Solar radiation is the energy emitted by the sun in the form of electromagnetic waves. It travels about 150 million kilometers to reach Earth, arriving at an intensity of roughly 1,361 watts per square meter at the top of the atmosphere. That energy drives weather, ocean currents, plant growth, and nearly every natural process on the planet’s surface.
What Solar Radiation Is Made Of
The sun doesn’t emit just one type of energy. Its output spans a wide range of the electromagnetic spectrum, but three categories account for almost all of it. About 49% of solar radiation arrives as infrared energy, which you feel as heat. Roughly 43% falls in the visible light range, the narrow band your eyes can detect. Around 7% is ultraviolet radiation, responsible for sunburns and vitamin D production. The remaining fraction, less than 1%, consists of X-rays, gamma rays, and radio waves.
Each type of radiation behaves differently when it hits the atmosphere, the ground, or your skin. Infrared radiation warms surfaces and is central to how Earth retains heat. Visible light powers photosynthesis and illuminates the planet. Ultraviolet radiation carries the most energy per photon of the three main types, which is why even a small percentage can have outsized biological effects.
How Much Energy Reaches the Ground
The standard measure of solar energy arriving at Earth is called total solar irradiance, sometimes referred to as the solar constant. NASA’s most recent satellite measurements place it at 1,361.6 watts per square meter during the 2019 solar minimum. That value isn’t truly constant. It fluctuates by about 0.1% over an 11-year solar cycle, rising to roughly 1,362 watts per square meter during active phases and dipping to about 1,360 during quiet ones.
Not all of that energy makes it to the surface. Earth’s average albedo is about 0.3, meaning the planet reflects roughly one-third of incoming sunlight back into space. Fresh snow reflects 80 to 90% of sunlight, while open ocean absorbs almost all of it, reflecting only about 6%. Fresh asphalt reflects just 4%. These differences matter: as Arctic ice melts and dark ocean water replaces bright snow, the surface absorbs more energy, accelerating warming.
Several factors determine how intense solar radiation is at any given spot on the ground. Near the equator, sunlight strikes almost perpendicularly, concentrating energy over a smaller surface area. At higher latitudes, the same beam spreads across a wider area, delivering less heat per square meter. This is the fundamental reason tropical regions are warmer than polar ones, even though high-latitude locations get more daylight hours in summer. Altitude also plays a role: thinner atmosphere at elevation means less filtering, so solar radiation is more intense on a mountaintop than at sea level.
How UV Radiation Affects Your Skin
The ultraviolet portion of solar radiation splits into three subtypes based on wavelength, and each one interacts with your body differently.
UVC has the shortest wavelength and the highest energy, but the ozone layer absorbs it completely. It never reaches the ground under normal conditions. UVB penetrates the atmosphere in small amounts and reaches the outermost layer of your skin (the epidermis). This is the radiation responsible for sunburn, DNA damage in skin cells, and the production of reactive oxygen species, which are unstable molecules that can harm cell structures. UVB also triggers vitamin D synthesis, so brief exposure serves a biological purpose. UVA has a longer wavelength, penetrates deeper into the skin (reaching the dermis, the layer beneath the surface), and is the primary driver of premature aging. Chronic UVA exposure thickens the outer skin layer and breaks down collagen over time.
The World Health Organization uses the UV Index to communicate daily risk. A UV Index of 0 to 2 poses limited danger even for fair-skinned people, and no special protection is needed. At 3 to 7, you should seek shade during midday, wear a hat and shirt, and apply sunscreen. At 8 and above, the WHO recommends avoiding outdoor exposure during midday hours entirely. Protective clothing, sunscreen, sunglasses, and shade all become essential at that level.
Solar Radiation and Earth’s Climate
Solar radiation is the input side of Earth’s energy budget. The planet absorbs sunlight, warms up, and re-emits energy as infrared radiation. Greenhouse gases in the atmosphere, including carbon dioxide, methane, and water vapor, absorb some of that outgoing infrared energy and radiate it back toward the surface. This process, called the greenhouse effect, keeps Earth’s average temperature about 33°C warmer than it would be without an atmosphere.
When the balance between incoming solar energy and outgoing infrared energy shifts, the climate responds. Adding greenhouse gases to the atmosphere traps more outgoing heat, creating what scientists call radiative forcing. Each gas contributes a measurable amount of forcing, measured in watts per square meter, which allows researchers to rank their relative impact. The 0.1% variation in the sun’s output over its 11-year cycle produces a small forcing of its own, but it is far smaller than the forcing from accumulated greenhouse gas emissions over the industrial era.
How Solar Panels Convert Sunlight to Electricity
Solar panels exploit the fact that photons carry energy. Each panel contains cells made of semiconductor material, typically silicon. When photons from sunlight strike a cell, three things can happen: they bounce off, pass through, or get absorbed. Only absorbed photons contribute to electricity generation.
When enough photons are absorbed, they knock electrons loose from atoms in the semiconductor. The cell’s surface is specially treated during manufacturing so that these freed electrons naturally migrate toward the front. This creates an imbalance of electrical charge between the front and back of the cell, similar to the positive and negative terminals of a battery. Metal conductors on the cell collect the electrons, and when connected to a circuit, the flow of electrons becomes usable electricity. The process is silent, has no moving parts, and works with both direct and diffuse sunlight, though output drops on cloudy days or at steep sun angles.
Because solar radiation intensity varies by latitude, season, and cloud cover, the same panel produces very different amounts of electricity depending on where it’s installed. A rooftop system in Arizona will generate roughly twice the annual output of an identical system in Seattle, largely because of differences in solar angle and cloud frequency.