Solar radiation

What is solar radiation made of?

Solar radiation is composed of different types of electromagnetic waves, each defined by its wavelength and energy level. The three most relevant types for life on Earth are ultraviolet (UV) radiation, visible light, and infrared (IR) radiation.

Ultraviolet (UV) radiation

UV radiation has shorter wavelengths and higher energy than visible light. It is divided into three types:

• UVA: Penetrates deeply into the skin and is the most abundant UV radiation reaching Earth’s surface.
• UVB: Partially absorbed by the ozone layer. It supports vitamin D production but can also cause sunburn and DNA damage.
• UVC: The most energetic form, but it is almost entirely absorbed by the atmosphere and rarely reaches the ground.

Though UV radiation makes up a small fraction of total solar output, it plays an outsized role in human health, ecological processes, and atmospheric chemistry.

Visible light

This is the range of solar radiation that our eyes can detect, spanning colors from violet to red. Visible light powers photosynthesis, the process by which plants produce food and oxygen, forming the foundation of nearly every food chain. It also enables vision and contributes to the warming of surfaces it strikes.

Infrared (IR) radiation

IR radiation has longer wavelengths than visible light and is experienced as heat. It plays a central role in warming the Earth’s surface and atmosphere, especially through absorption and re-emission by greenhouse gases like carbon dioxide, methane, and water vapor. This contributes significantly to Earth’s energy balance and supports the planet’s habitable temperatures.

How is solar radiation different from sunlight and insolation?

It’s important to distinguish between solar radiation and related terms:

• Sunlight refers specifically to the solar radiation that reaches Earth’s surface. It includes a mix of visible light, some UV, and some IR. So, while all sunlight is solar radiation, not all solar radiation reaches the ground.
• Insolation is the amount of solar energy received by a specific area over a set period, often expressed in watts per square meter (W/m²). This term is especially useful in scientific studies and solar energy planning.

How does Earth's atmosphere affect solar radiation?

As solar radiation travels through the atmosphere, it undergoes several interactions that determine how much and what kind of energy ultimately reaches the surface.

Absorption

Certain atmospheric gases absorb specific wavelengths. For example:

• Ozone absorbs most UVB and nearly all UVC, shielding living organisms from harmful radiation.
• Water vapor and carbon dioxide absorb infrared radiation, helping to retain heat and maintain Earth’s temperature.

Scattering

Molecules and tiny particles (aerosols) scatter sunlight, especially shorter blue wavelengths—this is why the sky appears blue. Scattering reduces the intensity of direct sunlight but increases diffuse light, which still contributes to total illumination.

Reflection

Surfaces like clouds, snow, ice, and deserts reflect incoming solar radiation back into space. The percentage reflected is called albedo:

• High albedo: Snow and ice (reflect 80–90%)
• Low albedo: Forests and oceans (reflect 10–20%)

Albedo plays a key role in Earth's climate system by affecting how much energy is absorbed versus reflected.

What affects how much solar radiation reaches the surface?

Several factors influence the intensity and availability of solar radiation at any location:

• Time of day: Radiation is strongest at solar noon, when the Sun is highest in the sky.
• Latitude: Equatorial regions receive more consistent and direct solar radiation year-round. Polar regions receive less due to low Sun angles and extended periods of darkness in winter.
• Season: Earth’s axial tilt causes seasonal changes. In summer, the Sun is higher in the sky and days are longer, increasing solar exposure.
• Cloud cover: Clouds block or scatter sunlight, reducing direct radiation. However, some diffuse radiation still reaches the ground.
• Elevation: Higher altitudes receive more intense radiation due to thinner atmosphere and less absorption.

How is solar radiation measured?

Accurate measurement of solar radiation is crucial for climate science, agriculture, and solar energy systems. Several instruments are used:

• Pyranometers: Measure total solar radiation (direct + diffuse) on a flat surface.
• Pyrheliometers: Measure direct radiation from the Sun.
• Radiometers: Measure specific types of radiation, such as UV or IR.
• Satellites: Observe solar radiation on a global scale and support climate modeling.

Measurements are commonly expressed in:

• W/m² (watts per square meter): Indicates instantaneous power—how much energy is hitting a surface at a given moment.
• kWh/m²/day (kilowatt-hours per square meter per day): Represents total energy received in a day, useful for energy planning.

How does solar radiation drive Earth's systems?

Solar radiation is the primary energy input for Earth’s climate, weather, and ecosystems.

Climate and weather

Uneven heating of the Earth’s surface creates temperature gradients that generate wind, ocean currents, and convection. Solar energy drives the water cycle by causing evaporation, cloud formation, and precipitation. Seasonal and geographic variations in solar input explain weather patterns, including monsoons and El Niño events.

Earth’s energy balance

Solar radiation is absorbed by the Earth and re-emitted as infrared radiation. Greenhouse gases trap some of this heat, maintaining a stable climate. This natural greenhouse effect keeps Earth's average temperature well above freezing and supports a wide range of life. Without it, the planet would be inhospitably cold.

How does solar radiation affect life on Earth?

Solar radiation shapes the health and behavior of humans, plants, and animals.

Human health

• Benefits:
  
  • UVB enables the skin to produce vitamin D, important for bone strength and immune function.
  • Visible light regulates circadian rhythms, influencing sleep and mood.
• Risks:
  
  • Excessive UV exposure causes sunburn, skin aging, eye damage (e.g., cataracts), and increases the risk of skin cancer.
  • UV radiation is classified as a Group 1 carcinogen by the World Health Organization (WHO).

Protection includes sunscreen, UV-blocking sunglasses, protective clothing, and seeking shade during peak sunlight.

Ecosystems and agriculture

Solar radiation supports ecosystems primarily through photosynthesis, where plants convert light into chemical energy, forming the base of food webs.

In agriculture, sunlight influences:

• Growth rates and biomass
• Photoperiod, which affects flowering and fruiting
• Crop quality and yield

Too little light reduces productivity, while too much can cause photoinhibition or sunburn on leaves and fruit. Farmers manage exposure through spacing, shading, and sometimes artificial lighting to optimize yields.

Understanding solar radiation's impact

Solar radiation is more than just the warmth we feel on a sunny day—it is the engine of life, weather, and energy systems on Earth. It drives the water cycle, powers photosynthesis, and maintains a livable climate. It influences everything from crop yields to human health and offers a vast, renewable energy resource for the future.

As we face challenges like climate change, energy transition, and food security, understanding and managing solar radiation becomes ever more important. Harnessing its power wisely can help us build a more sustainable, resilient, and life-supporting planet.