Unit AS 1: Physical Geography — Section 3: Atmosphere and Weather
Welcome to The Processes That Shape Our Weather and Climate! Weather and climate affect every single aspect of our lives, from what we wear in the morning to global agricultural patterns. In this section of your CCEA AS Level course, we will explore the fundamental physical engines of our planet: how energy arrives from the Sun, moves around the Earth, forms clouds and rain, and brings diverse weather systems across the British Isles.
Don't worry if atmospheric physics feels a little intimidating at first! We will break every concept down into bite-sized, logical steps with simple analogies and memory tricks to help you succeed in your AS 1 exam.
---1. The Global Energy Balance and Atmospheric Structure
To understand weather, we must first understand the stage where it all happens: Earth's atmosphere and its incoming energy.
The Structure of the Atmosphere
The atmosphere is a layered envelope of gases surrounding the Earth. Moving upward from the ground, the four primary layers are:
• Troposphere: The lowest layer (from ground level up to roughly \(10\text{--}15\text{ km}\)). This is where virtually all weather occurs, containing most of the water vapour and atmospheric mass.
• Stratosphere: The layer above the troposphere, containing the ozone layer which absorbs harmful ultraviolet radiation.
• Mesosphere: The cold middle layer where temperatures decrease with altitude.
• Thermosphere: The uppermost layer, where temperatures rise due to absorption of intense solar radiation.
Quick Mnemonic to Remember the Order (Lowest to Highest):
"Today's Sun Makes Temperature" (\(\mathbf{T}\)roposphere \(\to\) \(\mathbf{S}\)tratosphere \(\to\) \(\mathbf{Mesosphere}\) \(\to\) \(\mathbf{T}\)hermosphere).
Solar vs. Terrestrial Radiation
Energy drives our entire climate system, and it comes in two distinct forms:
• Solar Radiation (Insolation): Short-wave radiation emitted by the Sun. Because the Sun is extremely hot, its energy travels as high-energy, short wavelengths that pass relatively easily through atmospheric gases.
• Terrestrial Radiation: Long-wave radiation emitted by the Earth. The Earth absorbs short-wave insolation, warms up, and radiates that heat back out toward space at much cooler temperatures as low-energy, long-wave radiation.
The Greenhouse Effect
The natural greenhouse effect is essential for life on Earth. Naturally occurring greenhouse gases in the troposphere—primarily water vapour (\(\text{H}_2\text{O}\)), carbon dioxide (\(\text{CO}_2\)), and methane (\(\text{CH}_4\))—allow short-wave solar radiation to pass through to the ground, but absorb outgoing long-wave terrestrial radiation. These gases then re-radiate heat back toward the Earth's surface (a process called counter-radiation), keeping our planet habitable.
Albedo: Surface Reflectivity
Albedo is the measure of the reflectivity of a surface, expressed as the percentage of incoming solar radiation that is reflected back into space without being absorbed.
• Fresh Snow: \(80\%\text{ to }90\%\) (Very high albedo — reflects most sunlight)
• Forests: \(10\%\text{ to }20\%\) (Low albedo — absorbs most sunlight)
• Dark Soil: \(10\%\) (Very low albedo — absorbs nearly all sunlight)
The Global Energy Balance Equation
The Earth maintains a balance between incoming and outgoing energy, which can be expressed through the net radiation formula:
\(\text{Net Radiation} = (\text{Insolation} - \text{Reflection}) - (\text{Terrestrial Radiation} - \text{Counter-radiation})\)
Common Exam Pitfall: Students often write that the atmosphere is heated directly from above by sunlight. Remember: The air in the troposphere is primarily heated from below by long-wave terrestrial radiation emitted by the Earth's surface!
Key Takeaway for Section 1: Incoming solar energy arrives as short-wave radiation, is absorbed by the surface, and is re-emitted as long-wave terrestrial radiation, which heats the troposphere from the ground up.
---2. Heat Transfers and Global Atmospheric Circulation
The Equator receives intense, direct insolation all year, creating a net energy surplus. The Poles receive slanted, dispersed insolation and have high albedo, resulting in a net energy deficit. To stop the Equator from overheating and the Poles from freezing completely, the planet constantly transfers heat from the tropics toward the poles.
Mechanisms of Heat Transfer
• Vertical Transfers:
1. Conduction: Direct heat transfer via physical contact between the warm ground and the lowest air molecules.
2. Convection: Warm, less dense air rises, carrying heat upward into the troposphere.
3. Latent Heat Transfer: Energy absorbed during evaporation at the surface is transported upwards and released as heat when water vapour condenses to form clouds.
• Horizontal Transfers (Advection):
1. Winds: Large-scale atmospheric movements carrying warm air poleward and cold air equatorward.
2. Ocean Currents: Surface and deep ocean currents redistributing thermal energy globally.
The Tri-Cellular Model and Global Pressure Belts
Earth's global atmospheric circulation is organized into three distinct cells in each hemisphere:
1. The Hadley Cell (0° to 30° N/S):
• Intense solar heating at the Equator causes warm air to expand and rise vigorously, creating a band of low pressure called the Inter-Tropical Convergence Zone (ITCZ).
