Understanding Solar Radiation and Irradiance

Solar Radiation Explained

• Solar radiation is electromagnetic waves of light from the Sun, not harmful radiation like gamma rays.
• Solar radiation comprises visible light, infrared light, and ultraviolet light.
• Solar irradiance measures the amount of solar energy entering Earth, measured in watts per square meter or kilowatt-hours per square meter.

Solar Irradiance

• Solar irradiance data can be mapped to show energy absorption across regions, like the USA.
• This data is usually measured per year in kilowatt-hours per square meter, but daily measurements are often in watts per square meter.

Sun's Electromagnetic Spectrum

• The Sun emits energy across the electromagnetic spectrum, from radio waves to gamma rays, though mostly in the visible light portion.
• The visible light portion of the spectrum is what humans can perceive; red to violet.
• Infrared light has lower frequencies than visible light, while ultraviolet light has higher frequencies.

Different Types of Solar Radiation

• The Sun emits most of its energy as visible light.
• The atmosphere blocks some ultraviolet light.
• A significant amount of infrared light reaches Earth’s surface.
• Infrared Rays: Invisible light with lower frequency than visible light, higher than microwaves, ranging from 750 nm to 2.5 μm. Felt as warmth from heated objects.
• Visible Rays: Light visible to the human eye, in the middle of the electromagnetic spectrum, between infrared and ultraviolet. Wavelengths range from approx 400-750 nm, and are further categorized into colours by wavelength: Red (700 nm), Orange (600 nm), Yellow (550 nm), Green (500 nm), Blue (450 nm), Violet (400 nm).
• Ultraviolet Rays: Invisible light with higher frequency than visible light. UV light is grouped as UV-A (harmless, above 300 nm) and UV-B and UV-C (potentially harmful, below 300 nm, causing sunburns and, at high levels, potentially skin cancer).

Effects of Solar Radiation on Earth

• Incoming solar radiation provides warmth, sustains the water cycle, and contributes to wind patterns.
• Without the Sun, Earth would be significantly colder.
• Earth's seasons are caused by the tilt of Earth's axis, not variations in sunlight or distance from the Sun.
• Earth itself emits infrared radiation, and some of this radiation is absorbed by the atmosphere. This is the "greenhouse effect".
• Human activities have changed the composition of Earth’s atmosphere causing an increase in greenhouse gases, which trap more outgoing infrared radiation and lead to global warming.