Diurnal Energy Budget and Urban Climates
Diurnal Energy Budget
Definition: The balance between incoming solar radiation and outgoing terrestrial radiation during a 24-hour period.
Incoming Solar Radiation (Insolation):
During the day, the Earth receives energy from the Sun.
This energy warms the surface.
Known as solar radiation or insolation.
Outgoing Radiation:
At night, the Earth loses heat to the atmosphere and space through radiation (mostly infrared).
The amount of heat lost depends on cloud cover, humidity, and the Earth's surface.
Key Points:
The energy budget regulates temperature.
At night, temperatures decrease because no new solar energy is received, but the Earth continues to lose heat.
Albedo
Definition: The reflectivity of a surface, measuring how much sunlight is reflected rather than absorbed.
High Albedo Surfaces:
Light-colored surfaces (like ice and snow) have high albedo.
Reflect most of the sunlight.
Result in lower temperatures.
Low Albedo Surfaces:
Dark-colored surfaces (like forests or oceans) have low albedo.
Absorb more sunlight.
Warm the surface.
Why it Matters:
Albedo helps control the Earth's temperature.
Areas with high albedo (e.g., polar regions) reflect most solar radiation.
Areas with low albedo (e.g., deserts) absorb more solar radiation, contributing to higher temperatures.
Latitudinal Pattern of Radiation
Excess Radiation:
The Sun's energy is most intense at the equator due to direct overhead sunlight.
Leads to an energy excess in the tropics, creating warm temperatures.
Deficit Radiation:
Near the poles, sunlight is spread over a larger area and is less intense.
Creates an energy deficit in polar regions, leading to colder temperatures.
Why it Matters:
These differences create temperature gradients.
Influence global circulation patterns, including wind and ocean currents.
Need for Atmospheric Transfers
The Earth needs to transfer energy from areas of excess (equator) to areas of deficit (poles).
These transfers are carried out by:
Atmospheric Circulation:
Warm air rises at the equator and moves toward the poles.
Cooler air from the poles moves toward the equator, creating global wind patterns.
Ocean Currents:
Oceans also help transfer heat.
Warm water moves from the equator towards the poles.
Cold water moves from the poles toward the equator.
Result: This global circulation helps balance the Earth's temperature.
Pressure Variations
Definition: Air pressure is the weight of the atmosphere pressing down on Earth's surface.
High Pressure Areas:
Regions where air is sinking.
Create clear skies and calm weather.
Low Pressure Areas:
Occur where air is rising.
Lead to cloud formation and stormy weather.
Pressure Differences:
Result of temperature variations.
Warm air is less dense and creates low pressure.
Cold air is denser, creating high pressure.
Surface Pressure Belts
Equatorial Low Pressure Belt:
At the equator, the sun heats the air, causing it to rise and create low pressure.
Known as the Intertropical Convergence Zone (ITCZ).
Subtropical High Pressure Belt:
Around 30° north and south of the equator, air that rises at the equator cools and sinks, creating high pressure areas.
Subpolar Low Pressure Belt:
At about 60° latitude, warm air from the subtropics meets cold air from the poles, creating low pressure areas.
Polar High Pressure:
At the poles, cold air sinks, creating high pressure.
Surface Wind Belts
Trade Winds:
Blow from the subtropical high-pressure areas towards the equator, in both the Northern and Southern Hemispheres.
Deflected by the Earth's rotation (Coriolis effect).
Westerlies:
In the middle latitudes (30°–60°), winds blow from the west towards the east.
Driven by pressure differences between high-pressure belts and low-pressure areas.
Polar Easterlies:
Near the poles, winds blow from the east to the west.
Explaining Variations in Temperature, Pressure, and Winds
Temperature:
Varies with latitude.
The equator receives more direct sunlight, leading to higher temperatures.
The poles receive less sunlight, leading to cooler temperatures.
Pressure:
Warm air rises at the equator, creating low pressure.
Cooler air sinks at the poles, creating high pressure.
These differences drive winds.