• As this air rises, it cools, forms towering clouds, and produces heavy tropical rainfall.
• In the upper troposphere, this air flows poleward, cooling and becoming denser until it sinks around \(30^\circ\text{N}\) and \(30^\circ\text{S}\), creating the Subtropical High-Pressure Belts (regions of dry, clear skies where major deserts are located).
• Air then returns to the Equator at ground level as the trade winds.
2. The Polar Cell (60° to 90° N/S):
• Extremely cold, dense air sinks over the Poles (\(90^\circ\text{N/S}\)), producing Polar High Pressure.
• This cold air flows equatorward at ground level, meeting warmer air around \(60^\circ\text{N/S}\), where it is forced to rise at the Subpolar Low-Pressure Belt.
3. The Ferrel Cell (30° to 60° N/S):
• The mid-latitude cell driven mechanically between the Hadley and Polar cells.
• Air sinking at the Subtropical High (\(30^\circ\)) moves poleward along the surface and is forced upwards at the Subpolar Low (\(60^\circ\)) along the polar front.
The Coriolis Effect
Because the Earth rotates on its axis, moving air and ocean currents are deflected:
• Deflected to the right in the Northern Hemisphere.
• Deflected to the left in the Southern Hemisphere.
Key Takeaway for Section 2: Heat transfers balance the global energy budget. The Tri-Cellular Model (Hadley, Ferrel, Polar cells) combined with the Coriolis effect drives planetary winds and determines global pressure zones (ITCZ, Subtropical Highs, Subpolar Lows, Polar Highs).
---3. Lapse Rates and Atmospheric Stability
Understanding lapse rates is essential for explaining why clouds form on some days while skies remain clear on others.
What is an Adiabatic Process?
An adiabatic temperature change happens entirely due to changes in pressure, without any heat being added or removed from the surrounding air:
• As a parcel of air rises, atmospheric pressure decreases. The parcel expands, and expanding requires energy, which causes the temperature of the parcel to cool.
• When air sinks, pressure increases, the parcel is compressed, and it warms.
Common Exam Pitfall: Rising air does not cool because it is "farther from the hot ground." It cools because lower pressure allows it to expand (adiabatic cooling).
The Three Key Lapse Rates
1. Environmental Lapse Rate (ELR):
• The actual rate at which the surrounding, stationary background air decreases in temperature with altitude.
• Average value: \(6.5^\circ\text{C}\text{ per }1000\text{ m}\) (or \(0.65^\circ\text{C}\text{ per }100\text{ m}\)).
2. Dry Adiabatic Lapse Rate (DALR):
• The rate at which an unsaturated parcel of rising air cools as it ascends.
• Fixed value: \(9.8^\circ\text{C}\text{ per }1000\text{ m}\) (often rounded to \(10^\circ\text{C}\text{ per }1000\text{ m}\)).
3. Saturated Adiabatic Lapse Rate (SALR):
• The rate at which a saturated parcel of rising air (at \(100\%\) relative humidity, where condensation is actively occurring) cools.
• Variable value: Typically \(4^\circ\text{C}\text{ to }9^\circ\text{C}\text{ per }1000\text{ m}\).
• Why is the SALR slower than the DALR? Because as water vapour condenses into liquid droplets, it releases latent heat into the parcel, partially offsetting the cooling effect of expansion.
Atmospheric Stability States
To determine if the air is stable or unstable, compare the ELR of the surrounding atmosphere to the cooling rates of the rising parcel (DALR and SALR):
1. Absolute Stability (\(\text{ELR} < \text{SALR}\)):
• The environmental air cools very slowly with height.
• Any rising parcel (whether dry or saturated) will always be cooler and denser than the surrounding air.
• Result: The parcel sinks back down. No vertical cloud growth; calm, clear, or flat stratified conditions.
2. Absolute Instability (\(\text{ELR} > \text{DALR}\)):
• The environmental air cools very rapidly with height.
• Any rising parcel will always be warmer and less dense than the surrounding air.
• Result: The parcel continues to rise freely on its own buoyancy, creating strong vertical convection, towering cumulonimbus clouds, and heavy rain or thunderstorms.
3. Conditional Instability (\(\text{SALR} < \text{ELR} < \text{DALR}\)):
• The state of the atmosphere is "conditional" upon whether the air is dry or saturated.
• The parcel is stable when dry (because \(\text{ELR} < \text{DALR}\)), but becomes unstable if forced to rise high enough to reach saturation/condensation level (because \(\text{ELR} > \text{SALR}\)).
Key Takeaway for Section 3: If a rising parcel is warmer than the surrounding air, it keeps rising (instability). If it is cooler, it sinks back down (stability). Condensation releases latent heat, which slows the rate of cooling from the DALR (\(9.8^\circ\text{C}/1000\text{m}\)) to the SALR (\(4\text{--}9^\circ\text{C}/1000\text{m}\)).
