Chapter 9 Solar Radiation, Heat Balance And Temperature

We live at the bottom of a vast ocean of air known as the atmosphere, which surrounds the Earth. This gaseous envelope is essential for life, providing the air we breathe. While we are constantly surrounded by air, we primarily perceive its movement as wind. The atmosphere is a complex mixture of gases, water vapour, and dust particles, crucial for sustaining life on our planet.

The Earth receives almost all of its energy from the Sun. This incoming solar energy drives atmospheric processes and determines Earth's temperature. The Earth simultaneously radiates energy back into space. Over long periods, the amount of energy received generally balances the amount lost, preventing the Earth from continuously heating up or cooling down significantly.

However, the distribution of solar energy across the Earth's surface is uneven, both spatially (from place to place) and temporally (over time). This unequal heating leads to variations in atmospheric pressure, which in turn causes air movement (winds) and ocean currents. These movements redistribute heat across the globe, influencing weather and climate patterns. Understanding how the atmosphere is heated and cooled, and how temperature is distributed, is fundamental to comprehending Earth's climate system.

Solar Radiation

The energy that reaches the Earth from the Sun is primarily in the form of short wavelengths of electromagnetic radiation. This incoming solar energy received at the Earth's surface is called insolation (short for incoming solar radiation).

Because the Earth is a sphere-like shape (a geoid), the Sun's rays arrive at the top of the atmosphere at varying angles, becoming more oblique (slanted) towards the poles. 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 Earth's atmosphere is approximately $1.94$ calories per square centimeter per minute ($1.94 \, cal/cm^2/min$), often referred to as the solar constant (though it varies slightly).

The amount of insolation received at the top of the atmosphere also varies slightly throughout the year due to changes in the Earth's distance from the Sun during its elliptical orbit. The Earth is furthest from the Sun (at aphelion, approx. 152 million km) around July 4th and nearest to the Sun (at perihelion, approx. 147 million km) around January 3rd. Consequently, the Earth as a whole receives slightly more insolation in early January than in early July. However, the effects of this orbital variation on daily weather and temperature are generally less significant than other factors, particularly the distribution of land and sea and atmospheric circulation patterns.

Variability Of Insolation At The Surface Of The Earth

The amount and intensity of insolation reaching the Earth's surface vary significantly depending on several factors, changing throughout the day, seasonally, and yearly:

• Rotation of the Earth on its Axis: This causes the daily cycle of day and night, resulting in varying exposure to sunlight and thus varying insolation amounts from sunrise to sunset.
• Angle of Inclination of the Sun's Rays: This is a crucial factor and is primarily determined by the latitude of a place and the Earth's tilt. At higher latitudes, the Sun's rays are more oblique (slanting) relative to the surface, while near the equator, they are more direct (closer to vertical). Slanting rays deliver less energy per unit area compared to vertical rays because the same amount of energy is spread over a larger surface area.
• Length of the Day: The duration of daylight varies with latitude and season due to the Earth's axial tilt. Longer daylight hours in summer mean a place receives more total insolation during the day than in winter when days are shorter. The Earth's axis is tilted at approximately $66.5^\circ$ to the plane of its orbit, or $23.5^\circ$ relative to a line perpendicular to the orbit, which is the main reason for seasonal variations in day length and the angle of solar rays at different latitudes.
• Transparency of the Atmosphere: The presence of clouds, water vapour, dust, and other particles in the atmosphere can absorb, reflect, or scatter incoming solar radiation, reducing the amount that reaches the surface. Areas with less cloud cover, like subtropical deserts, tend to receive more insolation.
• Configuration of Land (Aspect): The orientation of a slope relative to the Sun (its aspect) affects the intensity of insolation received. Slopes facing towards the Sun receive more direct insolation than slopes facing away or flat surfaces.

While atmospheric transparency and land aspect have local influences, the angle of the sun's rays (due to latitude and axial tilt) and the length of the day are the dominant factors determining the amount of insolation received at different locations and times of year.

The Passage Of Solar Radiation Through The Atmosphere

As incoming solar radiation travels through the Earth's atmosphere before reaching the surface, it interacts with atmospheric components:

• The atmosphere is largely transparent to the shortwave radiation from the Sun, particularly visible light.
• However, certain gases in the troposphere, such as water vapour and ozone, absorb some parts of the solar spectrum, especially near-infrared radiation.
• Small particles suspended in the troposphere, like dust and aerosols, cause scattering of incoming solar radiation. Scattering redirects light in various directions, both back towards space and downwards towards the Earth's surface. This scattering of visible light is responsible for phenomena like the blue color of the sky (scattering of blue light more effectively than other colors) and the reddish hues of sunsets and sunrises (when light passes through a thicker layer of atmosphere, scattering away blue light).

