Welcome to Atmosphere and Weather!

Hello future geographers! The atmosphere is the engine that drives all our weather, from gentle rain to powerful hurricanes. This chapter, "Atmosphere and Weather," is foundational for Core Physical Geography (Paper 1). It might seem complex with all the energy flows and phase changes, but we will break it down using clear analogies and step-by-step processes. Mastering this content will give you a vital understanding of the air around us and how humans are changing it. Let's dive in!

Section 1: The Diurnal Energy Budget (2.1)

The Diurnal Energy Budget refers to the balance of energy (inputs and outputs) happening at the Earth's surface over a 24-hour period. Think of it like your personal bank account for heat!

1.1 Energy Inputs (The Deposits)

The main input is the energy from the Sun: Incoming Solar Radiation.

  • Shortwave Solar Radiation (SWR): This is the heat energy coming from the Sun. It is called 'shortwave' because the hot Sun emits radiation at short wavelengths.

1.2 Energy Outputs and Transfers (The Withdrawals)

Once the SWR reaches the surface, it is either reflected, stored, or transferred away.

Key Processes in the Energy Budget:

1. Reflection (Lost Energy)

  • Reflected Solar Radiation: Energy bounced straight back into space.
  • Albedo: The reflectivity of a surface. For example, fresh snow has a high albedo (reflects up to 90% of SWR), while dark asphalt has a low albedo (reflects only 5-10%).

2. Absorption and Storage (The Savings)

  • Energy Absorbed into the Surface and Subsurface: Heat that soaks into the ground, rock, or water. This energy is released later, usually at night.

3. Heat Transfer (The Spending)

  • Longwave Radiation (LWR): The Earth's surface, being much cooler than the Sun, radiates heat back into the atmosphere at longer wavelengths. This is crucial for keeping the atmosphere warm.
  • Sensible Heat Transfer (SHT): The movement of heat through convection (warm air rising) or conduction (heat moving through touch). You can *feel* this heat—it's sensible!
  • Latent Heat Transfer (LHT): Heat energy stored or released when water changes state (phase changes).
Focus: Latent Heat Transfer (LHT)

LHT is energy that is "hidden" (latent) within water molecules.

  • Evaporation: Liquid water turns to gas (vapour). This process uses energy from the surface, causing cooling. This is why sweating cools you down.
  • Dew: At night, the surface cools rapidly (radiation cooling). Water vapour condenses back into liquid dew, releasing latent heat. This is absorbed energy returned to Earth (the surface gets a little warmth back).
Quick Review: The Balance Equation

The surface energy balance should theoretically equal zero over 24 hours (energy in = energy out).

\(SWR_{in} - SWR_{out} - LWR_{out} \pm SHT \pm LHT = 0\)

Key Takeaway for Diurnal Budget: The balance between SWR (input) and LWR/SHT/LHT (outputs/transfers) controls local temperature fluctuations over a day. Surfaces with low albedo and high moisture content manage energy very differently from dry, reflective surfaces.

Section 2: The Global Energy Budget (2.2)

While the diurnal budget looks at one small spot over a day, the Global Energy Budget looks at the whole planet over a year. The Earth maintains a stable average temperature because, overall, energy input balances energy output. However, this balance is not equal everywhere!

2.1 Latitudinal Pattern: Excesses and Deficits

If the Earth maintained perfect balance everywhere, we would have no weather systems!

  • Radiation Excesses: Occur between the tropics (0° to 30° N/S). Sunlight hits the Earth directly, concentrating the energy over a smaller area. More SWR is received than LWR is radiated out.
  • Radiation Deficits: Occur near the poles (60° to 90° N/S). Sunlight hits the Earth at a steep angle, spreading the energy over a larger area. More LWR is radiated out than SWR is received.


Did you know? If there were no mechanisms to transfer this heat, the equator would boil and the poles would freeze solid.

2.2 Atmospheric and Oceanic Transfers

The imbalance is corrected by large-scale transfers of heat (energy) from the areas of excess (tropics) towards the areas of deficit (poles).

