Welcome to A1: Atmosphere and Weather Systems!
Hello Geographers! This chapter, Atmosphere and Weather Systems, is vital because the atmosphere is the engine that drives all weather, climate, and many of the hazards we face on our Contested Planet. Understanding how energy moves and air circulates helps us predict storms, manage resources, and address global issues like climate change.
Don't worry if terms like 'Coriolis Effect' sound intimidating—we're going to break them down into easy, bite-sized pieces. Let's explore the air around us!
Quick Review: Why is this important?
The atmosphere controls the distribution of heat and moisture globally. This dictates where people can live, what crops they can grow, and the frequency of damaging weather events, making it a central theme in global development and resource security.
Section 1: The Structure and Composition of the Atmosphere
The atmosphere is a thin layer of gases surrounding Earth, held in place by gravity. It protects us from solar radiation and helps maintain a stable temperature.
Composition of the Atmosphere (Simplified)
The air we breathe is mostly:
- Nitrogen (N\(_2\)): About 78%
- Oxygen (O\(_2\)): About 21%
- Argon and other gases: About 1%
The Layers of the Atmosphere
Imagine the atmosphere as a giant, invisible onion, with distinct layers defined by how temperature changes with altitude. We focus mainly on the bottom two:
- The Troposphere
- The Stratosphere
- (Above these are the Mesosphere and Thermosphere, but the Troposphere and Stratosphere are most relevant to A-Level weather systems.)
- This is the lowest layer, extending about 8 to 15 km up.
- All weather happens here.
- Temperature generally decreases with height—this is known as the Environmental Lapse Rate (ELR).
- Analogy: If you hike up a tall mountain, the air gets colder; you are experiencing the lapse rate.
- Extends up to about 50 km.
- Temperature generally increases with height (due to the absorption of UV radiation by the Ozone Layer).
- The air here is very stable, which is why commercial planes often fly here to avoid turbulence.
Quick Review: The Lapse Rate
The Lapse Rate describes the rate at which temperature drops with increasing altitude. On average, this is about 6.5°C per 1,000 metres in the Troposphere. This drop is fundamental to cloud and rain formation.
Section 2: The Global Energy Balance and Heat Transfer
The Earth’s temperature is remarkably stable because the amount of energy coming in (from the Sun) generally balances the amount of energy going out (radiated from the Earth). This is the Global Energy Balance.
2.1 Incoming Solar Radiation (Insolation)
Insolation is the solar energy reaching the Earth. Not all of it makes it to the surface:
- Reflection: Roughly 30% of incoming energy is immediately reflected back to space.
- Absorption: The remaining energy is absorbed by the atmosphere, clouds, and the Earth's surface.
What is Albedo?
Albedo is the measure of how reflective a surface is. It is expressed as a percentage:
- High Albedo (e.g., fresh snow or thick clouds) reflects lots of energy (up to 90%).
- Low Albedo (e.g., dark oceans, forests, or asphalt roads) absorbs lots of energy.
2.2 Outgoing Terrestrial Radiation (The Greenhouse Effect)
The Earth heats up and radiates energy back into space as long-wave radiation (heat). This is different from the Sun’s short-wave radiation.
Certain gases (like water vapour, CO\(_2\), and methane)—known as Greenhouse Gases (GHGs)—trap some of this outgoing long-wave heat, warming the lower atmosphere. This natural process is the Greenhouse Effect, which keeps the planet habitable.
2.3 Latitudinal Energy Imbalance
The global energy balance is true for the planet as a whole, but not for every location:
- Equator/Tropics: Receive a net energy surplus (more energy comes in than goes out) because the sun's rays are concentrated (high angle).
- Poles: Experience a net energy deficit (more energy goes out than comes in) because the sun's rays are spread out (low angle).
Key Takeaway: Without mechanisms to transfer heat from the tropics to the poles, the tropics would continue to heat up indefinitely, and the poles would freeze solid.
2.4 Mechanisms of Heat Transfer
The atmosphere and oceans work together to transfer heat poleward.