---4. Weather Systems in the British Isles
The weather across the British Isles is famously variable because it sits at the crossroads of several major air masses and atmospheric boundaries.
Air Masses Affecting the British Isles
An air mass is a large body of air with relatively uniform temperature and moisture characteristics acquired from its source region:
• Maritime Tropical (mT): Originates over the warm Atlantic Ocean (Azores). Brings warm, moist air, cloudy skies, mild winters, and humid summer weather with drizzle.
• Maritime Polar (mP): Originates over the cold North Atlantic. Brings cool, moist, unstable air with heavy showers and squalls.
• Continental Tropical (cT): Originates over North Africa and Southern Europe. Brings hot, dry, sunny conditions in summer.
• Continental Polar (cP): Originates over Northern Europe/Siberia. Brings very cold, dry conditions in winter (severe frosts) and warm, dry weather in summer.
• Arctic Maritime (Am): Originates over the Arctic ice cap/Greenland. Brings bitterly cold, direct northerly winds with heavy snow showers in winter.
Mid-Latitude Depressions (Low Pressure Systems)
A depression is an area of low atmospheric pressure formed where warm tropical air (mT) meets cold polar air (mP) along the polar front.
Structure and Weather Sequence of a Depression:
1. Ahead of the Warm Front:
• Pressure falls gradually.
• High, thin clouds appear first (Cirrus), thickening into mid-level Altostratus, and finally low, thick rain-bearing Nimbostratus.
• Prolonged, steady precipitation begins.
2. The Warm Sector:
• The wedge of warm Maritime Tropical air between the warm and cold fronts.
• Temperatures rise, rain eases to drizzle or stops, and skies remain overcast with low stratus clouds.
• Pressure stabilizes.
3. The Cold Front:
• Dense cold air undercuts the warm sector, forcing warm air to rise steeply.
• Large towering Cumulonimbus clouds develop, bringing sudden heavy, intense rain showers and gusty winds.
• Temperatures drop rapidly.
4. Behind the Cold Front (Cold Sector):
• Atmospheric pressure rises rapidly, clouds break up to give sunny intervals, accompanied by scattered showers.
5. Occlusion:
• Because cold fronts move faster than warm fronts, the cold front eventually catches up with the warm front, lifting the entire warm sector off the ground to form an occluded front.
Anticyclones (High Pressure Systems)
An anticyclone is a large system of high atmospheric pressure characterised by gently sinking (subsiding) air.
• General Characteristics: Sinking air warms adiabatically, preventing condensation and cloud formation. Isobars are widely spaced, resulting in light, gentle winds.
• Wind Direction: In the Northern Hemisphere, winds blow clockwise and outward around an anticyclone.
• Summer Anticyclones: Produce long, hot, sunny days with clear blue skies (heatwaves). Nights are mild and clear, though occasional late-afternoon thunderstorms can occur.
• Winter Anticyclones: Produce cold, crisp, sunny days. With no cloud cover at night, rapid terrestrial radiation loss leads to severe overnight frosts, radiation fog, and persistent low stratus known as "anticyclonic gloom".
Quick Summary Comparison Table:
Feature | Depression (Low Pressure) | Anticyclone (High Pressure)
• Air Movement: Rising air (converging at surface) | Sinking/subsiding air (diverging at surface)
• Wind Circulation (N. Hemisphere): Anticlockwise and inward | Clockwise and outward
• Isobar Spacing: Closely packed (strong winds) | Widely spaced (light winds/calm)
• Typical Weather: Cloudy, unsettled, windy, precipitation | Clear skies, dry, settled conditions
Key Takeaway for Section 4: The British Isles' weather is driven by five distinct air masses. Depressions bring low pressure, fronts, changing cloud sequences, and rain; anticyclones bring high pressure, light clockwise winds, and clear, settled conditions (hot in summer, cold and frosty in winter).
---Exam Revision Checklist
Before sitting your AS 1 examination, make sure you can comfortably:
• Define short-wave insolation, long-wave terrestrial radiation, and albedo (with example values).
• Explain the natural greenhouse effect and the vertical layers of the atmosphere.
• Draw and annotate the Tri-Cellular Model (Hadley, Ferrel, Polar cells) and locate key global pressure belts.
• Define and contrast the \(\text{ELR}\) (\(6.5^\circ\text{C}/1000\text{m}\)), \(\text{DALR}\) (\(9.8^\circ\text{C}/1000\text{m}\)), and \(\text{SALR}\) (\(4\text{--}9^\circ\text{C}/1000\text{m}\)).
• Distinguish between absolute stability, absolute instability, and conditional instability.
• Name the 5 air masses affecting the British Isles and describe their source characteristics.
• Describe the cross-section, cloud progression, and weather changes across a mid-latitude depression and compare them to summer and winter anticyclones.