Only a portion of the original insolation at the top of the atmosphere actually reaches the Earth's surface; the rest is either absorbed or scattered by the atmosphere.

Spatial Distribution Of Insolation At The Earth’s Surface

The actual amount of solar radiation received at the Earth's surface varies significantly across different regions. On average, insolation can range from around 320 Watt/m² ($320 \, W/m^2$) in tropical areas to as low as 70 Watt/m² ($70 \, W/m^2$) near the poles.

• The highest amounts of insolation are often received in subtropical desert regions. This is because these areas typically have very low cloud cover, allowing most of the incoming solar radiation to reach the surface.
• Paradoxically, the equator receives slightly less insolation than the tropics because the equatorial regions often have higher cloudiness due to convection and frequent rainfall, which reduces the amount of sunlight reaching the surface.
• Generally, at the same latitude, continents receive more insolation than oceans because land surfaces heat up faster and often have less cloud cover than adjacent ocean areas.
• Seasonal variations are also significant; middle and high latitudes receive much less insolation in winter compared to summer due to shorter day length and a lower sun angle.

Heating And Cooling Of Atmosphere

The atmosphere is primarily heated indirectly by the Earth's surface, which absorbs solar radiation. Heat is transferred within the atmosphere and between the Earth and space through various processes.

Once the Earth's surface absorbs shortwave solar radiation, it gets heated and then radiates energy back to the atmosphere and space in the form of longwave terrestrial radiation. The atmospheric layers closest to the surface are heated by this radiated energy, as well as by direct contact and vertical/horizontal air movements.

Mechanisms of heat transfer within the atmosphere and from the Earth's surface include:

• Conduction: The transfer of heat through direct contact between substances or molecules. Heat flows from a warmer body to a cooler body until temperatures equalize. Conduction is most effective in heating the lowest layers of the atmosphere directly in contact with the warm Earth's surface.
• Convection: The transfer of heat through the vertical movement of air. When air near the surface is heated, it becomes less dense and rises as thermal currents, carrying heat upwards into the atmosphere. Convective heat transfer is primarily confined to the troposphere.
• Advection: The transfer of heat through the horizontal movement of air, i.e., by winds. Advection is generally more significant for heat distribution across different regions than vertical convection. In mid-latitudes, much of the daily temperature variation is influenced by the advection of warm or cold air masses. A local example is the 'loo', a hot wind in northern India during summer, which is a result of advective heat transfer.

Terrestrial Radiation

The Earth's surface, having absorbed incoming shortwave solar radiation, becomes a source of energy itself and emits energy back towards the atmosphere and outer space as longwave radiation (also called terrestrial radiation). This is the primary way the atmosphere is heated from below.

• Atmospheric gases, particularly greenhouse gases like carbon dioxide ($CO_2$), water vapour, methane, etc., are efficient absorbers of this longwave terrestrial radiation. This absorption process traps heat within the atmosphere.
• The heated atmosphere, in turn, also radiates energy, some of which goes back towards the Earth's surface (the greenhouse effect) and some outwards into space.

Through these processes of absorption and radiation, the atmosphere gains heat from the Earth below and distributes it, eventually radiating a balanced amount back into space, ensuring that the Earth-atmosphere system maintains a relatively stable average temperature over time.

Heat Budget Of The Planet Earth

For the Earth as a whole to maintain a relatively stable average temperature over long periods, the total amount of energy received from the Sun must be balanced by the total amount of energy lost back to space. This balance is referred to as the Earth's heat budget or energy balance.

Considering the total incoming solar radiation at the top of the atmosphere as 100 units, the energy transfer can be accounted for as follows:

• Approximately 35 units are lost to space through reflection and scattering even before reaching the Earth's surface. This reflected portion is called the albedo of the Earth. About 27 units are reflected by clouds, and 2 units are reflected by snow and ice surfaces on the ground. The remaining 6 units are scattered by the atmosphere.
• The remaining 65 units of incoming solar radiation are absorbed by the Earth-atmosphere system. Of these, 14 units are absorbed directly by atmospheric gases and particles, and 51 units are absorbed by the Earth's surface (land and oceans).
• The Earth's surface, having absorbed 51 units, radiates this energy upwards as longwave terrestrial radiation. About 17 units of this terrestrial radiation pass directly through the atmosphere and escape to space.
• The remaining 34 units of terrestrial radiation are absorbed by the atmosphere (mainly by greenhouse gases). Additionally, the atmosphere receives heat from the Earth's surface through convection and turbulence (9 units) and through the release of latent heat during the condensation of water vapour (19 units). These amount to $34 + 9 + 19 = 62$ units of heat transferred from the Earth's surface to the atmosphere. *Correction: The text says 34 units absorbed by atm from terrestrial radiation, 9 through convection/turbulence, 19 through latent heat. Total received by atmosphere from surface = 34 + 9 + 19 = 62 units.*
• The atmosphere itself, having absorbed 14 units of solar radiation and 62 units of heat from the Earth's surface, radiates a total of 48 units of energy back into space.