1. Atmospheric Transfer:
- Advection: Horizontal movement of air (wind) carrying heat.
- Convection: Vertical movement of air. Hot air rises, carrying heat energy upwards and polewards via the global circulation cells (discussed next!).
- Warm ocean currents (like the Gulf Stream) move vast amounts of tropical heat toward the mid and high latitudes.
Section 3: Global Atmospheric Circulation
The uneven heating of the Earth drives large-scale, continuous movements of air called the Global Atmospheric Circulation. This system of wind and pressure belts transfers heat and moisture across the globe.
3.1 Pressure Belts and Thermal vs. Dynamic Forces
Air moves from areas of High Pressure (H) to areas of Low Pressure (L).
- Low Pressure: Air rises (as it is heated). Rising air cools, condenses, and usually leads to clouds and rain (e.g., the Equator).
- High Pressure: Air sinks (as it cools). Sinking air warms and becomes stable, resulting in clear skies and dry conditions (e.g., deserts).
Pressure belts are either thermally (caused by temperature) or dynamically (caused by movement/sinking air) driven.
3.2 The Three-Cell Model
This simplified model shows three major circulation cells in each hemisphere:
1. The Hadley Cell (0° to 30° Latitude)
- Driven by intense heating at the Equator (thermal).
- Air rises at the Equator, creating the Inter-Tropical Convergence Zone (ITCZ)—a low-pressure belt with heavy rainfall.
- Air flows polewards high in the atmosphere, cools, and sinks around 30° N/S.
- Sinking air creates the high-pressure belts known as the Subtropical Highs (where the world's major deserts are found).
2. The Ferrel Cell (30° to 60° Latitude)
- This cell is dynamically driven (it acts like a gear, pushed by the Hadley and Polar cells).
- Surface winds here flow polewards and meet cold air from the Polar cell around 60° N/S.
3. The Polar Cell (60° to 90° Latitude)
- Driven by extreme cold at the Poles (thermal).
- Cold, dense air sinks at the Pole (Polar High), flows towards 60° N/S, and rises again when it meets warmer air from the Ferrel cell (Polar Front/Subpolar Low).
3.3 The Coriolis Effect and Global Winds
If the Earth didn't spin, wind would blow straight from High pressure to Low pressure. However, the Earth’s rotation deflects winds: this is the Coriolis Effect.
The Rule:
- In the Northern Hemisphere, winds are deflected to the right.
- In the Southern Hemisphere, winds are deflected to the left.
This deflection creates the main global wind belts:
1. Trade Winds: Blow from the Subtropical Highs back towards the ITCZ (Equator).
2. Westerlies: Blow from the Subtropical Highs towards the poles (affecting mid-latitudes like Europe and North America).
3.4 Jet Streams
These are fast-flowing 'rivers' of air (up to 300 km/h) located high in the Troposphere.
- They form along the boundaries between the circulation cells where temperature contrast is greatest (e.g., the Polar Front Jet Stream).
- Jet streams steer mid-latitude weather systems (depressions) and are key to understanding short-term weather forecasting.
Section 4: Mid-Latitude Weather Systems
The mid-latitudes (30° to 60°) are highly contested areas due to the frequent clash of warm, tropical air and cold, polar air, creating dynamic and often unstable weather.
4.1 Mid-Latitude Depressions (Low Pressure Systems)
These are also called extratropical cyclones or 'lows'. They are responsible for most of the wet, windy weather in the mid-latitudes.
Formation (Simplified Polar Front Theory)
1. A boundary exists between cold polar air and warm tropical air (the Polar Front). 2. A ripple or wave forms along this boundary. 3. Low pressure develops as warm air is forced to rise over the cold air. 4. The system rotates (anti-clockwise in the N. Hemisphere) due to the Coriolis effect, developing distinct Fronts.
