The phenomena that brighten dawn and dusk (2016)

Dawn and dusk colors arise from simple geometry and selective scattering as the spinning Earth makes the sun appear to rise and set (≈0.25° per minute). When the sun is near the horizon, sunlight traverses a much longer, slanted path through a thin atmospheric shell concentrated within roughly 10 miles of the surface; the horizon can be 50-100 miles away, so sunlight at the horizon passes about 40 times more air than at noon. Visible sunlight (400-700 nm) is a mix of wavelengths that the atmosphere filters: short wavelengths (blue/violet) scatter out more readily, leaving longer wavelengths (red, orange, yellow) to dominate the direct light at sunrise and sunset.

Specific twilight features are predictable outcomes of that filtering. The rising deep-blue band opposite the sun is literally the Earth’s shadow in the atmosphere, climbing into the sky after sunset and contracting before dawn. Immediately above it sits the Belt of Venus, an orchid-to-lavender band produced when backscattered red light from high-altitude dust mixes with the residual blue of the sky. Alpenglow is a rosy, indirect illumination of snow or ice on mountains away from the sun, caused by low-altitude backscattering from particles and surface-reflected light; clarity and cloud cover determine how strongly these effects appear.