.
Polar cell (60°–90°) – cold air descends at the poles, rises at the polar front.
Seasonal & diurnal variations – result from the tilt of the Earth’s axis and the day‑night cycle, influencing temperature gradients, pressure systems and consequently precipitation patterns.
2. Vertical Structure of the Atmosphere (syllabus 2.2.1 – intro)
Troposphere (0 – ≈12 km) – zone of almost all weather; temperature generally decreases with height (environmental lapse rate ≈ 6.5 °C km⁻¹).
Tropopause – boundary between the troposphere and stratosphere; marks the limit of deep convection and therefore the maximum height of most precipitation particles (e.g., hail).
Stratosphere (≈12 – 50 km) – contains the ozone layer; temperature increases with height due to absorption of ultraviolet radiation.
Understanding the temperature profile from the surface to the tropopause is essential for predicting the form of precipitation that will reach the ground.
Moisture moves between the Earth’s surface and the atmosphere through a small set of fundamental processes. The syllabus requires you to know each process and the direction of change.
Evaporation – liquid water → water vapour (oceans, lakes, rivers, moist ground).
Condensation – water vapour → liquid water when air cools to its dew point; forms cloud droplets on cloud‑condensation nuclei (CCN).
Freezing – liquid water → ice crystals (occurs when droplets are below 0 °C).
Deposition (direct vapour‑to‑ice) – water vapour → ice without passing through the liquid phase; important for the initial formation of ice crystals in very cold clouds.
Melting – ice crystals → liquid water when they fall into a layer of air above 0 °C.
Sublimation – ice ↔ water vapour without becoming liquid (e.g., surface ice disappearing on a sunny day).
Related concepts (useful for deeper study, not examined)
Transpiration – water vapour released from plant leaves; together with evaporation it forms evapotranspiration.
Advection – horizontal transport of moist air masses.
Convection – vertical transport driven by surface heating; creates up‑drafts that can lead to cloud formation.
Warmer air can hold more water vapour; the capacity roughly doubles for every 10 °C increase in temperature. This explains why tropical regions experience the most intense rainfall.
4. Causes of Precipitation (syllabus 2.2.2)
Precipitation forms when moist air is forced to rise, cools adiabatically and reaches saturation. The three principal lifting mechanisms are:
Frontal uplift – air is forced upward along the boundary between contrasting air masses (cold front, warm front, occluded front).
Orographic uplift – moist air is driven up the windward side of a mountain range, cooling as it rises.
Convective uplift – intense surface heating creates buoyant up‑drafts; typical of tropical thunderstorms and summer storms.
Radiative cooling of the surface or near‑surface air at night can also bring the air to its dew point, producing fog or dew (see Section 5).
5. Types of Precipitation and Related Phenomena (syllabus 2.2.3)
Type (as listed in the syllabus)
Formation mechanism
Typical atmospheric temperature profile
Representative global example (LIC / MIC / HIC)
Clouds (visible condensate)
Condensation of water vapour onto cloud‑condensation nuclei when air reaches saturation.
Cool, moist layer at the lifting point; droplets remain < 0.02 mm so they do not fall.
Cumulus over the Amazon rainforest (tropical, LIC)
Rain
Coalescence of liquid droplets in warm clouds (T > 0 °C) until they become heavy enough to fall.
Entire cloud column above 0 °C; no freezing layer.
Steady stratiform rain from a warm front over the United Kingdom (temperate, HIC)
Snow
Deposition of ice crystals in sub‑freezing clouds; crystals grow by aggregation and fall as snowflakes.
Cloud temperature below 0 °C from base to tropopause.
Snowfall from a cold front over the Canadian Prairies (continental, MIC)
Hail
Repeated accretion of super‑cooled water onto ice nuclei within strong up‑drafts of cumulonimbus clouds.
Deep convective cloud with a warm (T > 0 °C) layer aloft and vigorous up‑drafts that keep hailstones suspended.
Large hailstones in a super‑cell thunderstorm over the Great Plains, USA (mid‑latitude, HIC)
Dew
Condensation of water vapour directly onto cool surfaces (e.g., grass) when surface temperature falls below the dew point, usually at night.
Stable, near‑surface layer cooled by radiative loss; air remains unsaturated aloft.
Morning dew on crops in the Sahel, Africa (dry‑land, LIC)
Fog
Condensation of water vapour onto suspended particles when a shallow layer of air near the ground becomes saturated, often through radiative cooling.
Cool, moist air close to the surface; cloud base at ground level.
Coastal fog in the San Francisco Bay area, USA (maritime, HIC)
Linking Lifting Mechanisms to Precipitation Types
Frontal uplift – usually produces steady rain (warm front) or snow (cold front) depending on the vertical temperature profile.
Orographic uplift – heavy rain on windward slopes; snow on leeward slopes when the air remains below 0 °C.
Convective uplift – intense, short‑lived showers, thunderstorms and hail.
Radiative cooling – creates dew and fog when the surface cools faster than the air above.
6. Linking Moisture Processes to Climate Change (syllabus 2.3)
Climate change modifies the moisture cycle in several ways:
Increased atmospheric water‑vapour capacity – a 1 °C rise allows about 7 % more water vapour, intensifying the hydrological cycle and leading to more extreme rainfall events.
Shift in lifting mechanisms – warmer oceans enhance convection, increasing the frequency of tropical thunderstorms and associated hail.
Changes in temperature profiles – higher tropopause heights in a warming climate allow deeper convection, potentially raising the altitude at which hail forms.
Altered frontal patterns – poleward migration of the jet stream can change the location and intensity of frontal uplift, affecting regional rain and snow distribution.
Impacts on non‑precipitating phenomena – more frequent radiative cooling events in some regions may increase fog occurrence, while reduced night‑time cooling in others may diminish dew formation.
7. Key Concepts for Revision (Scale, Place, Change over Time)
Scale – from micro‑scale (dew on a leaf) to meso‑scale (convective thunderstorms) to macro‑scale (continental rain belts).
Place – illustrate with global examples (LIC, MIC, HIC) to show how latitude, altitude and proximity to oceans influence precipitation type.
Change over time – seasonal shifts in lifting mechanisms (e.g., summer convection vs. winter frontal rain) and long‑term trends linked to climate change.
8. Suggested Diagram for Revision
Vertical cross‑section of the atmosphere showing:
Cloud formation at the lifting point.
Pathways to rain, snow and hail via frontal, orographic and convective uplift.
Radiative‑cooling processes that produce dew and fog at the surface.
The tropopause as the upper limit for deep convection.
Your generous donation helps us continue providing free Cambridge IGCSE & A-Level resources,
past papers, syllabus notes, revision questions, and high-quality online tutoring to students across Kenya.