Sources of Energy (AQA A-Level Geography): Revision Notes
Sources of energy
Introduction
Water and wind shape most desert landforms through their continuous action. To understand how desert systems function, we need to examine the energy sources that power these processes. Three key inputs drive hot desert geomorphological systems: insolation, wind, and run-off. These energy sources work together with sediment (primarily sand-sized particles) to create the distinctive features we associate with desert landscapes.
Understanding these three energy sources is fundamental to comprehending how desert systems operate and evolve over time. Each plays a distinct but interconnected role in shaping desert landscapes.
Insolation
What is insolation?
Solar energy from the Sun is a vital input in desert systems. Many processes occurring in hot deserts result from temperature changes and differences, making the Sun's energy crucial for understanding why these environments behave as they do.
The angle of incidence
In low to mid-latitude hot desert environments, the Sun sits almost directly above during the day throughout the year. This means there are approximately 12 hours of daylight year-round. Because the Sun is positioned high in the sky at these latitudes, solar radiation strikes the surface at a steep angle.
[IMAGE
]The angle of incidence refers to the angle at which solar radiation hits the Earth's surface. A high angle of incidence means the Sun's energy is concentrated onto a smaller area, causing more intense heating compared to higher latitudes where the Sun is lower in the sky.
Effects in arid environments
The impact of solar energy differs between moist and arid environments:
-
In moist environments: The Sun's energy evaporates water on and beneath the surface, converting heat energy into latent heat that rises into the atmosphere. This process keeps the ground relatively cooler.
-
In arid environments: The lack of moisture means the ground becomes excessively warm. More energy is available to heat both the ground surface and the air in direct contact with it, creating the characteristically hot conditions.
The concept of latent heat is key to understanding why moist environments stay cooler. When water evaporates, it absorbs heat energy and carries it away into the atmosphere, acting as a natural cooling mechanism that deserts lack.
Winds
Wind as an energy source
Wind serves as an important energy input in hot deserts, acting as both an energy source and a driver of processes within desert systems. The movement of air tends towards achieving dynamic equilibrium.
Geographic context
Because of their mid-latitude location along the sub-tropical high-pressure belt, many large hot desert areas experience localised winds that blow outwards towards the desert's edge. These winds function as agents of erosion and transport.
Examples of local winds
Different desert regions have their own characteristic wind patterns:
-
Kharif: Blows in the Somali-Chalbi desert in the Horn of Africa from June to September
-
Irifi: Easterly winds that blow in the Western Sahara around March
-
Harmattan: Famous winds in the Sahara that blow in winter months from November to March
-
Shamal: Blows over the Arabian Gulf from May to early July
-
Haboobs: More localised winds generated in the Sudanese Sahara
Although desert winds are generally not stronger than winds elsewhere on average, their effects are particularly notable because of the abundance of loose sediment available for transport and the lack of vegetation to slow wind speeds near the ground surface.
Run-off
Characteristics of desert rainfall
Rainfall in deserts is characteristically patchy and unpredictable - often described as 'spotty'. It is frequently very localised and temporally unpredictable. Despite arid and hot desert areas being defined as receiving less than 250 mm of rainfall annually, some hyper-arid areas recording almost no rain can still experience intense storms. These storms can produce huge amounts of rainfall in very localised areas.
Overland flow
Combined with the baked ground and limited vegetation cover, swift surface water movement becomes possible. This overland flow (also called run-off) can be a significant agent of erosion and sediment transportation.
Overland flow refers to the rapid movement of water across the land surface during and after rainfall events. In deserts, the lack of vegetation and hard ground surface means water flows quickly rather than soaking in, giving it considerable erosive power.
Sediment sources, cells and budgets in hot deserts
Sediment dominance
Hot desert landscapes are often dominated by loose sediment. Deserts serve as stores of this sediment, and much of it is transported and deposited within the desert boundary itself, forming what is known as a sediment cell.
Inputs of sediment
There are several origins, or inputs, of sediment into hot deserts:
-
From weathering: Sediment may simply be derived from the weathering of the underlying parent material (the bedrock beneath the desert surface)
-
Fluvial origin: Sediment may be fluvial in origin, meaning rivers bring the sediment into the deserts from surrounding areas
The concept of a sediment cell helps us understand desert systems as self-contained units where sediment is continuously cycled through weathering, transport, and deposition processes. Understanding sediment sources helps explain how desert landforms develop and change over time.
Remember!
Key Points to Remember:
- Three key energy sources drive hot desert systems: insolation, wind, and run-off
- The high angle of incidence in low-mid latitude deserts concentrates solar energy, creating intense surface heating
- In arid environments, lack of moisture means solar energy heats the ground and air directly rather than being absorbed by evaporation
- Localised winds (such as kharif, harmattan, and shamal) act as agents of erosion and transport in desert regions
- Desert rainfall is unpredictable and localised, but intense storms can produce rapid overland flow that significantly erodes and transports sediment