Chapter 8 Solar Radiation, Heat Balance And Temperature

We exist within a vast ocean of air that surrounds our planet. While we constantly breathe, we only feel this air when it moves as wind. This atmospheric envelope, composed of numerous gases, is essential for supporting life on Earth.

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The primary source of energy for the Earth's climate system is the **sun**. The Earth absorbs energy from the sun and, in turn, radiates energy back into space. Over time, the Earth maintains a relatively stable average temperature because the amount of incoming solar energy is balanced by the amount of outgoing energy.

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However, the distribution of heat received across the Earth's surface is not uniform. Variations in incoming solar energy create differences in atmospheric pressure, leading to the movement of air (wind) and the transfer of heat from warmer to cooler regions. Understanding how the atmosphere is heated and cooled, and how temperature is distributed, is crucial to comprehending weather and climate.

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Solar Radiation

The energy that the Earth receives from the sun is called **insolation**, which is short for **incoming solar radiation**. This energy arrives predominantly in the form of **short wavelengths**.

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Because the Earth is roughly spherical (a geoid), the sun's rays strike the top of the atmosphere at varying angles. Also, the Earth intercepts only a tiny fraction of the total energy emitted by the sun. On average, the amount of solar energy received at the top of the atmosphere is about **1.94 calories per square centimeter per minute** ($1.94 \text{ cal/cm}^2\text{/min}$), which is equivalent to approximately 1361 Watts per square meter ($1361 \text{ W/m}^2$) - this value is known as the solar constant, although it's not truly constant.

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The distance between the Earth and the sun changes slightly throughout the year as the Earth orbits. This variation causes minor fluctuations in the amount of solar output received at the top of the atmosphere:

• On or around July 4th, the Earth is farthest from the sun (approx. 152 million km). This position is called **aphelion**.
• On or around January 3rd, the Earth is closest to the sun (approx. 147 million km). This position is called **perihelion**.

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Consequently, the Earth receives slightly more insolation in early January than in early July. However, this effect on the total solar output is generally less significant than other factors (like land-sea distribution and atmospheric circulation) in determining daily weather changes.

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Variability Of Insolation At The Surface Of The Earth

The amount and intensity of insolation reaching the Earth's surface vary considerably due to several factors. These variations occur on daily, seasonal, and annual timescales:

1. Rotation of the Earth on its axis: Causes day and night, leading to daily variations in solar radiation received at any given location.
2. Angle of inclination of the sun’s rays: The angle at which sunlight strikes the surface, which is primarily controlled by latitude and the Earth's axial tilt.
3. Length of the day: Determined by latitude and the season, influencing how long a location is exposed to sunlight.
4. Transparency of the atmosphere: Affected by cloud cover, dust, pollution, etc., which can absorb, scatter, or reflect insolation.
5. Configuration of land (Aspect): The direction a slope faces relative to the sun influences the amount of insolation received.

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While atmospheric transparency and land aspect have some influence, the most significant factors determining the variability of insolation are the Earth's axial tilt, its rotation, and its revolution around the sun, which together determine the angle of the sun's rays and the length of daylight at different latitudes and times of the year.

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The Earth's axis is tilted at an angle of **$66.5^\circ$** to the plane of its orbit. This tilt, combined with the Earth's spherical shape, causes the angle at which the sun's rays strike the surface to vary with latitude.

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At lower latitudes (closer to the equator), the sun's rays are more direct or vertical. At higher latitudes (closer to the poles), the rays are more oblique or slant. This angle has two main effects on the amount of energy received per unit area:

• **Area Covered:** Vertical rays are concentrated over a smaller area compared to slant rays, which spread the same amount of energy over a larger surface. Therefore, energy received per unit area is higher where rays are more vertical.
• **Path Length through Atmosphere:** Slant rays travel a longer distance through the atmosphere compared to vertical rays. This longer path means more solar radiation is absorbed, scattered, and reflected by atmospheric components (gases, dust, clouds) before reaching the surface, reducing the amount of energy that actually reaches the ground.

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The Passage Of Solar Radiation Through The Atmosphere

As solar radiation travels through the atmosphere, its intensity and spectral composition are altered before it reaches the Earth's surface. The atmosphere is largely **transparent to the shortwave solar radiation**.

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However, certain atmospheric components interact with this radiation:

• Within the troposphere, gases like **water vapour** and **ozone**, as well as other trace gases, absorb some of the incoming solar radiation, particularly in the near-infrared portion of the spectrum. The ozone layer in the stratosphere is especially effective at absorbing harmful ultraviolet (UV) radiation.
• Very small suspended particles (like dust, aerosols) in the troposphere cause **scattering** of visible light in all directions – back into space and towards the Earth's surface. This scattering is responsible for the color of the sky; blue light is scattered more effectively than red light, making the sky appear blue. At sunrise and sunset, sunlight passes through a longer path in the atmosphere, and most of the blue light is scattered away, leaving the longer wavelengths (red and orange) to reach our eyes.