Thus, the total outgoing radiation to space is the 35 units reflected initially + 17 units of terrestrial radiation escaping directly + 48 units radiated from the atmosphere = $35 + 17 + 48 = 100$ units. This balance confirms that the total energy received from the Sun is matched by the total energy returned to space, maintaining the Earth's overall temperature stability.

Diagram showing the flow of incoming solar radiation and outgoing terrestrial radiation and their interactions with the Earth and atmosphere, illustrating the planet's energy balance.

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

Although the Earth system as a whole is in energy balance, this balance does not hold true for every location on the planet. Due to the varying angle of incoming solar radiation and other factors, different latitudes receive different amounts of energy. This creates a significant imbalance:

• Areas in the tropics and subtropics (roughly between 40° North and 40° South latitude) receive more solar radiation than they lose through terrestrial radiation, resulting in a net radiation surplus.
• Areas in the middle and high latitudes, particularly near the poles, lose more energy through radiation than they receive from the sun, resulting in a net radiation deficit.

Graph illustrating that latitudes near the equator have a surplus of incoming solar energy compared to outgoing radiation, while higher latitudes have a deficit.

This latitudinal energy imbalance is a primary driver of atmospheric and oceanic circulation. Heat energy from the surplus regions in the tropics is transported polewards by winds and ocean currents. This poleward heat transfer prevents the tropics from continuously heating up and the polar regions from continuously cooling down, playing a vital role in regulating global climate.

Temperature

When solar radiation interacts with the atmosphere and the Earth's surface, it creates heat. Temperature is a measure of the intensity of this heat – specifically, it is the degree or measure of hotness or coldness of a substance or location. While heat is the total internal energy due to molecular motion, temperature is a scale that quantifies this energy level.

Specific heat is the amount of energy (in calories or joules) required to raise the temperature of one gram of a substance by one degree Celsius ($1^\circ C$). Different substances have different specific heats, meaning they require varying amounts of energy to change their temperature (e.g., water has a high specific heat, meaning it takes a lot of energy to heat up compared to land).

Plank's Law in physics states that the hotter an object is, the more energy it radiates, and the shorter the dominant wavelength of that radiated energy. This explains why the hot Sun emits shortwave radiation and the cooler Earth emits longwave radiation.

Factors Controlling Temperature Distribution

The air temperature at any given place on Earth is influenced by several interacting factors:

• Latitude: As discussed earlier, latitude determines the angle of the Sun's rays and the length of the day, which are the main controls on the amount of insolation received. Places closer to the equator (lower latitudes) generally receive more intense insolation and therefore tend to have higher temperatures than places closer to the poles (higher latitudes).
• Altitude (Elevation): Temperature generally decreases with increasing height above sea level. This is because the atmosphere is primarily heated by longwave radiation from the Earth's surface below. Therefore, as you move further away from this primary heat source, the air tends to be cooler. The average rate at which temperature decreases with height in the troposphere is called the normal lapse rate, which is approximately $6.5^\circ C$ per 1,000 meters ($1000 \, m$).
• Distance from the Sea (Continentality): Land and water have different thermal properties, particularly their specific heat. Water has a much higher specific heat than land, meaning it requires significantly more energy to raise its temperature. Water also heats up and cools down more slowly than land and heat is distributed throughout a water body. As a result, temperatures over oceans are much more stable and have less variation (annual and diurnal) compared to temperatures over continents. Inland locations, far from the moderating influence of the sea, experience greater temperature extremes (hotter summers and colder winters). This effect is known as continentality. Places located near the coast experience a more moderate climate due to the influence of sea breezes (cooling effect during the day) and land breezes (warming effect during the night in some cases).
• Air Masses and Ocean Currents: Large bodies of air (air masses) and currents of water in the oceans (ocean currents) transport heat over vast distances, influencing the temperatures of the regions they move into. Areas affected by warm air masses or warm ocean currents experience higher temperatures than their latitude might suggest, while areas influenced by cold air masses or cold ocean currents experience lower temperatures. For example, the Gulf Stream, a warm ocean current, keeps the temperatures of Western Europe milder in winter than other locations at similar latitudes.
• Local Aspects: Local factors such as the slope and orientation (aspect) of the land, vegetation cover, and surface type (e.g., urban areas vs. forests) can cause local variations in temperature. Slopes facing the sun receive more insolation and are warmer than shaded slopes.

