Frosty morning and Blue Skies

 

Why is the sky so blue in the winter?

The sky is blue because of the way sunlight scatters in the air. Sunlight is composed of different colors of light, each with a different wavelength. When sunlight reaches the Earth, some of it is absorbed by the surface, and some of it is reflected back into the sky. However, before it reaches our eyes, it has to pass through the air, which contains molecules of gas, dust, water vapor, and other particles. These particles can scatter some of the sunlight in different directions, depending on the wavelength of the light and the size of the particle.

The science of scattering

There are two main types of scattering that affect the color of the sky: Rayleigh scattering and Mie scattering.

Rayleigh scattering occurs when the particles are much smaller than the wavelength of the light. Rayleigh scattering affects shorter wavelengths more than longer wavelengths, meaning that blue light is scattered more than red light. This is why the sky appears blue on a clear day, as the scattered blue light reaches our eyes from all directions.

The effect of seasons

The color of the sky is not constant, but rather changes with the seasons and the weather. There are two main factors that influence the amount and type of scattering that occurs in the atmosphere: the humidity level and the sun angle.

Humidity level

The humidity level is the amount of water vapor in the air, which varies depending on the temperature and the precipitation. In general, the air is drier and colder in the winter, and wetter and warmer in the summer. This means that there are fewer water droplets and other particles in the air in the winter, which reduces the amount of Mie scattering. As a result, the sky appears more clear and blue, as the Rayleigh scattering dominates

Sun angle

The sun angle is the position of the sun in the sky, which changes throughout the day and the year. In general, the sun is lower in the sky in the winter, and higher in the sky in the summer. This means that the sunlight has to travel through more air in the winter, which increases the amount of Rayleigh scattering4. As a result, the sky appears more blue, as the scattered blue light becomes more intense.

A Shropshire Lad