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The portion of insolation that reaches the Earth's surface includes both direct sunlight and diffuse radiation (scattered sunlight).

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Spatial Distribution Of Insolation At The Earth’s Surface

The amount of insolation received at the Earth's surface varies geographically:

• Generally, insolation decreases from tropical regions towards the poles due to the angle of the sun's rays and day length variations. The average annual insolation can range from about 320 Watt/m² in the tropics to only about 70 Watt/m² at the poles.
• However, the **maximum insolation** is often received not directly at the equator, but in the **subtropical desert regions**. This is because these areas typically have very clear skies with minimal cloud cover, allowing more direct sunlight to reach the surface compared to the often cloudy equatorial regions.
• At the same latitude, land masses generally receive more insolation than ocean surfaces, particularly in certain seasons, due to differences in how land and water absorb and reflect radiation.
• Seasonal variations are significant, especially in middle and higher latitudes. These regions receive much less radiation in winter compared to summer, primarily due to shorter day lengths and lower sun angles.

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Heating And Cooling Of Atmosphere

The atmosphere itself is not directly heated by the shortwave solar radiation passing through it as much as the Earth's surface is. The primary way the atmosphere gains heat is indirectly from the Earth's surface through several processes after the surface absorbs solar energy.

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The Earth's surface, heated by incoming shortwave insolation, becomes a source of energy itself and radiates heat upwards in the form of **longwave terrestrial radiation**. This is the main process that heats the atmosphere from below. Other processes include:

• **Conduction:** Heat transfer through direct contact. The lowest layer of air in contact with the warm Earth surface gets heated. This heat is then transferred to the air layers immediately above through molecular collision. Conduction is most important in heating the lowermost atmosphere layers.
• **Convection:** Heat transfer through the vertical movement of fluids (air or water). When the air in contact with the warm surface is heated, it becomes less dense and rises as thermal currents, carrying heat upwards. This process of vertical heat transfer is called convection and is primarily confined to the troposphere.
• **Advection:** Heat transfer through the **horizontal movement of air (wind)**. This is often more significant than vertical convection in distributing heat across different regions. Advection causes day-to-day variations in weather, especially in mid-latitudes. Local winds, like the 'loo' in northern India during summer, are examples of heat transfer by advection.

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Terrestrial Radiation

The Earth's surface, having absorbed solar radiation, re-emits energy as **longwave radiation**. This is the main source of heat for the atmosphere. Atmospheric gases, particularly **greenhouse gases** like water vapour and carbon dioxide, are strong absorbers of this longwave terrestrial radiation.

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By absorbing the Earth's outgoing heat, the atmosphere warms up. The atmosphere then also radiates energy, some back towards the Earth's surface (further warming it - the greenhouse effect) and some outwards into space.

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Through these processes of absorption and radiation, the Earth-atmosphere system achieves a thermal equilibrium, where the total amount of incoming solar energy is balanced by the total amount of outgoing energy radiated back to space. This balance maintains the Earth's average temperature.

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Heat Budget Of The Planet Earth

The **Heat Budget** (or Heat Balance) refers to the perfect balance between the total amount of solar radiation received by the Earth-atmosphere system and the total amount of energy returned to space. This balance ensures that the Earth maintains a relatively stable average temperature over long periods.

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Let's consider the incoming solar radiation at the top of the atmosphere as 100 units. As this energy passes through the atmosphere, some of it is lost before reaching the surface:

• Approximately **35 units** are reflected back to space even before reaching the ground. This reflected portion is called the **albedo** of the Earth. Of these 35 units:
  • 27 units are reflected by clouds.
  • 2 units are reflected by snow and ice cover on the Earth's surface.
  • The remaining 6 units are scattered by the atmosphere (reaching the ground as diffuse radiation, but eventually some is lost to space). *Note: The text states 35 reflected back before reaching the surface, 27 from clouds, 2 from snow/ice. This implies 6 units are reflected/scattered by other atmospheric elements before reaching the surface and counted in this 35. Let's stick to the numbers provided.* So, **35 units are reflected back to space**.

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The remaining **65 units** are absorbed:

• **14 units** are absorbed directly by the atmosphere (by gases like ozone, water vapour, etc.).
• **51 units** are absorbed by the Earth's surface (land and oceans).

