Solar Radiation, Heat Balance And Temperature

Solar Radiation

Variability Of Insolation At The Surface Of The Earth

Insolation, or incoming solar radiation, is the energy that drives Earth's climate system. The amount of insolation received at the Earth's surface is not uniform and varies significantly due to several factors:

• Earth's Revolution Around the Sun: The Earth orbits the Sun in an elliptical path. While the distance does vary slightly, this is a minor factor in seasonal temperature changes compared to the tilt of Earth's axis.
• Earth's Axial Tilt: This is the primary reason for seasons. The Earth's axis is tilted at approximately 23.5 degrees relative to its orbital plane. This tilt causes different hemispheres to receive more direct sunlight at different times of the year.
• Revolution and Inclination of Axis: As the Earth revolves around the Sun, the angle at which solar rays strike the surface changes. When a hemisphere is tilted towards the Sun, it receives more direct rays (higher angle of incidence), leading to longer days and warmer temperatures (summer). When tilted away, it receives less direct rays (lower angle of incidence), resulting in shorter days and cooler temperatures (winter).
• Length of Day: The duration of daylight hours directly correlates with the amount of insolation received. Longer days mean more time for solar energy to reach the surface.
• Transparency of the Atmosphere: The presence of clouds, dust, aerosols, and other atmospheric constituents can scatter, absorb, or reflect solar radiation before it reaches the surface. A clearer atmosphere allows more insolation to pass through.
• Latitude: Insolation is most intense at the equator, where solar rays strike the surface at a nearly perpendicular angle. As latitude increases towards the poles, the angle of incidence decreases, and the rays spread over a larger area, reducing their intensity.

The Passage Of Solar Radiation Through The Atmosphere

As solar radiation (shortwave radiation) travels from the Sun to the Earth, it interacts with the atmosphere in several ways:

• Scattering: Air molecules and aerosols scatter solar radiation in all directions. Shorter wavelengths (blue and violet) are scattered more effectively than longer wavelengths (red and orange). This phenomenon, known as Rayleigh scattering, is why the sky appears blue. Scattering reduces the intensity of direct solar radiation.
• Absorption: Certain atmospheric gases absorb specific wavelengths of solar radiation.
  • Ozone (O₃) in the stratosphere absorbs most of the harmful ultraviolet (UV) radiation.
  • Water Vapour (H₂O) and Carbon Dioxide (CO₂) absorb some infrared radiation within the solar spectrum.
  • Oxygen (O₂) and Nitrogen (N₂) absorb very short wavelengths (like extreme UV).
• Reflection: Clouds, aerosols, and the Earth's surface (especially snow and ice) reflect a portion of incoming solar radiation back into space. This reflected radiation is known as albedo.
• Transmission: The portion of solar radiation that passes through the atmosphere without being scattered, absorbed, or reflected is called direct solar radiation. The sum of direct and diffuse (scattered) radiation reaching the surface is termed global radiation.

On average, about 30% of incoming solar radiation is reflected back to space (planetary albedo), 25% is scattered back to space by clouds and aerosols, and about 45% is absorbed by the Earth's surface and atmosphere. This absorbed energy is what warms the planet.

Spatial Distribution Of Insolation At The Earth’s Surface

The amount of solar radiation received at the Earth's surface varies spatially due to a combination of factors:

• Latitude: As mentioned, insolation is highest at the equator and decreases towards the poles. This is due to the angle of incidence and the path length through the atmosphere. At the poles, solar rays strike at a very low angle and must travel through a much thicker layer of atmosphere, leading to significant scattering and absorption. During polar winters, there is no daylight at all.
• Altitude: At higher altitudes, the atmosphere is thinner and less effective at scattering and absorbing solar radiation. Therefore, higher elevations generally receive more intense insolation than lower elevations at the same latitude, assuming clear skies.
• Cloud Cover: Regions with persistent cloud cover (e.g., tropical rainforests, mid-latitude storm tracks) receive less insolation than clear, arid regions.
• Surface Albedo: The reflectivity of the surface affects how much solar radiation is absorbed. Light-coloured surfaces like snow and ice have high albedo and reflect most of the incoming radiation, absorbing very little. Darker surfaces like forests and oceans have low albedo and absorb more radiation.
• Season: Due to the tilt of the Earth's axis, the distribution of insolation changes throughout the year, leading to distinct seasons.