The Role of Fronts
Fronts are boundaries where two air masses meet:
- Warm Front: Warm air gently slides up and over the colder, denser air mass. This results in broad areas of light, continuous rain or drizzle, often preceded by high clouds (cirrus).
- Cold Front: Cold air is denser and wedges aggressively underneath the warm air, forcing it rapidly upwards. This leads to intense rainfall, heavy showers, thunderstorms, and a rapid drop in temperature after the front passes.
- Occluded Front: Occurs in the later stages of the depression when the faster-moving cold front catches up to the warm front, lifting the warm air mass entirely off the ground. The weather becomes complex but usually colder and showery.
Did You Know?
The process of a low-pressure system forming, developing, and eventually dying out is called Frontal Depressions or Cyclogenesis.
4.2 Anticyclones (High Pressure Systems)
These are areas where air is slowly sinking (subsidence).
- Sinking air warms up and dries out, preventing cloud formation.
- They are large, slow-moving, and bring stable weather.
Associated Weather:
- Summer: Clear skies, intense sunshine, high temperatures, often leading to drought or heatwaves.
- Winter: Clear skies, but extreme cold overnight (due to heat radiating easily), often resulting in frost, fog, or pollution build-up (smog).
Common Mistake to Avoid:
Do not confuse the direction of spin! Depressions (Lows) spin anti-clockwise in the Northern Hemisphere. Anticyclones (Highs) spin clockwise in the Northern Hemisphere. This is reversed in the Southern Hemisphere.
Section 5: Tropical Weather Systems – Tropical Revolving Storms (TRS)
Tropical storms, known by different regional names (Hurricanes, Typhoons, Cyclones), are among the most powerful weather hazards on Earth, posing massive threats to coastal communities.
5.1 Conditions Required for TRS Formation
A tropical revolving storm needs five critical ingredients to form:
- Warm Sea Water: Ocean temperatures must be at least 26.5°C down to a depth of 50 metres. This provides the massive energy (latent heat) needed for the storm.
- Low Wind Shear: Winds must not change speed or direction significantly with height. High wind shear rips the storm apart.
- Rapid Evaporation/High Humidity: Abundant moisture in the air.
- Low Pressure Disturbance: A starting point, like a cluster of thunderstorms.
- Coriolis Effect: They must form far enough from the Equator (usually 5° to 20° latitude) for the Coriolis force to be strong enough to initiate rotation. (They cannot form exactly on the Equator).
5.2 Structure and Characteristics of a TRS
A mature TRS is a massive system (200–700 km wide) with extremely low central pressure.
- The Eye: The calm, clear, low-pressure centre (10–50 km wide). Air is sinking here, warming up, so there are no clouds or rain.
- The Eyewall: A ring immediately surrounding the eye. This is where the fastest winds, highest rainfall, and most intense storm activity occur (rising air).
- Spiral Rain Bands: Bands of thunderstorms spiralling outwards from the centre, bringing heavy rain and gusts of wind.
5.3 Hazards Associated with Tropical Storms
TRS systems generate multiple, devastating hazards:
- High Winds: Can reach speeds over 250 km/h, causing structural damage and destroying infrastructure (e.g., electricity lines).
- Intense Rainfall: Leads to widespread flash flooding and river flooding, causing landslides and contaminating water supplies.
- Storm Surge: The most dangerous hazard. It is a temporary rise in sea level caused by the combination of the low central pressure and strong onshore winds pushing water towards the coast.
Impact on the Contested Planet:
The hazards of TRS systems disproportionately affect vulnerable, low-lying coastal populations, requiring massive international aid efforts and challenging governments' ability to provide infrastructure and safety. This constant threat dictates migration patterns, coastal defence spending, and resource allocation.
Final Encouragement
You’ve covered the entire workings of the atmosphere, from energy coming in to massive storms spinning out! Remember that global circulation is simply the planet trying to balance its heat budget. If you understand the flow (rise -> cool -> rain; sink -> warm -> dry), the complex systems will make sense. Keep reviewing the diagrams of the three-cell model and the structure of fronts—you've got this!