Atmospheric processes and circulation, the weather systems they produce, urban climates, and the causes, evidence and management of climate change.

What this dot point is asking

WJEC wants you to explain the atmospheric processes that drive weather and climate, from the global energy budget and circulation to the mid-latitude and tropical weather systems they produce, account for distinctive urban climates, and assess the causes, evidence and management of climate change. Strong answers sequence processes precisely (for example the passage of a depression), distinguish weather from climate, and reach a balanced judgement on climate change grounded in evidence.

The answer

The energy budget and atmospheric circulation

Because the Earth is a sphere, insolation is concentrated near the equator and spread thinly at the poles, creating the latitudinal energy imbalance that powers all weather. The atmosphere redistributes this heat through a three-cell circulation in each hemisphere: the Hadley cell (rising air and rain at the equator, sinking dry air at about $30^\circ$ creating the desert belt), the Ferrel cell in the mid-latitudes, and the Polar cell. Where the cold polar air of the Polar cell meets the warmer air of the Ferrel cell, the polar front forms, and it is along this front that mid-latitude depressions are born, which is why Britain's weather is so changeable.

Weather systems

A depression (mid-latitude cyclone) is a low-pressure system that forms at the polar front where warm and cold air meet. As it passes over Britain it brings a sequence of weather: thickening cloud and steady rain at the warm front, milder air in the warm sector, then heavy showery rain at the cold front as cold air undercuts the warm air, followed by cooler, brighter conditions. An anticyclone is a high-pressure system of sinking, stable air giving settled weather, hot and clear in summer, cold, frosty or foggy in winter. Tropical weather systems, including tropical cyclones, form over warm oceans and bring intense rainfall and wind.

Urban climates and climate change

Cities create their own climate. The urban heat island makes built-up areas several degrees warmer than the surrounding countryside, because dark surfaces absorb heat, buildings release stored and waste heat, and there is little vegetation to cool the air by evapotranspiration. Cities also alter rainfall and air quality, with higher concentrations of particulates and pollutants. At the global scale, climate change is driven by the enhanced greenhouse effect: rising concentrations of carbon dioxide and methane from fossil fuels and land-use change trap more outgoing longwave radiation and warm the lower atmosphere.

Examples in context

Example 1. The passage of an Atlantic depression over Wales. Mid-latitude depressions form along the polar front in the North Atlantic and track north-eastwards across the UK, giving Wales its famously changeable, wet weather. As one passes, Cardiff and the Welsh coast typically experience falling pressure, thickening cloud and steady rain at the warm front, a milder cloudy warm sector, then a band of heavy rain and gusty winds at the cold front, clearing to bright, blustery showers behind. This sequence is the standard UK example for explaining how a depression produces an ordered set of weather changes from a single low-pressure system.

Example 2. The evidence and management of global climate change. Global mean temperature has risen by roughly $1.1\,^\circ\text{C}$ since pre-industrial times, recorded by instruments and confirmed by ice cores, tree rings, retreating glaciers and rising sea level. The dominant cause is the enhanced greenhouse effect from fossil-fuel emissions and deforestation. Management combines mitigation, cutting emissions through renewable energy, carbon pricing and international agreements such as the Paris Agreement, with adaptation, including flood defences and drought-resistant crops. This case illustrates both the strength of the evidence for human-caused warming and the dual strategy needed to respond to it.

Try this

Q1. Define the term urban heat island. [2 marks]

• Cue. The tendency for a built-up urban area to be several degrees warmer than the surrounding rural area, owing to heat absorption by surfaces, waste heat and limited vegetation.

Q2. Explain why air sinks at about $30^\circ$ latitude in the Hadley cell. [3 marks]

• Cue. Air rises and loses moisture at the equator, then moves polewards aloft and cools; at around $30^\circ$ it becomes dense and sinks, warming and drying as it descends, which suppresses cloud and creates the desert belt.