General Pattern: The highest annual insolation occurs in subtropical desert regions due to clear skies and high solar angles, while the lowest occurs at the poles.

Heating And Cooling Of Atmosphere

Terrestrial Radiation

While the Sun provides energy to the Earth in the form of shortwave radiation, the Earth itself radiates energy back into space in the form of longwave (infrared) radiation. This process is known as terrestrial radiation.

• Nature of Radiation: All objects with a temperature above absolute zero emit thermal radiation. The Earth's surface, warmed by absorbed solar radiation, emits infrared radiation. The intensity and wavelength distribution of this radiation depend on the surface temperature, following Planck's Law. The Earth's average surface temperature is around 15°C (59°F), causing it to emit radiation primarily in the infrared spectrum (wavelengths longer than visible light).
• Greenhouse Effect: Certain atmospheric gases, known as greenhouse gases (GHGs) – primarily water vapour (H₂O), carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and ozone (O₃) – are largely transparent to incoming shortwave solar radiation but are strong absorbers of outgoing longwave terrestrial radiation. When these gases absorb terrestrial radiation, they re-emit it in all directions, including back towards the Earth's surface and lower atmosphere. This process effectively traps heat, warming the planet beyond what it would be without an atmosphere. This is the natural greenhouse effect, essential for maintaining habitable temperatures on Earth.
• Selective Absorption: Greenhouse gases absorb specific wavelengths of infrared radiation. For example, CO₂ absorbs strongly in the 12-18 micrometer range, while water vapour absorbs over a broader range, especially in the atmospheric window regions.
• Cooling: The atmosphere also cools itself through radiation. While the lower atmosphere (troposphere) is heated from below by the Earth's surface (conduction, convection, and absorption of terrestrial radiation), the upper layers are heated more by the absorption of solar radiation (stratosphere) or cool by radiating heat into space.

Heat Budget Of The Planet Earth

The Earth's heat budget is the balance between the total amount of solar energy received by the Earth and the total amount of energy radiated back into space. For the Earth's temperature to remain relatively stable over long periods, the amount of energy absorbed must equal the amount of energy radiated away.

Energy Input (Incoming Solar Radiation - Insolation):

• The Sun emits energy constantly. The amount of energy received by the Earth per unit area at the top of the atmosphere, perpendicular to the solar rays, is called the solar constant, approximately 1361 W/m².
• However, due to the Earth's spherical shape, solar rays strike the Earth's surface at varying angles, and only a portion of the incoming radiation is absorbed.

Energy Output (Outgoing Terrestrial Radiation):

• The Earth radiates energy back into space as longwave (infrared) radiation.
• The amount of radiation emitted depends on the Earth's temperature.

The Balance:

• Albedo: About 30% of incoming solar radiation is reflected back to space by clouds, atmospheric particles, and the Earth's surface (especially ice and snow). This reflected energy is not absorbed and does not contribute to warming.
• Absorption: The remaining 70% is absorbed by the atmosphere (about 20%) and the Earth's surface (about 50%).
• Radiative Equilibrium: For the Earth's average temperature to be stable, the energy absorbed must be balanced by the energy radiated. The Earth's effective radiating temperature (if it had no atmosphere) would be about -18°C (0°F). However, the greenhouse effect raises the average surface temperature to a much more habitable 15°C (59°F).

Heat Budget Equation:

Total Incoming Solar Radiation = Total Outgoing Terrestrial Radiation

(Absorbed Solar Radiation) = (Emitted Terrestrial Radiation)

This balance is crucial for maintaining Earth's climate.

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

While the Earth as a whole maintains a heat balance, the distribution of absorbed solar radiation and emitted terrestrial radiation varies significantly across different latitudes and surfaces. This variation leads to differences in net heat budget, driving atmospheric and oceanic circulation.

