Three main types of electromagnetic radiation reach Earth’s surface: visible light, some infrared radiation, and some ultraviolet radiation. A narrow band of radio waves also passes through the atmosphere from space, though the sun’s energy at the surface is dominated by visible and near-infrared light. Everything else, including gamma rays, X-rays, and most far-infrared radiation, gets absorbed or scattered by the atmosphere before it can reach the ground.
The atmosphere acts like a selective filter. Scientists call the gaps where radiation passes through “atmospheric windows,” and understanding these windows explains why life on Earth receives the energy it does while staying shielded from the radiation that would destroy it.
The Optical Window: Visible Light
Visible light, the narrow band of wavelengths your eyes can detect, passes through the atmosphere almost entirely. It spans roughly 380 to 700 nanometers, from violet at the short end to red at the long end. This is the single largest source of solar energy reaching the ground, and it’s no coincidence that human vision evolved to use exactly the wavelengths the atmosphere lets through most freely.
The atmosphere does scatter some visible light. Short wavelengths (blue and violet) scatter more than long ones (red and orange), which is why the sky looks blue. But scattering redirects the light rather than eliminating it. On a clear day, the vast majority of visible sunlight reaches the surface either directly or as diffuse light bouncing around overhead.
Ultraviolet Radiation: Mostly Filtered
UV radiation sits just below visible light on the spectrum, ranging from 100 to 400 nanometers, and the atmosphere treats its three subtypes very differently.
- UVA (315 to 400 nm) passes through the ozone layer essentially unaffected. It makes up about 95% of the UV radiation that reaches the ground.
- UVB (280 to 315 nm) is strongly absorbed by stratospheric ozone, but a small fraction gets through. It accounts for roughly 5% of surface UV and is the primary cause of sunburn.
- UVC (100 to 280 nm) is almost entirely absorbed by the ozone layer and upper atmosphere. It is not present in natural sunlight at the surface.
Altitude changes the balance. For every 1,000 meters of elevation gain, UV radiation intensity increases by about 12%. At high-altitude locations, you’re exposed to meaningfully more UVB because there’s less atmosphere overhead to filter it. This is why sunburn happens faster in the mountains even when temperatures feel cool.
Infrared Radiation: A Partial Window
Near-infrared radiation, the wavelengths just longer than visible red light, reaches the surface in large quantities. Together with visible light, it makes up most of the sun’s energy at ground level. You can’t see near-infrared, but you feel it as warmth on your skin.
Farther into the infrared spectrum, the atmosphere becomes much less transparent. Water vapor, carbon dioxide, and ozone absorb strongly across broad infrared bands. There is a notable gap in this absorption, roughly between 8 and 14 micrometers, called the infrared atmospheric window. This window matters more for outgoing radiation than incoming: it’s the channel through which heat radiated by Earth’s surface escapes to space, playing a central role in regulating the planet’s temperature. Greenhouse gases work by narrowing this window, trapping more of that outgoing heat.
Radio Waves: The Other Window
Far from the visible spectrum, the atmosphere opens a second major window for radio waves. Frequencies from about 5 megahertz to over 300 gigahertz (wavelengths from nearly 100 meters down to about 1 millimeter) pass through relatively freely. This radio window is why ground-based radio telescopes can observe the universe without needing to be in space, and it’s the foundation of all terrestrial radio communication, Wi-Fi, and cellular networks.
Below about 5 MHz, the ionosphere reflects radio waves back rather than letting them through. Above 300 GHz, water vapor in the atmosphere starts absorbing strongly. So the radio window, like the optical window, has defined edges.
What the Atmosphere Blocks Completely
Gamma rays and X-rays, the highest-energy forms of electromagnetic radiation, never reach the surface. They interact with atoms in the upper atmosphere through processes that strip electrons from gas molecules, transferring their energy into the air long before they get anywhere near the ground. This is why X-ray and gamma-ray telescopes must operate from satellites in orbit.
Most far-infrared and microwave radiation outside the radio window is also absorbed, primarily by water vapor. The atmosphere is effectively opaque across wide stretches of these wavelengths.
How Clouds and Conditions Change the Picture
Even within the atmospheric windows, what actually reaches the surface on any given day depends heavily on local conditions. Clouds can weaken incoming solar radiation by up to 80%, depending on their type, thickness, and water content. Low and mid-level clouds like stratus and stratocumulus, which are rich in liquid water droplets, block the most sunlight. High, thin clouds like cirrus, composed mostly of ice crystals, let considerably more through.
Cloud cover doesn’t just reduce light uniformly. It shifts the balance between direct and diffuse radiation. Broken clouds can actually increase the diffuse component through lateral scattering, sometimes creating brief moments where ground-level light intensity spikes above what a clear sky would deliver. Aerosols, dust, and pollution add another layer of filtering, scattering and absorbing light in ways that vary by region and season.
The overall amount of solar radiation reaching the surface has fluctuated over recent decades, influenced by changing aerosol levels from industrial emissions, shifts in cloud patterns, and interactions between all of these factors. These trends vary significantly by region, with some areas brightening as air pollution declines and others dimming as cloud cover increases.