TERRESTRIAL RADIATION

Terrestrial radiation refers to the longwave infrared radiation emitted by the Earth’s surface and atmosphere. This radiation plays a crucial role in the Earth’s energy balance and climate system, influencing temperatures and weather patterns.

Basics of Terrestrial Radiation

1. Definition:
   • Terrestrial radiation is the infrared radiation that the Earth emits as it cools down. It is also known as longwave radiation.
2. Wavelength:
   • Typically ranges from 4 to 100 micrometers (µm).
3. Source:
   • The Earth’s surface, atmosphere, and clouds emit terrestrial radiation.

Mechanism of Terrestrial Radiation

1. Absorption and Emission:
   • The Earth absorbs shortwave solar radiation, which heats the surface.
   • This absorbed energy is then re-emitted as longwave infrared radiation.
2. Greenhouse Effect:
   • Certain gases in the atmosphere, known as greenhouse gases (e.g., carbon dioxide, water vapor, methane), absorb terrestrial radiation and re-emit it in all directions, including back towards the Earth’s surface.
   • This process traps heat in the atmosphere, warming the planet. This natural greenhouse effect is essential for maintaining life-friendly temperatures on Earth.

Energy Balance

1. Incoming Solar Radiation:
   • The Earth receives energy from the Sun in the form of shortwave radiation.
2. Outgoing Terrestrial Radiation:
   • To maintain energy balance, the Earth must emit an equal amount of energy back into space as longwave radiation.
3. Net Radiation:
   • The difference between incoming solar radiation and outgoing terrestrial radiation determines whether the Earth is warming or cooling.

Factors Affecting Terrestrial Radiation

1. Surface Temperature:
   • Warmer surfaces emit more terrestrial radiation.
   • Example: Deserts emit large amounts of infrared radiation due to high surface temperatures.
2. Surface Characteristics:
   • Different surfaces (water, soil, vegetation) have different emissivities, affecting the amount of radiation they emit.
   • Example: Water bodies generally have higher emissivities compared to bare soil, leading to more efficient radiation.
3. Atmospheric Composition:
   • Greenhouse gases absorb and re-emit terrestrial radiation, influencing the overall radiation budget.
   • Example: Regions with high concentrations of greenhouse gases (e.g., urban areas with pollution) experience greater atmospheric absorption and re-emission.
4. Cloud Cover:
   • Clouds absorb and emit longwave radiation, impacting the radiation balance.
   • Example: Cloudy nights are often warmer than clear nights because clouds trap terrestrial radiation and re-emit it back to the surface.

Examples of Terrestrial Radiation

Example 1: Desert Environment

• Region: Sahara Desert
• Characteristics: High daytime temperatures lead to significant absorption of solar radiation.
• Terrestrial Radiation: At night, the desert emits large amounts of terrestrial radiation due to the high surface temperature. However, with minimal moisture and greenhouse gases, much of this radiation escapes directly to space, leading to rapid cooling at night.

Example 2: Urban Heat Island

• Region: Metropolitan areas (e.g., New York City)
• Characteristics: High levels of concrete and asphalt absorb solar radiation and have high thermal masses.
• Terrestrial Radiation: These materials emit substantial terrestrial radiation at night. The presence of buildings and pollution increases the concentration of greenhouse gases and particulates, trapping more radiation and causing the urban heat island effect, where cities are warmer than surrounding rural areas.

Example 3: Polar Regions

• Region: Arctic and Antarctic
• Characteristics: Low-angle sunlight results in less absorbed solar radiation.
• Terrestrial Radiation: Despite low surface temperatures, the polar regions emit terrestrial radiation. However, snow and ice have high albedo, reflecting much of the incoming solar radiation, and the cold atmosphere results in less emitted radiation compared to warmer regions.

Importance of Terrestrial Radiation

1. Climate Regulation:
   • Terrestrial radiation is a key component of the Earth’s energy budget, regulating the planet’s temperature.
2. Weather Patterns:
   • Variations in terrestrial radiation affect atmospheric temperatures and pressure, influencing weather systems and patterns.
3. Agriculture and Ecosystems:
   • Understanding terrestrial radiation helps in managing agricultural practices and predicting ecosystem responses to temperature changes.
4. Climate Change:
   • Increased greenhouse gas concentrations enhance the greenhouse effect, altering the balance of terrestrial radiation and contributing to global warming.

Conclusion

Terrestrial radiation is a fundamental aspect of the Earth’s energy balance and climate system. It represents the Earth’s way of losing energy it gains from the Sun, and it plays a critical role in maintaining temperatures conducive to life.