• Latitudinal Variation:
  • Low Latitudes (Equator to Tropics): Receive more direct insolation and have a positive net heat budget over the year. They absorb more solar energy than they radiate away.
  • High Latitudes (Poles): Receive less direct insolation and have a negative net heat budget over the year. They radiate away more energy than they absorb.
  This latitudinal gradient in net heat is the fundamental driver for global atmospheric and oceanic circulation, as heat is transported from the tropics towards the poles.
• Surface Characteristics:
  • Albedo: Surfaces with high albedo (e.g., snow-covered land) reflect more solar radiation and absorb less, leading to a lower net heat gain. Surfaces with low albedo (e.g., oceans, dark soils) absorb more radiation, resulting in a higher net heat gain.
  • Vegetation: Dense vegetation can influence the heat budget through shading (reducing absorption) and evapotranspiration (cooling effect).
  • Water Bodies: Oceans have a high heat capacity and can store large amounts of heat, moderating regional temperatures and influencing the heat budget through evaporation and currents.
• Seasonal Variation: The net heat budget at any location changes significantly between summer and winter due to variations in insolation (angle and duration of daylight) caused by the Earth's axial tilt.

These variations in the net heat budget are responsible for creating different climate zones and driving weather patterns.

Temperature

Factors Controlling Temperature Distribution

The distribution of temperature across the Earth's surface is influenced by a complex interplay of factors that affect the amount of solar radiation absorbed and retained:

• Latitude: This is the most significant factor. As latitude increases, the angle of incidence of solar rays decreases, spreading the energy over a larger area and reducing its intensity. Additionally, the path length through the atmosphere increases, leading to more scattering and absorption. This results in generally warmer temperatures at the equator and cooler temperatures at the poles.
• Altitude: Temperature decreases with increasing altitude in the troposphere. This is due to the normal lapse rate, where the atmosphere is heated primarily from the Earth's surface. As altitude increases, the air becomes thinner and less able to absorb heat, and the surface is further away from the primary heat source.
• Distance from the Sea (Continentality): Water heats up and cools down much more slowly than land (due to water's high specific heat capacity). Coastal regions experience more moderate temperatures with smaller annual and diurnal ranges (maritime climate) compared to inland areas, which experience greater temperature extremes (continental climate).
• Ocean Currents: Warm ocean currents can significantly warm the coastal areas they flow past, raising temperatures compared to adjacent regions at the same latitude. Conversely, cold ocean currents can cool coastal areas. For example, the Gulf Stream moderates the climate of Western Europe, making it warmer than regions at similar latitudes in North America.
• Prevailing Winds: Winds blowing from warmer regions (e.g., from over oceans in summer) can raise local temperatures, while winds from colder regions (e.g., from continental interiors in winter) can lower them.
• Aspect: The direction a slope or land surface faces influences the amount of direct sunlight it receives. In the Northern Hemisphere, south-facing slopes receive more direct sunlight and are therefore warmer than north-facing slopes. The opposite is true in the Southern Hemisphere.
• Topography: Mountain ranges can block the movement of air masses, influencing temperature. Valleys may trap warmer or cooler air depending on the conditions.
• Cloud Cover and Albedo: High cloud cover reduces the amount of insolation reaching the surface, leading to cooler daytime temperatures. Areas with high albedo (like snow and ice) reflect more solar radiation, keeping them cooler.
• Human Activities: Urban areas, with their heat-absorbing surfaces (concrete, asphalt) and reduced vegetation, often experience higher temperatures than surrounding rural areas (the urban heat island effect).

Distribution Of Temperature

The distribution of temperature across the Earth is typically shown on isothermal maps, which use lines of equal temperature (isotherms).

• Equatorial Regions: Experience consistently high temperatures throughout the year due to direct insolation.
• Subtropical Regions: Have warm to hot temperatures, with higher annual ranges than equatorial regions.
• Mid-Latitudes: Experience distinct seasons with significant variations in temperature between summer and winter.
• Polar Regions: Experience extremely low temperatures year-round, with very short periods of milder temperatures.