Winds:
Winds blow from high-pressure areas to low-pressure areas due to pressure differences.
The Earth's rotation (Coriolis effect) deflects winds, causing them to curve rather than flow in a straight line.
Factors Affecting Air Movements
Pressure Gradients:
Air moves from areas of high pressure to low pressure.
The stronger the difference, the stronger the wind.
Coriolis Effect:
Due to Earth's rotation, moving air is deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Friction:
The Earth's surface (mountains, cities, forests) slows down wind speeds, especially near the surface.
Temperature Differences:
Warm air rises, and cold air sinks.
These temperature differences help drive atmospheric circulation.
General Circulation Model (GCM)
Definition: A simplified model that explains the movement of air around the globe based on temperature, pressure, and wind patterns.
Hadley Cell:
Warm air at the equator rises and moves toward the poles.
As it cools, it sinks around 30° latitude, creating the trade winds and subtropical high-pressure areas.
Ferrel Cell:
In the middle latitudes (30°–60°), air is forced to rise at low-pressure areas and sinks at high-pressure areas.
This is where the westerlies occur.
Polar Cell:
Cold air sinks at the poles and moves toward the equator, creating the polar easterlies.
Coriolis Effect:
This model also accounts for the Coriolis effect, which makes winds curve as they move across the Earth's surface.
Cambridge A Level Geography - 15 Mark Essay
Examiners look for:
Clear understanding of the topic
Accurate knowledge of urban climate effects due to human activity (e.g. urban heat island, air pollution, changes in wind, rainfall, humidity).
Explain why temperature may be significant, but also compare it to other effects.
Balanced argument
Consider other effects besides temperature (like wind patterns, rainfall, air pollution).
Evaluate, not just explain.
A balanced conclusion is essential (e.g. “While temperature is important, other factors such as rainfall and pollution may be more significant in some cities… ”)
Use of examples
At least one example is required.
Specific named examples of cities (e.g. London, Tokyo, Mumbai, New York) will strengthen your answer.
Use data or case study information when possible (e.g. "In central London, temperatures can be 5°C higher than surrounding rural areas").
Structure and clarity
A clear introduction, paragraphs with topic sentences, and a conclusion.
Logical flow: don’t jump randomly between points.
Model Essay Structure:
Introduction (2–3 sentences)
Briefly define urban climates.
Acknowledge that temperature is a key impact (urban heat island).
State that you will evaluate this in relation to other effects.
Example: Urban climates refer to the climatic characteristics that are different in urban areas compared to surrounding rural areas. Human activity has a strong impact, especially through the creation of urban heat islands which increase temperatures. However, other changes like altered rainfall patterns, pollution levels, and reduced wind speeds must also be considered when assessing significance.
Main Body (Organised into clear paragraphs)
Paragraph 1: Human activity and temperature (urban heat island)
Explain why temperature is affected by urban surfaces, buildings, reduced vegetation.
Use an example: “In Tokyo, summer temperatures can be 7°C higher in the city centre than in nearby rural areas due to extensive concrete and low albedo surfaces.”
Mention the impacts: heat stress, energy demand for cooling, health risks.
Paragraph 2: Human activity and rainfall/humidity
Urban areas increase convection due to rising heat — can cause more thunderstorms and higher rainfall.
Mention particulate matter (pollution) helping condensation and rainfall.
Example: “In Manchester, rainfall is 5–10% higher than surrounding areas due to urban influence.”
Paragraph 3: Human activity and air quality
Increased pollution levels: smog, temperature inversions trap pollutants.
Discuss health effects: respiratory problems, reduced visibility.
Example: “In Delhi, high pollution levels often exceed WHO limits, leading to severe health warnings.”
Paragraph 4: Human activity and wind patterns
Tall buildings disrupt wind — leads to urban canyons, turbulence, or wind shadows.
Can reduce natural cooling and worsen heat/pollution effects.
Conclusion (Strong evaluative judgement)
Return to the question: Is temperature the most significant?
