👋 Welcome to Diurnal Energy Budgets!
This chapter is all about how the Earth’s surface gains, loses, and moves energy throughout a single day. Think of it as the planet's daily banking system for heat!
Understanding this is absolutely crucial because these energy transfers drive temperature changes, create wind, fuel the water cycle, and ultimately determine the weather we experience.
Don't worry if the terms seem technical at first. We will break down every concept using simple analogies, making sure you can master this core part of Core Physical Geography!
1. Understanding the Diurnal Energy Budget (The Daily Bank Account)
Diurnal simply means 'daily'. The Diurnal Energy Budget is the balance (or imbalance) of all energy inputs and outputs at the Earth's surface over a 24-hour period.
The Core Concept: Net Radiation (\(Q^*\))
The total available energy at the surface is called Net Radiation (\(Q^*\)). If \(Q^*\) is positive, the surface is heating up. If it’s negative, the surface is cooling down.
The energy budget equation explains where this net energy goes:
$$Q^* = H + L_{E} + G$$
- \(Q^*\): Net Radiation (The total energy available).
- \(H\): Sensible Heat Transfer (Heat you can feel, transferred to the air).
- \(L_{E}\): Latent Heat Transfer (Heat used or released when water changes state, e.g., evaporation).
- \(G\): Ground Heat Transfer (Heat absorbed or lost by the soil/subsurface).
Think of it like this: If you have £100 (\(Q^*\)), you might spend £40 on coffee (\(H\)), £30 on water (\(L_{E}\)), and save £30 (\(G\)). The total energy available must equal the total energy used or stored.
Quick Review: Energy Types
We deal with two main types of radiation in the budget:
- Shortwave Radiation (K): Energy coming from the sun (visible light, UV). It carries huge amounts of energy.
- Longwave Radiation (L): Energy coming from the Earth’s surface and atmosphere (infrared/thermal). This is how the Earth cools down.
Key Takeaway: The diurnal energy budget is a balance equation showing how net radiation (\(Q^*\)) is distributed between heating the air (\(H\)), changing water states (\(L_{E}\)), and heating the ground (\(G\)).
2. Shortwave Radiation: The Input and Loss
This section deals with the energy coming into the system from the sun (Shortwave) and how much of it is immediately bounced back.
2.1 Incoming Solar Radiation (\(K \downarrow\))
This is the primary energy source. It is the amount of shortwave solar radiation that reaches the Earth's surface. This input is entirely dependent on the sun's position.
- It peaks around noon when the sun is highest.
- It is zero at night.
2.2 Reflected Solar Radiation (\(K \uparrow\)) and Albedo
Not all solar energy is absorbed; some is immediately reflected back into space.
Albedo is the key term here. It is the percentage of incoming shortwave radiation that is reflected by a surface.
High Albedo: Highly reflective surfaces (e.g., fresh snow, thick clouds, light-coloured sand). They reflect a large percentage (up to 95%) and absorb little energy.
Low Albedo: Poorly reflective surfaces (e.g., dark asphalt, thick forest canopy, deep water). They absorb a large percentage (as low as 5%) and reflect little.
Analogy: Imagine wearing a black t-shirt (low albedo) or a white t-shirt (high albedo) on a sunny day. The black shirt gets much hotter because it absorbs more shortwave radiation.
⚠️ Common Mistake Alert
Remember, albedo only relates to shortwave (solar) radiation. The colour of a surface doesn't affect how much longwave radiation it emits!
Key Takeaway: Incoming energy comes from the sun (\(K \downarrow\)). How much is actually used depends heavily on the surface’s albedo, which determines how much is reflected (\(K \uparrow\)).
3. Longwave Radiation and Ground Heat (\(L\) and \(G\))
Once the surface absorbs shortwave energy, it heats up and begins to re-radiate energy back out. This is primarily longwave radiation.
