Solar Radiation, Heat Balance, and Temperature

Learning Outcomes

1. Understand the variability of insolation at the Earth’s surface.
2. Recognize the processes involved in the heating and cooling of the atmosphere.
3. Learn the components of the Earth’s heat budget.
4. Identify factors controlling temperature distribution and inversion phenomena.

Solar Radiation and Insolation

Solar radiation, referred to as insolation, is the primary source of energy for Earth’s atmosphere and surface. The Earth’s energy balance relies on the shortwave radiation it receives from the sun and the longwave radiation it emits back into space. This continuous cycle of absorption and radiation ensures that the Earth neither warms up nor cools down excessively.

Variability of Insolation at Earth’s Surface

The energy received from the sun is not uniform across the Earth’s surface, which is attributed to various factors:

1. Earth’s Rotation: The Earth rotates on its axis, causing daily variations in the intensity of insolation at any point.
2. Angle of Sun’s Rays: The angle at which the sun’s rays hit the Earth varies by latitude, with steeper angles at higher latitudes, resulting in lower insolation.
3. Length of Day: The duration of daylight affects the amount of solar radiation received. Longer days result in more insolation.
4. Atmospheric Transparency: The atmosphere’s ability to transmit sunlight also varies based on factors like cloud cover and the presence of gases or particles.
5. Topography: The configuration of the land, including its aspect and elevation, influences the intensity of insolation received.

The Passage of Solar Radiation through the Atmosphere

Solar radiation passes through the atmosphere largely unimpeded. However, various atmospheric components absorb, reflect, and scatter radiation:

1. Absorption by Gases: Water vapor, ozone, and other gases in the troposphere absorb near-infrared radiation.
2. Scattering: Small particles in the atmosphere scatter sunlight, which contributes to the coloration of the sky. For example, Rayleigh scattering is responsible for the blue sky, while longer wavelengths such as red are scattered during sunrise and sunset.

Note: The Earth receives about 1.94 calories per square centimeter per minute at the top of the atmosphere.

Spatial Distribution of Insolation

The distribution of solar radiation across the Earth’s surface is not uniform. Insolation varies significantly from the equator to the poles. Maximum insolation is found in the subtropical deserts where cloud cover is minimal, while polar regions receive the least insolation. During winter, middle and higher latitudes experience a significant reduction in solar radiation, contributing to colder temperatures.

Heating and Cooling of the Atmosphere

The Earth’s surface heats the atmosphere, but the heating and cooling processes involve multiple mechanisms:

1. Conduction: Heat is transferred from the Earth’s surface to the atmosphere through direct contact between the land and air molecules. This process heats the lower atmosphere but is less effective for upper layers.
2. Convection: The vertical movement of heated air from the surface to higher altitudes spreads heat throughout the atmosphere. This process, known as convective transfer, is restricted to the troposphere.
3. Advection: Heat is also transferred horizontally by the movement of air masses, which plays a significant role in temperature variations, especially in middle latitudes. For instance, the hot winds known as loo in northern India are a result of advection.

Terrestrial Radiation and Heat Balance

After the Earth’s surface absorbs solar radiation, it radiates this energy back to the atmosphere in the form of longwave radiation. Atmospheric gases such as carbon dioxide and water vapor absorb this energy, warming the atmosphere. This process is known as terrestrial radiation.

The Earth maintains its temperature by balancing the energy it receives with the energy it radiates:

1. Heat Budget: The Earth’s heat budget ensures that the total amount of energy received from the sun equals the energy radiated back into space. Of the energy that reaches the Earth’s atmosphere, about 35% is reflected back, and the remaining 65% is absorbed by the Earth’s surface and atmosphere.

Note: The reflection of solar radiation is termed albedo, with 27% of the reflected radiation coming from clouds.

Variation in the Net Heat Budget

There is a surplus of radiation near the tropics, while polar regions face a deficit. The excess heat from tropical areas is transferred towards the poles, preventing continuous warming in the tropics and excessive cooling in polar regions. This transfer of heat ensures a global balance of temperatures.

Factors Controlling Temperature Distribution

The temperature at any location on Earth depends on several factors:

1. Latitude: The amount of solar radiation received varies with latitude. Equatorial regions receive more heat, while polar regions receive less due to the angle of incoming solar rays.
2. Altitude: Temperature decreases with increasing altitude. The normal lapse rate is about 6.5°C per 1000 meters.
3. Proximity to Water Bodies: Water heats and cools more slowly than land. As a result, coastal areas experience moderated temperatures, while inland areas experience more extreme temperatures.
4. Ocean Currents: Warm ocean currents raise the temperature of nearby landmasses, while cold currents reduce temperatures.
5. Air-Mass Movement: The movement of warm or cold air masses can significantly affect local temperatures.

Global Distribution of Temperature

Temperature distribution across the globe is typically represented using isotherms, which are lines that connect points of equal temperature. Isotherms tend to run parallel to latitudes but are influenced by factors like land-sea contrasts and ocean currents. For instance, in January, the North Atlantic is warmer due to the Gulf Stream, while Siberia experiences extremely cold temperatures.

Inversion of Temperature

Under normal circumstances, temperature decreases with altitude. However, during temperature inversion, this pattern is reversed, and the air at higher altitudes is warmer than the air at the surface. Temperature inversion typically occurs:

1. During Cold Winter Nights: When the surface cools rapidly, trapping cold air beneath warmer air.
2. In Mountain Valleys: Air drainage causes cold air to accumulate in valleys, leading to temperature inversion.

Important Note: Inversion can trap dust and pollutants, leading to smog formation, particularly in urban areas.

Comprehensive Temperature Comparison

Processes Maintaining the Earth’s Heat Balance

The Earth-atmosphere system maintains its heat balance through a combination of processes:

1. Absorption: Solar radiation is absorbed by the Earth’s surface and atmospheric components like water vapor.
2. Reflection and Scattering: A portion of solar radiation is reflected back into space, while scattering by atmospheric particles contributes to phenomena like the blue sky.
3. Radiation: The Earth radiates energy back to the atmosphere in longwave forms, which is absorbed by greenhouse gases.
4. Heat Transfer: Conduction, convection, and advection work together to transfer heat within the atmosphere, distributing it across different regions and layers.

Inversion of Temperature: Impact and Causes

Temperature inversion occurs when surface air is cooler than the air above, leading to various atmospheric effects:

1. Air Stability: Inversion creates a stable atmosphere, preventing the vertical mixing of air.
2. Fog Formation: Inversions can cause the accumulation of fog, especially in valleys and low-lying areas.
3. Air Pollution: Trapped pollutants below the inversion layer can result in smog and poor air quality.

MCQ

Question: Which of the following processes primarily heats the atmosphere?
(a) Shortwave solar radiation
(b) Reflected solar radiation
(c) Longwave terrestrial radiation
(d) Scattered solar radiation
Answer: (c) Longwave terrestrial radiation