Total absorbed = $14 + 51 = 65$ units. (Incoming 100 - Reflected 35 = Absorbed 65).

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Now, the Earth radiates energy back outwards as longwave terrestrial radiation. The 51 units absorbed by the Earth's surface are re-radiated. Of these 51 units:

• **17 units** are radiated directly back into space through the atmospheric window (wavelengths that are not absorbed by greenhouse gases).
• The remaining **34 units** are absorbed by the atmosphere. This absorption happens through:
  • Direct absorption by atmospheric gases (6 units).
  • Transfer through convection and turbulence (9 units).
  • Transfer through latent heat of condensation (energy released when water vapour condenses - 19 units).

Total absorbed by the atmosphere from Earth radiation = $6 + 9 + 19 = 34$ units.

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The atmosphere has now absorbed a total of $14$ units (from solar) + $34$ units (from Earth) = **48 units**. The atmosphere, in turn, radiates this absorbed energy back into space.

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So, the total energy radiated back to space is:

• 17 units radiated directly from the Earth's surface.
• 48 units radiated from the atmosphere.

Total outgoing radiation = $17 + 48 = 65$ units.

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Since the total energy received (65 units absorbed insolation) equals the total energy radiated back to space (65 units), the Earth-atmosphere system maintains a **heat balance**. (This refers to Figure 8.2 illustrating the heat budget components).

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Variation In The Net Heat Budget At The Earth’s Surface

While the Earth as a whole is in heat balance, there are significant geographical variations in the net radiation balance at the surface. The amount of incoming solar radiation is not equal to the amount of outgoing terrestrial radiation at every latitude.

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Observations show that there is generally a **surplus of net radiation** (more incoming than outgoing) in the tropical and subtropical regions, roughly between $40^\circ$ North and $40^\circ$ South latitudes. Conversely, there is a **deficit of net radiation** (more outgoing than incoming) in the regions closer to the poles. (This refers to Figure 8.3 illustrating this latitudinal variation).

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This imbalance doesn't cause the tropics to continuously heat up or the poles to freeze solid because heat is constantly being transferred from the surplus regions (tropics) to the deficit regions (poles) through atmospheric circulation (winds) and oceanic circulation (ocean currents). This global heat transfer system helps to moderate temperatures across the planet.

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Temperature

**Temperature** is a measure of the degree of hotness or coldness of a substance or place. It quantifies the intensity of heat, which is itself a form of energy related to the movement of molecules within a substance.

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Factors Controlling Temperature Distribution

The temperature of the air at any specific location is influenced by several interacting factors:

1. **Latitude:** This is the most fundamental control. As discussed under insolation, latitude determines the angle of the sun's rays and the length of daylight, which directly affects the amount of solar energy received and thus the temperature. Temperatures generally decrease with increasing latitude.
2. **Altitude (Elevation):** As the atmosphere is mainly heated from below by the Earth's surface, air temperature generally decreases with increasing height above sea level. This rate of decrease, approximately $6.5^\circ\text{C}$ per 1,000 meters, is known as the **Normal Lapse Rate**. Places at higher elevations are typically cooler than places at lower elevations at the same latitude.
3. **Distance from the sea (Continentality):** Land and water heat up and cool down at different rates. Land heats up and cools down much faster and to a greater extent than water. Therefore, coastal areas experience a more moderate temperature range (less difference between summer and winter, day and night) because the large body of water nearby regulates temperatures. Inland areas, far from the moderating influence of the sea, experience greater temperature extremes. This effect is known as **continentality**.
4. **Air-masses and Ocean Currents:** The passage of large bodies of air with uniform temperature and humidity characteristics (**air masses**) influences local temperatures. A region will experience warming when under the influence of a warm air mass and cooling under a cold air mass. Similarly, warm ocean currents circulating along a coast can raise the temperature of nearby land areas, while cold ocean currents can lower them.
5. **Local aspects:** Factors like the slope and orientation of the land (aspect), vegetation cover, and the presence of urban areas can also influence local temperatures. For example, slopes facing the sun receive more insolation and are warmer.

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Distribution Of Temperature

The global pattern of temperature distribution is often represented using **isotherms**, which are lines drawn on a map connecting locations that have the same temperature. Studying maps showing isotherms for different months (like January and July) reveals the influence of the factors controlling temperature.

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Generally, isotherms tend to run parallel to lines of latitude, reflecting the primary control of latitude on temperature. However, deviations from this parallel pattern are significant and reveal the influence of other factors, particularly the distribution of land and sea and ocean currents.

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(This refers to Figure 8.4 (a) and (b) showing global temperature distribution in January and July).