Make a judgement: It often is, but other impacts are also very important, depending on the city and the context.
Example: Overall, while increased temperature due to urban heat islands is a very visible and serious effect of human activity on urban climates, other impacts such as altered rainfall patterns, poor air quality, and wind disruptions are also significant. In many cities, pollution-related health risks may even outweigh temperature impacts. Therefore, temperature is one of the most important effects, but not always the most significant in all urban areas.
Tips to Get 15/15
✅ Directly answer the question ("how far do you agree?")
✅ Show depth and range of knowledge
✅ Give clear examples with specific detail
✅ Evaluate, don’t just describe
✅ Use topic sentences to guide your answer
✅ Write a balanced, justified conclusion
Example 15-mark Answer to the question: “The most significant effect of human activity on urban climates is on temperature.” With the aid of one or more examples, how far do you agree with this statement?
Introduction
Urban climates are local climate conditions that are altered due to human activities in cities. One major impact is the increase in temperature, known as the urban heat island (UHI) effect. However, urban areas also experience changes in rainfall, wind, and air quality, which may be equally or more significant depending on the city. This essay will assess the extent to which temperature is the most significant effect of human activity on urban climates.
Paragraph 1: Human activity and increased temperature (Urban Heat Island)
Human activity strongly increases temperature in urban areas due to factors such as dense buildings, concrete surfaces, low vegetation, and vehicle emissions. These surfaces absorb more heat during the day and release it slowly at night, causing cities to remain warmer than rural areas. This is known as the urban heat island effect. For example, in central London, urban temperatures can be up to 5°C higher than surrounding rural zones. This can cause increased energy demand for air conditioning, heat stress, and greater health risks, especially for the elderly and vulnerable populations. Therefore, the temperature change is a significant and easily measured impact of human activity.
Paragraph 2: Human activity and rainfall patterns
However, urban climates are also affected in other ways. The heat generated by cities causes more rising air and convection, which can lead to increased cloud formation and local rainfall, especially in summer. Cities also release large amounts of dust and pollutants, which act as condensation nuclei, helping raindrops form. For instance, in Manchester, UK, urban areas receive around 5–10% more rainfall than surrounding areas. This can lead to more frequent storms and localised flooding. In some cities, changes in rainfall may cause more damage than higher temperatures, especially when infrastructure cannot cope.
Paragraph 3: Human activity and air quality
Another major impact of human activity is the increase in air pollution in urban areas. Traffic, industry, and energy production release pollutants such as nitrogen dioxide and fine particles (PM2.5), which reduce air quality. In cities like Delhi, pollution levels often exceed World Health Organization (WHO) limits. This causes serious health problems, including asthma, lung disease, and heart issues. During temperature inversions, cold air is trapped under a layer of warm air, preventing pollutants from escaping, making the situation worse. In cities like this, air pollution may have a greater direct impact on human health than higher temperatures.
Paragraph 4: Human activity and wind patterns
Tall buildings in urban areas can also affect wind patterns. They block wind and create areas of low air movement, which prevents heat and pollutants from being dispersed. This effect is known as the canyon effect. It can make urban areas feel more uncomfortable and can increase the impact of heat and pollution. In contrast, in some cities, buildings can funnel wind, creating dangerous wind gusts. Therefore, changes to wind patterns are another important impact of urban development.
Conclusion
In conclusion, temperature is a very significant effect of human activity on urban climates, especially because it affects energy demand, comfort, and health. However, other impacts like air pollution, rainfall increases, and changes to wind patterns are also very important and, in some cities, may have more direct and harmful effects on the population. Therefore, while temperature is one of the most important effects, it is not always the most significant. The answer depends on which city is being studied and what effects are most dangerous or costly in that context.
✅ Why this is a full-mark answer:
✔ Shows strong knowledge of urban climate processes.
✔ Discusses temperature first, then other effects.
✔ Uses named examples with specific data (London, Manchester, Delhi).
✔ Provides a balanced evaluation.
✔ Ends with a clear, justified conclusion.