Physics: How Ozone Closes Earth's Infrared Window

The particulate nature of matter explains how the microscopic behaviour of gas molecules governs macroscopic phenomena like Earth’s energy balance. A central idea is that each atmospheric gas absorbs infrared radiation only at specific wavelengths, determined by its molecular vibrations and rotations. This selectivity creates the “atmospheric window,” a spectral gap from roughly 8 to 12 μm where outgoing terrestrial radiation can escape to space, keeping the planet cool. Within this window, however, certain trace gases still intercept energy. Ozone (O₃) possesses a vibrational absorption band centred near 9.6 μm, which partially closes the window by trapping a slice of outgoing radiation. This contrasts with water vapour (H₂O), which absorbs strongly across most infrared wavelengths but less so inside the window, and carbon dioxide (CO₂), whose principal band sits near 15 μm, outside the window. Methane (CH₄) also has bands elsewhere. The relationship between wavelength and energy follows E = hc/λ, so shorter wavelengths carry more energy per photon. Understanding which gas absorbs where—and how those bands overlap with the window—is essential for modelling radiative forcing, climate feedbacks, and the greenhouse effect.