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The deviations are particularly noticeable in the **Northern Hemisphere** because it has a much larger proportion of land area compared to the Southern Hemisphere. The continentality effect and ocean current influences are more pronounced here.

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In **January** (winter in the Northern Hemisphere, summer in the Southern Hemisphere):

• Isotherms tend to bend **northward over the oceans** (as oceans are warmer than land in winter) and **southward over the continents** (as land is much colder than oceans).
• A clear example is the North Atlantic, where the warming influence of the Gulf Stream and North Atlantic Drift causes isotherms to bend significantly northward.
• Over large continents like Eurasia, temperatures drop sharply inland, causing isotherms to dip far southward (e.g., the severe cold in the Siberian plains).
• In the Southern Hemisphere, which is mostly ocean, isotherms are more parallel to latitudes and show a more gradual temperature change.

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In **July** (summer in the Northern Hemisphere, winter in the Southern Hemisphere):

• Isotherms generally run more parallel to latitudes overall, but still show some bending.
• Isotherms tend to bend **southward over the oceans** (as oceans are cooler than land in summer) and **northward over the continents** (as land heats up much more than oceans).
• High temperatures are observed over subtropical continental regions (e.g., in Asia, along $30^\circ$ N).
• The Southern Hemisphere isotherms remain relatively parallel to latitudes due to the dominance of oceans.

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The **Range of Temperature** is the difference between the maximum and minimum temperatures recorded over a period (e.g., daily range, annual range). The **annual range of temperature** (difference between the mean temperature of the warmest and coldest months) is particularly influenced by continentality.

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(This refers to Figure 8.5 illustrating the annual range of temperature).

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Figure 8.5 shows that the highest annual temperature range (over $60^\circ\text{C}$) occurs over the interior of continents in the Northern Hemisphere (e.g., northeastern Eurasia), directly demonstrating the effect of continentality. The lowest range (around $3^\circ\text{C}$) is found over the oceans in tropical regions, where temperatures are moderated by water and seasonal variations in insolation are smaller.

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Inversion Of Temperature

Normally, air temperature decreases with increasing altitude (the normal lapse rate). However, sometimes this pattern is reversed, and temperature increases with height for a limited atmospheric layer. This phenomenon is called **Temperature Inversion** or **Inversion of Temperature**. While usually temporary, it can significantly impact local weather and air quality.

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An ideal situation for a **surface inversion** (inversion near the ground) is a **long winter night with clear skies and still air**. Under clear skies, heat absorbed by the Earth during the day is rapidly radiated away into space at night. With still air, there is little vertical mixing. The ground surface cools quickly, and the air immediately above it also cools by conduction. The air higher up, however, retains its heat longer, resulting in a layer of colder air trapped beneath warmer air. Over polar regions, where nights are very long in winter and the surface is snow/ice covered (high albedo), temperature inversion can be a normal occurrence throughout the year.

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Surface inversions create stable atmospheric conditions in the lower layers, as the warm air above acts like a lid preventing the cold air below from rising. This trapping effect means that smoke, dust, and other pollutants emitted at the surface cannot disperse upwards and accumulate near the ground. This can lead to dense fogs, especially in the morning during winter, as water vapour condenses in the cold, still air trapped beneath the inversion layer. Surface inversions typically break up a few hours after sunrise as the sun begins to warm the ground and the trapped cold air.

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**Inversion also commonly occurs in hilly and mountainous areas due to air drainage**. At night, the air in contact with the higher slopes cools rapidly. Being denser and heavier, this cold air flows downslope under the influence of gravity, pooling in valleys and depressions at lower elevations. Warmer air, being lighter, is displaced upwards, sitting on top of the pool of cold air in the valley bottom. This process, called **air drainage**, results in temperature inversion in the valleys (valleys are colder than the slopes above). This natural phenomenon can protect sensitive crops on the slopes from frost damage that might occur in the colder valley bottom.

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Some fundamental physics principles relate to radiation and heat:

• Plank's Law: States that any object with a temperature above absolute zero emits electromagnetic radiation. Hotter objects radiate more energy overall, and the peak wavelength of their radiation is shorter. The sun, being much hotter than Earth, emits mostly shortwave radiation, while Earth emits longwave radiation.
• Specific Heat: The amount of heat energy required to raise the temperature of one gram of a substance by one degree Celsius. Water has a high specific heat compared to land, meaning it requires much more energy to raise its temperature. This property contributes significantly to the moderating effect of oceans on coastal climates.

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Exercises

Multiple Choice Questions

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Answer The Following Questions In About 30 Words

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Answer The Following Questions In About 150 Words

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Project Work

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