3.1 Longwave Radiation Transfer
Outgoing Longwave Radiation (\(L \uparrow\)): The heat emitted by the Earth's surface itself. All objects above absolute zero emit longwave radiation. The hotter the surface, the more it emits.
Incoming Longwave Radiation (\(L \downarrow\)): This is thermal energy emitted back down towards the surface by the atmosphere (specifically clouds, water vapour, and greenhouse gases). This is the key process of the natural greenhouse effect.
3.2 Energy Absorbed into the Surface and Subsurface (\(G\))
Ground Heat Transfer (\(G\)) is the energy used to heat up the soil, rock, or water beneath the surface.
During the Day (Positive \(G\)): Energy flows downwards, heating the subsurface. \(G\) is a *loss* from the immediate surface budget.
During the Night (Negative \(G\)): Energy flows upwards from the warmer subsurface back to the surface, slowing down the surface cooling. \(G\) becomes an *input* to the immediate surface budget.
Analogy: Think of a beach. During the day, the sand gets hot because it absorbs energy (\(G\) is positive). At night, the sand releases that stored heat, keeping the beach air slightly warmer than the air far above it (\(G\) is negative).
Key Takeaway: The Earth constantly loses heat as outgoing longwave radiation (\(L \uparrow\)). Ground heat transfer (\(G\)) acts as a temporary storage battery, soaking up heat during the day and releasing it at night.
4. Non-Radiative Transfers: Sensible and Latent Heat
The remaining energy is not transferred by radiation (waves) but by moving molecules or changing the state of water. These are called turbulent transfers.
4.1 Sensible Heat Transfer (\(H\))
This is the heat energy transferred by conduction (touching the surface) and convection (vertical movement of air).
It is "sensible" because it directly changes the temperature of the air, which we can feel.
The transfer happens when the air right next to the hot surface warms up, becomes less dense, and rises (convection). This removes heat from the surface.
In hot, dry conditions (like a desert), sensible heat transfer (\(H\)) is the dominant way the surface loses energy because there is very little moisture for latent heat transfer.
Analogy: If you put your hand directly above a hot pavement, you feel the heat rising. That heat is sensible heat transfer (H).
4.2 Latent Heat Transfer (\(L_{E}\))
Latent heat is energy that is hidden or stored when water changes its state (phase change).
Evaporation: The most common type. When liquid water turns into water vapour, it requires a lot of energy. This energy is taken *from the surface*, cooling it down. This is the main way surfaces in humid or vegetated environments lose heat during the day.
Dew and Condensation: At night, when water vapour condenses back into liquid (dew), it releases the stored latent heat back to the surface, slightly reducing cooling.
The syllabus specifically mentions evaporation and dew (which relates to condensation/absorbed energy returned to Earth).
Analogy: When you sweat, the evaporation of that water from your skin uses your body heat. This latent heat process is why sweating cools you down so effectively.
Key Takeaway: Sensible heat (\(H\)) heats the air directly. Latent heat (\(L_{E}\)) is heat used or released during the phase change of water (crucially, evaporation cools the surface).
5. The Diurnal Cycle: Day vs. Night
The energy budget changes dramatically between day and night, primarily due to the presence or absence of shortwave radiation (\(K \downarrow\)).
5.1 The Daytime Energy Budget (Energy Surplus)
The budget is dominated by incoming shortwave radiation (\(K \downarrow\)).
- Input: High \(K \downarrow\) (Solar Radiation).
- Losses: Some \(K \uparrow\) (reflected by albedo) and constant \(L \uparrow\) (Earth emitting heat).
- Net Surplus (\(Q^*\) is positive): The surface gains energy faster than it loses it.
- Distribution: The surplus \(Q^*\) is partitioned into:
- Heating the air (\(H\)) - convection starts.
- Evaporation (\(L_{E}\)) - important in humid areas.
- Storage in the ground (\(G\)) - heating the subsurface.
The peak temperature usually occurs a few hours after noon, because this is when the stored heat (\(G\)) and sensible heat (\(H\)) transfers are highest, even though the solar input (\(K \downarrow\)) has just passed its peak.
