The Blue Sky in Porto

The atmosphere itself is colorless. The blue color of the sky is a picture created by the interaction of the atmospheric molecules, water droplets, the sun, etc..When sunlight enters the atmosphere, the light with long-wavelength, such as red light, has a large transmission power and can be transmitted to the ground through the atmosphere; while short-wavelength light like purple, blue, and cyan lights are easy to generate scattering when they hit the particles in the atmosphere. The scattered light fills the sky, making the sky appear blue.