Key Observations from Isothermal Maps:

• West-East Temperature Gradient in Mid-Latitudes: Temperatures generally decrease from west to east in the mid-latitudes of the Northern Hemisphere (e.g., North America, Eurasia) during winter. This is because westerly winds bring moderating maritime air from the Atlantic to Western Europe, while continental interiors of North America and Asia become much colder.
• North-South Temperature Gradient: A clear decrease in temperature from the equator towards the poles is evident on global temperature maps, primarily driven by latitude.
• Influence of Ocean Currents: Isotherms are deflected by ocean currents. They bend poleward along warm currents and equatorward along cold currents, indicating the moderating influence of oceans.
• Continentality: Landmasses show larger annual temperature ranges than oceans. The interiors of continents are generally colder in winter and hotter in summer than coastal regions at the same latitude.
• Altitude: Mountainous regions show lower temperatures than surrounding lowlands, even at similar latitudes.

Global Temperature Patterns:

Temperature generally decreases from the equator towards the poles. However, this pattern is modified by land-ocean distribution, ocean currents, prevailing winds, and altitude, creating complex regional variations.

Inversion Of Temperature

Definition: Temperature inversion is an atmospheric condition where temperature increases with altitude, rather than decreasing. This is contrary to the normal lapse rate observed in the troposphere, where temperature typically decreases with height.

Conditions for Inversion: Temperature inversions usually occur under conditions of clear skies, light winds, and long winter nights, which promote radiative cooling of the surface and the air layer immediately above it.

Types of Temperature Inversions:

1. Radiational Inversion:
   • Formation: Occurs on clear, calm nights, especially during winter. The ground surface rapidly loses heat by radiation. The air layer closest to the ground cools by conduction, becoming colder than the air above it.
   • Characteristics: Typically forms in valleys or low-lying areas where cold, dense air can accumulate. It is a temporary phenomenon, usually dissipating with the rising sun and increased atmospheric mixing.
   • Effects: Can lead to fog formation and trap pollutants near the surface.
2. Advectional Inversion:
   • Formation: Occurs when warm, moist air moves horizontally (advects) over a cooler surface (e.g., a cold ocean current, a snow-covered landmass). The lower layer of the warm air cools by contact with the cold surface, creating an inversion.
   • Examples: Coastal regions experiencing sea breezes in summer, or warm Chinook winds flowing over snow-covered mountains.
3. Frontal Inversion:
   • Formation: Occurs along weather fronts, particularly warm fronts. When a warm air mass overrides a colder, denser air mass, a stable layer with a temperature inversion is formed at the boundary.
   • Characteristics: The warm air is forced upwards, leading to cloud formation and precipitation.
4. Subsidence Inversion:
   • Formation: Occurs in high-pressure systems where large masses of air descend (subsidence). As the air sinks, it is compressed and warms adiabatically. The lower layers of this sinking air remain cooler and denser, while the upper layers become warmer, creating a stable inversion.
   • Characteristics: Often associated with stagnant air conditions and the trapping of pollutants over large areas.

Significance and Effects of Temperature Inversions:

• Atmospheric Stability: Inversions create very stable atmospheric conditions because the warmer, less dense air is trapped above the cooler, denser air. This inhibits vertical air movement (convection).
• Trapping of Pollutants: Due to the lack of vertical mixing, pollutants emitted near the ground (from vehicles, industries) can become concentrated in the lower atmosphere, leading to poor air quality and smog formation, especially during prolonged inversions.
• Fog and Low Clouds: Radiational inversions often lead to the formation of fog or low stratus clouds as the moist air near the surface cools to its dew point.
• Precipitation Patterns: Inversions can affect the type and intensity of precipitation. For example, in winter, rain can fall on top of an inversion layer, while the ground remains below freezing, leading to freezing rain.
• Weather Forecasting: Understanding temperature inversions is crucial for weather forecasting, particularly for predicting air quality, fog, and precipitation types.