Sunlight contains a range of visible wavelengths. As it passes through the atmosphere, molecules scatter shorter visible wavelengths more strongly than longer ones. Some of that scattered light reaches your eyes from directions away from the Sun. This process, called Rayleigh scattering, helps explain the blue appearance of a clear daytime sky. Although violet wavelengths are also strongly scattered, the spectrum of sunlight and the sensitivity of human vision contribute to our perception of blue. The explanation is more than simply saying that air is blue. Near sunrise or sunset, sunlight travels through a longer atmospheric path. More short-wavelength light is scattered out of the direct path, so warmer colours may become prominent. Clouds, dust, and other particles can change the appearance. Never look directly at the Sun during an observation. Practice: Compare the sky away from the Sun at different times. Record colour and cloud conditions, then explain why one observation does not represent every day.