Distribution Of Temperature

The global pattern of air temperature is often visualized using isotherms, which are lines drawn on a map connecting locations that have the same temperature at a given time or over a specific period (e.g., average monthly temperature). Studying maps of isotherms for different times of the year, such as January and July, helps illustrate temperature distribution patterns (Figures 9.4a and 9.4b).

Map showing lines of equal temperature (isotherms) for average surface air temperature in January.

Map showing lines of equal temperature (isotherms) for average surface air temperature in July.

Generally, isotherms tend to run roughly parallel to lines of latitude, reflecting the primary control of latitude on temperature. However, this parallel pattern is significantly disrupted by the distribution of land and sea and the influence of ocean currents, particularly in the Northern Hemisphere, which has a much larger proportion of landmass compared to the Southern Hemisphere.

• January Distribution (Winter in NH, Summer in SH): In the Northern Hemisphere winter, continents are much colder than oceans at the same latitude due to continentality. Isotherms tend to bend equatorward (southward) over landmasses and poleward (northward) over oceans. A prominent example is the northward bending of isotherms over the North Atlantic, influenced by the warm Gulf Stream and North Atlantic Drift currents. Inland areas of continents like Siberia experience extremely low temperatures, and isotherms bend sharply southward. In contrast, the Southern Hemisphere, dominated by oceans, shows isotherms that are much more parallel to latitudes, and temperature variations are more gradual.
• July Distribution (Summer in NH, Winter in SH): In the Northern Hemisphere summer, continents are much warmer than oceans. Isotherms tend to bend poleward (northward) over landmasses and equatorward (southward) over oceans. High temperatures ($>$ 30°C) are common over subtropical continental interiors (e.g., Asia and North America). In the Southern Hemisphere winter, the patterns are again more zonal (parallel to latitude) due to the oceanic dominance, with more gradual temperature changes.

The annual range of temperature, which is the difference between the mean temperature of the warmest month and the coldest month, highlights the effect of continentality (Figure 9.5). The largest annual temperature ranges (over 60°C) are found deep within the continents of the Northern Hemisphere, such as northeastern Eurasia, due to extreme cold in winter and significant heat in summer. The smallest annual ranges (as low as 3°C) are found near the equator over the oceans, where temperatures are consistently high throughout the year.

Map showing the difference between average temperatures of the warmest and coldest months, illustrating the annual temperature range across the globe.

Inversion Of Temperature

Under normal conditions, temperature decreases with increasing altitude in the troposphere (the normal lapse rate). However, sometimes, this relationship is reversed, and temperature increases with height for a certain layer. This phenomenon is called a temperature inversion or negative lapse rate.

Temperature inversions are relatively common, though often short-lived, but they significantly impact atmospheric stability and air quality.

One common type is Surface Inversion. Ideal conditions for a strong surface inversion include:

• A long winter night, allowing ample time for the ground to cool significantly by radiating heat.
• Clear skies, which allow heat to escape freely into space without being trapped by clouds.
• Still air, which prevents mixing and keeps the cold air concentrated near the surface.

Under these conditions, the ground becomes colder than the air above it, and it cools the layer of air directly in contact with it through conduction, creating a layer of cold air near the surface with warmer air above. In polar regions, surface inversions are often persistent throughout much of the year.

Surface inversions create very stable atmospheric conditions, as the cold, dense air is at the bottom. This stability inhibits vertical mixing. Consequently, pollutants like smoke and dust emitted near the ground are trapped beneath the inversion layer, accumulating and spreading horizontally. This can lead to reduced visibility and poor air quality, often manifested as dense fogs in the morning, particularly during winter. Surface inversions typically dissipate a few hours after sunrise as the sun warms the ground and the lowest layer of air, restoring normal lapse rate conditions.

Another type is Air Drainage Inversion, common in hilly and mountainous areas. On clear, calm nights, the ground on slopes cools rapidly. The air in contact with the cooled slopes also cools and becomes denser. This cold, heavy air then flows downslope under the influence of gravity, similar to how water flows. This downhill movement of cold air is called air drainage. The cold air accumulates in valley bottoms and low-lying areas, displacing the warmer air upwards. This results in warmer temperatures on the slopes above the valley floor and colder temperatures in the valley bottom, creating an inversion layer above the valley floor. Air drainage inversions can protect crops planted on slopes from frost damage, as the coldest air collects in the valley bottom.

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