Atmospheric Transfers (Wind Belts)

Heat is transferred by the global circulation of air:

  • Hadley Cells: Drive air movement between the equator and 30° N/S (hot air rises at the equator).
  • Ferrel Cells: Mid-latitude circulation (30° to 60° N/S).
  • Polar Cells: Near the poles (60° to 90° N/S).

These cells create the predictable wind belts (like the Trade Winds) that move large volumes of heated air.

Oceanic Transfers (Ocean Currents)

Water is excellent at storing and moving heat.

  • Warm Ocean Currents: Move heated water from the tropics poleward (e.g., the Gulf Stream/North Atlantic Drift keeps Western Europe much warmer than areas at similar latitudes).
  • Cold Ocean Currents: Move colder water from the poles toward the equator.

2.3 Seasonal Variations in Climate

The location and intensity of these wind belts and pressure systems shift throughout the year, causing seasons.

  • Influence of Latitude: Determines the intensity of SWR received (changing solar angle).
  • Land/Sea Distribution (Continentality): Land heats up and cools down quickly, leading to extreme seasonal temperature variations inland. Water heats and cools slowly, moderating temperatures in coastal areas.
  • Ocean Currents: Further modify temperatures, especially in winter.

Key Takeaway for Global Budget: The Earth uses wind belts and ocean currents to move heat from the energy surplus areas (tropics) to the energy deficit areas (poles), thereby regulating global temperatures.

Section 3: Weather Processes and Phenomena (2.3)

Weather phenomena depend heavily on the processes of atmospheric moisture and the mechanisms that cause air to rise and cool.

3.1 Atmospheric Moisture Processes (Phase Changes)

These processes involve water changing state, which is crucial for cloud and precipitation formation.

1. Vapour/Gas to Liquid/Solid: (Release Energy)

  • Condensation: Gas (vapour) turns to liquid (droplets/clouds). Requires cooling.
  • Freezing: Liquid turns to solid (ice).
  • Deposition: Gas turns directly to solid (e.g., forming frost).

2. Liquid/Solid to Vapour/Gas: (Absorb Energy)

  • Evaporation: Liquid turns to gas.
  • Melting: Solid turns to liquid.
  • Sublimation: Solid turns directly to gas (e.g., ice turning straight into vapour without melting first).

3.2 Causes of Precipitation (Air Uplift)

For most precipitation to occur, air must rise, cool down, and reach its dew point (where condensation begins). Here are the primary causes of this vertical uplift:

1. Convectional Uplift (The Bubble Lift)

  • The ground is heated strongly by the sun. The air directly above the ground becomes hot and less dense.
  • This air rises rapidly in a thermal column (a bubble), cools quickly, condenses, and forms tall cumulonimbus clouds, often resulting in heavy thunderstorms.

2. Frontal Uplift (The Collision)

  • Occurs when two large masses of air meet.
  • When warm air meets cold air, the less dense warm air is forced to rise up over the denser cold air mass.
  • This steady uplift leads to wide areas of cloud and often prolonged, steady rain.

3. Orographic Uplift (The Mountain Barrier)

  • Air is forced to rise as it encounters a physical barrier, usually a mountain range ('Oro' means mountain).
  • As the air rises up the windward side, it cools, condenses, and causes rain. The leeward side (rain shadow) remains dry.

4. Radiation Cooling (The Ground Chill)

  • This process occurs when the Earth’s surface loses heat very quickly, usually on clear nights.
  • The air right next to the surface cools below the dew point, but since there is no vertical movement, this forms ground-level moisture, such as dew and fog.

3.3 Types of Precipitation

Precipitation is any form of water falling from the atmosphere.

  • Clouds: Collections of tiny condensed water droplets or ice crystals suspended in the atmosphere.
  • Rain: Liquid water droplets that have coalesced and are too heavy to remain suspended.
  • Snow: Precipitation in the form of ice crystals, formed when condensation and deposition occur below freezing point.
  • Hail: Lumps of ice formed within strong convectional clouds (thunderstorms) where updrafts repeatedly carry frozen droplets higher until they become too heavy.
  • Dew: Liquid water droplets formed on surfaces due to radiation cooling below the dew point.
  • Fog: A cloud layer resting on the ground, formed usually through radiation cooling or the mixing of warm and cold air masses.