5.2 The Nighttime Energy Budget (Energy Deficit)
The budget is simplified by the absence of solar input.
- Input: \(K \downarrow\) is zero. Only \(L \downarrow\) (atmospheric counter-radiation) remains as an input.
- Loss: Constant \(L \uparrow\) (Earth still emitting heat) is the dominant output.
- Net Deficit (\(Q^*\) is negative): The surface loses energy rapidly, causing temperatures to drop.
- Subsurface Release: Stored heat from the ground (\(G\)) flows back up to the surface (negative \(G\)), slightly reducing the cooling rate.
- Dew Formation: Condensation releases latent heat back to the surface (absorbed energy returned to Earth), also slightly mitigating cooling.
Key Takeaway: Day is defined by the solar input (\(K \downarrow\)) leading to a positive net budget. Night is defined by the absence of solar input and continuous longwave loss (\(L \uparrow\)), leading to a negative net budget and surface cooling.
6. Factors Affecting the Diurnal Energy Budget
The partitioning of energy (how much goes to H, \(L_{E}\), or \(G\)) changes dramatically depending on environmental factors.
6.1 Incoming Solar Radiation (K↓) Factors
Latitude and Season: Affects the angle of the sun and the length of the day. A higher sun angle (summer, tropics) means more intense radiation over a smaller area.
Cloud Cover: Clouds are excellent reflectors (high albedo). On a cloudy day, \(K \downarrow\) reaching the surface is greatly reduced, lowering the daytime \(Q^*\) significantly.
6.2 Surface and Subsurface Factors (Albedo and G)
Surface Colour and Material: Determines albedo. Dark surfaces (e.g., asphalt) absorb more heat (lower \(K \uparrow\)) than light surfaces (e.g., concrete).
Thermal Properties of Surface: Materials like water or wet soil have a high specific heat capacity (they take longer to heat up and cool down) compared to dry soil or rock. This affects the magnitude of \(G\).
Vegetation Cover: A dense canopy increases reflection slightly (higher \(K \uparrow\)) but crucially prevents a large amount of solar radiation from reaching the ground, reducing \(G\).
6.3 Moisture Factors (Latent Heat LE)
The amount of water present is critical as it determines the potential for latent heat transfer.
Wet Surfaces: Where water is readily available (e.g., tropical rainforests, oceans, irrigated fields), a large proportion of \(Q^*\) will be used for evaporation (\(L_{E}\)). This keeps the surface temperatures cooler.
Dry Surfaces (Arid/Semi-Arid): Little water means little evaporation. Consequently, the energy that would have been used for \(L_{E}\) must go into heating the air (\(H\)) or the ground (\(G\)). This is why deserts experience huge diurnal temperature ranges.
Antecedent Moisture: The existing moisture content in the soil. Wetter soil prior to the day means more energy will go into latent heat rather than sensible heat.
6.4 Atmospheric Factors (Longwave L and H)
Wind: Strong winds increase sensible heat transfer (\(H\)) by rapidly mixing the warm air near the surface with cooler air above, promoting cooling.
Cloud Cover at Night: This is a crucial examination point! Clear nights have very high longwave loss (\(L \uparrow\)) because there are no clouds to emit longwave radiation back down (\(L \downarrow\)). This leads to rapid cooling and often frost formation.
A cloudy night, however, acts like a blanket, increasing \(L \downarrow\) and keeping temperatures warmer.
Memory Aid: The Partitioning Rule
How does the surface choose where to send its heat (\(Q^*\))?
If Wet 💧: Energy goes mainly to LE (Evaporation). Air stays cool. (Think humid tropical coast.)
If Dry 🔥: Energy goes mainly to H (Sensible Heat). Air gets very hot. (Think hot, dry desert.)
Final Key Takeaway: The factors of moisture availability, albedo, and cloud cover are the most critical controls on the daily budget, determining the balance between latent heat, sensible heat, and ground heat storage.