Key Takeaway for Weather Processes: Water changing phase releases or absorbs energy, and uplift mechanisms (convection, frontal, orographic) are required to cool air sufficiently to cause condensation and precipitation.

Section 4: The Human Impact on Climate (2.4)

Human activities have significantly impacted atmospheric processes at both the global scale (climate change) and the local scale (urban environments).

4.1 The Enhanced Greenhouse Effect and Global Warming

The Greenhouse Effect is a natural process where certain gases trap LWR radiated by the Earth, keeping the planet warm enough to support life.

The Enhanced Greenhouse Effect is the strengthening of this natural process due to human activities.

Causes and Evidence
  • Possible Causes: Primarily the burning of fossil fuels (releasing stored carbon dioxide, CO2) and deforestation (reducing the amount of CO2 absorbed by plants). Other gases include methane (CH4) and nitrous oxides.
  • Evidence of Global Warming: Rising global average temperatures, observed changes in precipitation patterns, melting glaciers and ice sheets, and rising sea levels.
Atmospheric Impacts
  • Increased frequency and intensity of extreme weather events (e.g., severe heatwaves, intense tropical storms).
  • Changes in global wind and pressure patterns (affecting distribution of rainfall).

4.2 Case Study: The Urban Area Climate

Urbanisation drastically modifies the local environment, creating a distinct microclimate known as the Urban Heat Island (UHI) effect, alongside changes in humidity, precipitation, and winds.

Temperature (Heat Island)
  • Effect: Urban areas are significantly warmer (especially at night) than surrounding rural areas.
  • Why?
    • Materials: Concrete and asphalt absorb and store more heat during the day than vegetation (low albedo surfaces).
    • Anthropogenic Heat: Heat generated by human activity (cars, air conditioners, factories).
    • Reduced LHT: Lack of vegetation means less evapotranspiration, reducing cooling.
Humidity
  • Urban humidity is generally lower because rainwater runs off quickly (drains and sewers), reducing surface water available for evaporation.
  • Exception: High humidity can occur immediately downwind of industrial cooling towers or large water bodies.
Precipitation
  • Urban areas typically experience higher precipitation and more storm events than rural surroundings.
  • Why? The UHI creates strong thermal convection (air rises readily), and the increased dust/pollution provides more condensation nuclei (tiny particles for water droplets to form upon).
Winds
  • Average wind speeds are generally lower within the urban canyon (streets) due to the frictional drag caused by tall, uneven buildings.
  • However, localised, high-speed winds can occur: the venturi effect (wind squeezed between tall buildings accelerates rapidly).
Memory Aid: Urban Climate Effects (U-H-P-W)

Think of Urban climates as generally having higher **T**emperature and **P**recipitation, but lower **H**umidity and slower **W**inds (apart from localized gusts).

Key Takeaway for Human Impact: At the global scale, human activity enhances the greenhouse effect, leading to warming. At the local scale, urbanisation creates a unique climate characterised by the Urban Heat Island and modified moisture and wind patterns.

Chapter Summary: What You Must Remember

The chapter on Atmosphere and Weather revolves around the movement of energy and water:

  • Energy Balance: Know the components (SWR, LWR, SHT, LHT, Albedo) and how they vary over a day (Diurnal) and across the globe (Global).
  • Global Transfers: Understand that wind belts and ocean currents balance tropical energy excesses with polar deficits.
  • Moisture Processes: Be able to define the six phase changes (evaporation, condensation, etc.) and explain that condensation requires cooling.
  • Uplift: Know the three main uplift mechanisms (Convectional, Frontal, Orographic) that cause large-scale precipitation.
  • Human Effects: Understand how human pollution drives global warming and how the physical structure of a city (materials, shape) creates the UHI effect.